Richard PottsHuman Origins Program,National Museum of NaturalHistory, SmithsonianInstitution, Washington, DC20560, U.S.A., and Divisionof Paleontology, NationalMuseums of Kenya,P.O. Box 40658, Nairobi,Kenya. E-mail:potts.rick@nmnh.si.edu

Anna K.BehrensmeyerDepartment of Paleobiology,National Museum of NaturalHistory, SmithsonianInstitution, Washington, DC20560, U.S.A. E-mail:behrensmeyer.kay@nmnh.si.edu

Peter DitchfieldDepartment of Geology,University of Bristol,Wills Memorial Building,Queens Road, Bristol BS8 1RJ,Great Britain. E-mail:p.ditchfield@bristol.ac.uk

Received 22 December 1997Revision received 2 June1999 and accepted6 June 1999

Keywords: early hominidbehavior, paleolandscape,land use, paleoecology,Olorgesailie, Acheulean,stone tools, fossilized bones,paleosol, fluvial deposition,taphonomy.

Paleolandscape variation and EarlyPleistocene hominid activities: Members 1and 7, Olorgesailie Formation, Kenya

Paleolandscape research tests for variation in the spatial distributionof hominid artefacts and establishes the association of hominidactivities with paleoenvironmental features over distances of 100s to1000s of meters. This approach requires (1) precise definition ofnarrow stratigraphic intervals based on sedimentary criteria that canbe documented over a broad area, and (2) excavation of theseintervals in order to establish taphonomic and paleoenvironmentalcontexts. In this report, excavations of three target intervals within theearly Pleistocene deposits (992 to 780 ka) of the Olorgesailie basin aredescribed. Assessment of time-averaging and paleolandscape struc-ture shows that each target interval represents a relatively brief period(�1000 yrs) and exhibits a unique distribution of environmentalfeatures (e.g., topographic gradients, channels, soil development).Stone artefacts and fossilized animal bones are distributed nonran-domly in each interval, and include clusters that were five to 293times more densely concentrated than the laterally equivalent back-ground scatter. A paleosol in upper Member 1 preserves a relativelycontinuous distribution of artefacts and fossils, in contrast with themore patchy distribution in two intervals of lower Member 7. Weinfer that the difference between the two members reflects a realvariation in hominid land use—either a response to local environ-mental differences or perhaps a change through time in hominidinteraction with the environment. By expanding the comparativeanalysis to diverse basins, it should be possible to test for broaderevolutionary change in hominid activities. Examples drawn from EastAfrican Pliocene and early Pleistocene sites suggest that evolutionarychange in land use entailed (1) wider ranging of hominids and longerdistances of stone transport, (2) expansion of tool-assisted behaviorsto a wider diversity of environmental settings, and (3) more stronglyfocused placement of particular artefact forms (e.g., bifaces) indifferent areas of the landscape in response to specific environmentalfeatures, such as lava outcrops, stream channels, and lake margins.

Journal of Human Evolution (1999) 37, 747–788Article No. jhev.1999.0344Available online at http://www.idealibrary.com on

0047–2484/99/110747+42$30.00/0

archeological remains are commonlyassumed to have a clustered distribution,this was not documented with in situevidence prior to the development oflandscape-scale excavations (Potts, 1989a,1994; Blumenschine & Masao, 1991;Rogers, 1997). The latter research has beendesigned to investigate thin stratigraphicintervals, such as a single paleosol, over a

Introduction

Many organisms distribute their activities inresponse to spatial variation in their habitats.One peculiarity of humans is the tendencyto leave materials on the landscape—eitherunplanned litter or purposeful collections ofobjects and structures—that is evidence oftheir overall use of space. Although early

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broad lateral exposure. The stage was set forthis new research approach by prior studiesof Plio-Pleistocene surface scatters and con-centrations (Isaac & Harris, 1980, 1997;Stern, 1991, 1993). In contrast to surfacesurveys, landscape-scale excavation strat-egies have the advantage of allowing re-covery of artefacts and fossilized bones inprecise stratigraphic and taphonomic con-text. As a result, hominid and nonhominideffects, preservation biases, and correlatedsedimentary and paleoenvironmental fea-tures may be documented over wide areas.Similar approaches have been developed bypaleontologists to investigate importantpaleoecological and evolutionary questionsin samples of fossil biotas representing shortintervals of time (Bown & Beard, 1990;Wing et al., 1993).

A central paleoanthropological questionconcerns the ways that hominids respondedto their surroundings. In a paleolandscapeapproach, this question leads to manyothers, such as: did hominids leave signs oftheir presence (artefacts or skeletal remains)in proportion to their use of particularresources or habitats? Or were the associ-ations with particular habitats essentiallyaccidental or random, a product of manyidiosyncratic discardings of artefacts?Furthermore, were the patterns of behaviorand ecological interaction documented inone stratigraphic interval evident also inmany other intervals, suggesting a specificniche that characterized early toolmakersover a long time? Or were these patternsdiverse over time and space, indicatingvaried responses to different settings?

Posing these questions is straightforwardbut answering them is difficult, requiringlong-term efforts by scientific teams. Amongother things, this type of research requires(1) point-sampling of large areas (typically1–100 km2) by excavation, (2) adequatespatial samples at each excavation point, (3)microstratigraphic documentation of laterallithofacies relationships within the target

interval, (4) analytical methods of detectingand accounting for preservation biases, (5)reliable geochemical and fossil proxies withwhich to characterize past environments infine spatial detail, and (6) comparison of thereconstructed hominid–environmental re-lationship in one stratigraphic level or basinto other instances of comparable scale. Thislast point allows one to evaluate the gener-ality of an interpretation and to test foradaptive patterns or evolutionary shifts inbehavior.

The Olorgesailie basin, where landscape-scale studies have been underway since1985, is noted for its dense clusters of stonehandaxes and other Acheulean bifaces.Located in southern Kenya, the basin lies onthe floor of the Gregory Rift and is boundedto the south by the Pliocene volcanic com-plex of Mount Olorgesailie and MountShanumu, and to the east and west by horstsof plateau lava (Magadi Trachyte), whichalso forms the regional bedrock. The�80 msection of lacustrine, fluvial, floodplain, andvolcanigenic sediments of the OlorgesailieFormation ranges from 1·2–0·2 Ma, and isoverlain by the late Pleistocene Oltepesibeds (Deino & Potts, 1990).

Stone handaxes were discovered atOlorgesailie by J. W. Gregory in 1919(Gregory, 1921). M. D. and L. S. B.Leakey’s re-discovery in 1942 led to 2 yearsof excavation (by Louis Leakey) and map-ping (by R. M. Shackleton), followed byLeakey’s presentation of the site at theFirst Pan-African Congress of Prehistory,Nairobi, in 1947. G. Ll. Isaac directedresearch there from 1961–65, focusing hisexcavations in the Site Museum area. Herefined the stratigraphic nomenclaturedeveloped by Shackleton and produced asynthetic picture of variation in Acheuleanbifaces and artefact assemblages (Isaac,1977, 1978). While Isaac’s research wascharacteristically broad, involving initialtaphonomic experiments (Isaac, 1967) andpaleoenvironmental studies (Isaac, 1978),

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the work from 1942 through 1965 largelypursued the discovery and analysis ofhandaxes. It turns out that handaxes aredistributed very patchily, almost entirelyconfined to the Site Museum area,which represents a small part of the basin(Figure 1).

A paleolandscape approach was devel-oped at Olorgesailie in order to systemati-cally evaluate spatial co-variations betweenecological parameters and hominid artefactswithin high resolution time intervals. Priorto 1985, archeological interpretation ofhominid behavior usually was based onsmall exposures of dense concentrations ofstone artefacts and animal bones. For theearly Pleistocene and Pliocene of Africa,such excavation sites seldom exceeded 15 min diameter and rarely sampled the samestratigraphic level (Leakey, 1971; Isaac,1997). Although intensive study of thesesites has been highly informative, theynecessarily reflect very limited, isolatedsamples of the spatial range and behavioralspectrum of early hominid activities.Debates about early hominid behavior,theoretical considerations, and distribu-tional studies of surface-eroded artefactsdemonstrated the need to widen the spatialscope of excavated information about earlyhominids and associated taphonomic andpaleoecological variables.

Since our initial survey, research atOlorgesailie has developed in two phases.The first (1986–93) involved excavationof a fossil- and artefact-rich paleosol inMember 1 of the Olorgesailie Formation.This paleosol is bracketed by a volcanic ash0·992 Ma (A5 ash: 3·5 m below thepaleosol) and a pumice gravel 0·974 Ma inMember 5 (13 m above the paleosol) (Potts,1989a,b; Deino & Potts, 1990) (Figure 2).Considerable emphasis was also given toestablishing a new chronological, strati-graphic, and paleoenvironmental frameworkfor the basin (Deino & Potts, 1990, 1992;Tauxe et al., 1992; Potts, 1994; Sikes, 1995;

Potts et al., 1996). The second phase (1994to present) has involved comparing paleo-landscape excavations in different timeintervals and testing for correlations be-tween, on the one hand, hominid artefactsand behavior and, on the other, evidence ofenvironmental change in the southernKenya rift (e.g., changes in lake level,paleodrainage patterns, the rate/extent oflandscape remodeling, turnover in animalcommunities, and volcanogenic and tectonicevents). The objective of this second phasehas been to document variations in hominidbehavior in relation to environmental vari-ables, and to see whether the variationsreflect diverse strategies of hominid land useand ecological interaction.

Our purpose here is to lay out the com-parative framework of paleolandscaperesearch, and to illustrate it with findingsfrom Members 1 and 7 of the OlorgesailieFormation. In this paper the spatial pattern-ing of stone artefacts, fossil bones, and sedi-mentary observations is used to assess therelationship between environmental par-ameters and hominid activity. Detailed iso-topic evidence of Member 1 vegetation ispresented in the accompanying paper bySikes et al. (1999), and more specific evi-dence of hominid behavior (e.g., cut marks)and habitats will be reported in laterpublications.

An evolutionary ecological framework

Behavioral evolution arises from the relativesuccess of one or more strategies of behaviorover alternatives, including earlier, ancestralstrategies. An evolutionary perspectiveon hominids therefore demands carefulattention to behavioral variations and howthey changed in relation to the surround-ings. Patterns of relationship betweenbehavior and ecological setting are also theprimary concern of paleolandscape research.Its central approach is to excavate acrossbroad areas to test for differences in hominid

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use of space, material resources, landscapefeatures, and habitats within a single,narrowly defined stratigraphic interval. Bycomparing paleolandscapes and basins ofdifferent times and places, there is thepotential to define alternative strategies ofhominid land use and to study how these

were expanded or altered over the course ofhominid evolutionary history.

In contrast with this approach is the ten-dency to base generalized interpretationsof hominid behavior on single sites or onmodels applicable to a single time intervalwithin a basin. The focus on finding the

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single ‘‘correct’’ model or reconstructionmay be an appropriate pursuit for a particu-lar archeological site or for remains from asingle stratigraphic level. Yet even consider-ation of multiple sites and times within abasin may lead to identifying a diverse rangeof foraging options and other behaviors(Potts, 1994; Monahan, 1996). By docu-menting the varied relationships thatoccurred between activities and environ-ments, archeological research can poten-tially make an important contribution toevolutionary studies of early hominids.

An evolutionary ecological frameworkmay thus usefully be construed as a multi-dimensional matrix. The axes of the matrixreflect time, geographic location, anddiverse environmental variables (e.g., habi-tat diversity, physical geographic features,variations in tectonic and volcanic activity,etc.). The goal is then to fill in the matrixwith information about how hominid tool-makers (where artefacts are the mainindicator of hominids) responded to thevariables represented by these axes, and toinvestigate when, where, and why theresponses of hominids became expanded orchanged in some fundamental way. Crucialat all steps of the comparative process—whether among excavation trenches, orstratigraphic levels, or different sedimentarybasins—is the development of testablehypotheses.

Hypotheses pertinent to paleolandscape studiesat OlorgesailieIn modern landscape ecology, the fundamen-tal question is how the complex spatialstructure of landscapes affects ecologicalpatterns and processes. The first questionthen concerns the definition of spatial vari-ation. To what extent is a particular land-scape homogeneous? Are there differencesbetween one place and another? Only byanswering this basic question can one moveto more interesting issues: how does land-scape structure affect foraging dynamics, the

movements of organisms, predation risks, orcompetition? These questions about processmay still be framed as tests of variation—i.e.,whether one place or sampling area is differ-ent from other such places in terms of themeasurable proxies for foraging, organismdistribution, predation, or competition(Wiens, 1995).

The comparative approach applied topaleolandscapes also involves the search fordifferences among areas. In this regard theidea of a null hypothesis, which refers tothe lack of a statistically significant differ-ence among compared entities, is useful.The compared entities may be excavatedtrenches, or may involve two or more paleo-landscapes at different stratigraphic levels oreven in distant sedimentary basins. The nullhypothesis means simply that the preser-vation of traces of hominid activity is essen-tially the same everywhere one has looked.At the paleolandscape scale of analysis(where distinct excavations are compared),this implies that the net result of artefactdiscard and taphonomic processes was ran-domly or evenly distributed rather thanpatchy or clustered. When comparing strati-graphic levels, the null hypothesis impliesthat, whether clustered, even, or random indistribution, the pattern of hominid activi-ties was the same at different times. Finally,when comparing different basins, this samehypothesis implies that different settingselicited no discernible variation in thepattern of artefact discard—a possibilitythat would have intriguing implicationsregarding early hominid behavior andevolution.

The distinction commonly made betweendense concentrations and background scat-ters requires showing the null hypothesis tobe false—that there is significant variation inthe areal density of remains. To establishthat hominids treated paleoenvironmentalfeatures (e.g., woodlands vs. grasslands,near-channel settings vs. distal floodplains,or distinct lithic sources) in different ways

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also requires falsifying the null hypothesis.This means showing that there is a patterneddifference in the distribution of stone arte-facts in relation to landscape features, andthat this difference is due to behavior andnot preservational bias. Differences in thekinds of stone artefacts, associated animalbones, or the number of hominid- andcarnivore-damaged bones, furthermore,may suggest variations in activity that can becorrelated with paleoecological variations inways that imply causal relationships.

Knowledge about a region leads to manyspecific questions. Olorgesailie, for example,was a lake basin with a prominent volcanicmountain complex and surrounding high-lands. Did hominids make use of lake fish,other animals attracted to the lake margin,or both? Did hominids preferentially leavecertain kinds of artefacts close to the lakeedge and different ones further way [similarto the biased occurrence of handaxes morethan 1 km from the lake deposits in Bed IIOlduvai (Hay, 1976)]? At Olorgesailie,handaxes are known to be distributed alongshallow paleochannels. Were handaxes alsoused and discarded on the terrain adjacentto the stream channels? Many differentstone sources were available to hominidtoolmakers, so the presence of stone arte-facts in some places and their absence inothers may reflect the distance of transport-ing rocks. If a given type of stone materialdoes not decrease in abundance as afunction of distance from source (over 102–103 m), what other variables (rock mechan-ical properties, taphonomic factors, habitatpreference) may have been important?Finally, the division of the landscape intoupland vs. lowland is a distinctive aspect ofthe Olorgesailie basin. Is there evidence thathominids inhabited the lake and streammargins and relied solely on the resources ofthe lowlands (where, due to sedimentation,materials were preserved)? Or is there evi-dence that hominids were attracted toMount Olorgesailie and the surrounding

highlands as their primary habitat, merelyvisiting the lowlands for food and otherresources?

At a broader temporal scale, we want tofind out whether the pattern of hominid landuse varied between separate stratigraphiclayers. Different patterns of artefact discardand bone–stone association might be causedby several factors, such as the amount oftime represented in each stratigraphic inter-val; the intensity with which hominids usedor visited a particular landscape transect; therate of artefact discard; the net rate of boneaccumulation and burial. Variation in any ofthese factors could lead to apparent differ-ences in patterns of hominid land use, evenif the actual behaviors did not differ signifi-cantly. Many of these factors are difficult tomeasure or estimate, but comparative analy-sis may help to show which factors weremost important in creating the observedspatial patterns.

Increase in the spatial scale of analysis is acharacteristic of paleolandscape research.Extending the scale of the paleolandscapeapproach to diverse basins could demon-strate responses of hominid toolmakers towidely different distributions of stone out-crops, fluvial vs. lake-dominated environ-ments, or other variations beyond theadaptive conditions that occurred in any onebasin. This in turn could reveal if any behav-ior patterns were more or less independentof environmental setting, and which weredependent.

Methods and field observations

Basics of the paleolandscape approachWe identify here six baseline requirementsof the paleolandscape approach developedat Olorgesailie.

1. Each target stratigraphic interval mustbe defined with the finest possible scale ofresolution, based on sedimentary criteriathat can be applied over a broad area(>0·5 km of outcrop length; >0·1 km2).

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The objective is to discern a sedimentaryunit that can be traced laterally over a con-siderable outcrop distance while also mini-mizing the amount of time represented bythe target interval. Estimation of time-averaging usually is based on a combinationof radiometric ages, known accumulationrates for sediments such as diatomites,and estimated rates of pedogenesis orchannel-filling.

Even the most narrowly defined strati-graphic interval (e.g., a single paleosol) isnot equivalent to an actual landscape, pastor present (Kidwell & Behrensmeyer, 1993).Although any single event of hominidactivity did occur on a land surface, thecomposite of such events and environmentalprocesses recorded in a given stratumencompasses a much longer period oftime. During the early work at Olorgesailie,L. S. B. Leakey referred to the artefact-bearing levels as ‘‘land surfaces’’ (e.g.,UM1p=‘‘Land Surface 3’’, M6/7s=‘‘LandSurfaces 6 and 7’’) and treated them asif they were actual landscapes equivalentto observed terrain in the present. Wehave adopted the term paleolandscape to dis-tinguish our research from the land surfaceconcept and to emphasize that each targetlayer integrates an amount of time overwhich climate, vegetation, and sedimentaryprocesses may have changed the landscapein subtle ways. The paleolandscape conceptreflects the many diverse environmental,behavioral, and taphonomic processes andevents that are represented in the combinedevidence in a given target interval.

2. Since each excavation represents apaleolandscape sample, it should be suf-ficiently large to allow recognition of sedi-mentary and taphonomic anomalies. Forexample, a 1 m2 excavation of a paleosolmay happen to hit a small patch of reworkedsediments containing an artefact clusterwhose composition has been altered bywater flow. Such a sample, therefore, mayskew the comparison with artefact samples

obtained from other excavations in the sametarget interval. In Member 1 at Olorgesailie,we have found that larger excavations(usually �4 m2) help to place atypicalpatches in their proper sedimentary context,which allows us to identify and compareexcavated samples that derive from a similardepositional setting.

3. Excavations (referred to here assites—i.e., excavated places, not solely denseclusters of remains) of the target interval aredistributed randomly or evenly over most ofthe lateral extent of the erosion slope.Choices about the exact placement of sitesare affected by vegetation cover, erosionalslope contour, and accessibility. Where un-usual or dense concentrations of artefactsor bones are found (e.g., Hyena Hill inUM1p), exceptionally large or clusteredexcavations may prove helpful in docu-menting taphonomic, paleoenvironmental,and/or activity boundaries.

4. The following features are recordedand measured: sedimentary structures (e.g.,bedding types and sediment textures, rootmarks, paleochannels, cross-stratification,burrows, game trails), stable isotope values,faunal remains (densities, element andspecies composition, taphonomic indica-tors), artefacts (densities, tool types, rawmaterials), and other environmental evi-dence (e.g., fossil pollen, phytoliths). Spatialvariation in each of these features is ana-lyzed, and correlations among the variablesare then used to document and to assesstaphonomic bias, environmental effects, andhominid behavior.

5. The density of artefact and faunal bonecollections in each excavated site shouldbe calculated in cubic meters rather thansquare meters. Standardizing for thevolume of sediment (number of items percubic meter) is necessary when comparingexcavations on a paleolandscape scale.

6. Taphonomic analysis, which testsfor an equivalence of taphonomic mode(Behrensmeyer, 1991; Behrensmeyer &

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Hook, 1992) over the paleolandscape,should precede the behavioral interpretationof clusters, background densities, or corre-lations between hominid remains and paleo-ecological variables. The effects of waterflow, downslope movement of materials,juxtaposition of remains in favorable burialsettings, and random associations betweenbones and artefacts are among the manyfactors that affect artefact densities, mimicthe results of hominid activity, or can bemisinterpreted as hominid attraction to aparticular landscape feature or resource.

Target intervalsThree stratigraphic intervals in the Olorge-sailie Formation are the focus of this report,one in Member 1 and two in Member 7(Figure 2). These intervals are depicted atdifferent scales in Figures 3–7 in order toshow the overall stratigraphic context of thepaleolandscapes over kilometer-scale dis-tances laterally as well as details of thelithologies representing 10–100 m scale vari-ation across these landscapes. Figures 4 and5, and Figures 6 and 7, are drawn at thesame scales (and vertical exaggeration) tofacilitate comparison between Members 1and 7.Upper Member 1 paleosol (UM1p):

yellow-brown to buff-white, rootmarkedsiltstone, typically 10–20 cm thick, rang-ing up to 120 cm thick. UM1p representsa paleosol interbedded with the diatomitesof Member 1 below and Member 2 above.It preserves rich assemblages of fossilizedbones and stone artefacts (Potts, 1989a,1994). The age of this interval is approxi-mately 990 ka (Deino & Potts, 1990).

Member 6 and Lower Member 7 sands(M6/7s): a complex of whitish greychannel sands, diatomaceous gravels, andsandy overbank deposits, 10–100 cmthick, in which the classic handaxe con-centrations of the Site Museum area(Locality C) are preserved. The Member6 sand, defined by Shackleton (1978) and

Isaac (1977, 1978) as a separate member,is a thin but remarkably continuous sheetsand which is not cut by lower Member 7channels but instead can be traced intothe upper sandy lenses of lower Member 7[Isaac’s unit 7c at Site DE/89 (Isaac,1977)] that fill channels cutting intoMember 5. Thus we now consider Mem-ber 6 to be part of the lower Member 7fluvial sands, and refer to them collec-tively as ‘‘M6/7s’’. Based on 40Ar/39Ardates (Deino & Potts, 1990), the esti-mated age of this interval is about 900 ka.

Lower Member 7 diatomaceous silts(LM7ds): 20–40 cm of white–buff di-atomaceous silts immediately overlyingM6/7s, which represent a period of fluc-tuating subaerial and subaquatic con-ditions during final filling and burial ofthe channel complex of lower Member 7[equivalent to Isaac’s unit 7e at SiteDE/89 (Isaac, 1977)]. Fossil and archeo-logical materials in LM7ds were buriedduring a phase in which lacustrine di-atomite and fluvial deposition alternatedprior to more stable lacustrine conditionshigher in Member 7. The age of thisinterval is probably near 900 ka.

A fourth stratigraphic interval was alsosampled during the excavation of Member7:Upper Member 7 paleosol complex

(UM7p): A well-developed greenishbrown paleosol, 5–100 cm thick, whichappears to be a composite of successivepedogenic units in some areas but whichalso occurs as a vertically continuous soilimmediately above LM7ds [equivalent toIsaac’s unit 7f at DE/89 (Isaac, 1977)].UM7p is not archeologically rich and wasnot a primary target of paleolandscapesampling; but its excavation in the processof reaching underlying strata affords avaluable comparison. Judging from thematurity of the soil, its development mayhave encompassed much of the time

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(194 ka) represented between Member 5(974 ka) and the Brunhes–Matuyamaboundary (780 ka) near the base ofMember 8 (Deino & Potts, 1990; Tauxeet al., 1992).

Upper Member 1We have divided the Olorgesailie basin intoseveral areas of sedimentary exposure, calledlocalities (see Figure 1). The target paleosolof upper Member 1 (UM1p) is exposed inLocalities A, C, and D, over an outcroplength of �4 km, as measured laterallyalong the erosional slope (Figures 1, 3). Thearea that encompasses the exposure ofUM1p totals �3·2 km2. The paleosol isvery poorly exposed in Locality D, and isbest observed in Locality A. Eighty-fiveexcavations were dug into UM1p, 70% ofwhich were �4 m2 in area (Table 1). Anunusually large number of excavations wereopened in an exposure known as Hyena Hillin order to establish the spatial limits ofcomplex clusters of stone artefacts and fossilbones. Spatial coordinates of all in situ arte-facts and faunal remains were measuredusing a Topcon EDM and tied into a single,basinwide coordinate system (Potts et al.,1996).

During excavation, the following sedi-mentary sequence was observed: white di-atomaceous siltstone (layer 5) overlain by aunit of yellow-brown silt, clay, and sand thatincluded the target paleosol (layer 4);covered by interbedded diatomite, buff

and grey siltstone, and sandy siltstone (layer3); capped by diatomite and reworkedtuffaceous sands (layer 2). UM1p representsmodest pedogenic alteration of diatomite,terrigenous silts and clays, pockets ofreworked volcanic ash, and lenses ofmedium to coarse, poorly sorted sand. Thislayer was strongly rootmarked and preservedburrows, footprints, and other evidence of atemporary land surface. In places (e.g.,Hyena Hill), the overlying layer 3 exhibitssome pedogenic alteration and preserves

fragmentary fossil bones and stone artefacts.These materials are not counted as part ofUM1p. In Locality A, layer 2 is mainlyrepresented by reworked tuffaceous sand,and, in Locality C, by massive diatomaceoussilts that contained ash pods and lensesequivalent to the tuffaceous sands in Local-ity A. The upper Member 1 sequence is thencovered by the diatomites and lacustrinesilts of Member 2.

UM1p is associated stratigraphically withtwo laterally extensive marker units, theMain Marker Ash below and the Evaporite/Carbonate Marker (E/C Marker) above(Figures 3, 4, 6). The combination of thesemarkers and the well defined sequence oflithologies in layers 2–5, plus the nearlycontinuous outcrops, ensure that UM1p iscorrectly identified in all sample sites as asingle lithostratigraphic unit. The E/CMarker is a 2–15 cm thick layer of calciumcarbonate overlying and filling shallow dessi-cation cracks formed on the underlyingdiatomite. This layer includes molds ofcubic halite (NaCl) crystals, and it appearsthat the calcium carbonate secondarilyreplaced an evaporitic crust formedduring a brief episode of widespread lakeregression and dessication. The E/CMarker is more continuous than the MainMarker Ash, and because it represents adepositional event producing a uniformlithofacies over a wide area, it can beused as a horizontal reference plane forplotting the paleotopography of the UM1p(Figures 3, 4, 6).

In most excavated archeological and geo-logical trenches, UM1p is a simple, welldefined bed, 10–40 cm in thickness, consist-ing of distinctive yellow-brown clay, graytuffaceous silt, and light yellow to whitediatomite. CaCO3 nodules are rare to com-mon (see Sikes et al., 1999); and there areabundant clay-filled, mostly vertical roottraces from <1·0–4 mm diameter and apoorly developed vertical ped structure.Outside of the channeled area between

757

Trenches 10 and 21, layer 4 consists of acombination of in situ, altered parentmaterial from layer 5 and overbank siltsfrom the channel or interfluve sheetwash.The contact of layer 4 on layer 5 is usuallysharp and irregular and appears to representthe downward limit of most pedogenicprocesses, particularly bioturbation, exceptwhere there are coarse sediments associatedwith obvious channel cut-and-fill features.Root traces and breccia-filled cracks in theunderlying diatomite are evidence that thewater table was at least periodically lowerthan the layer 4/5 contact and that layer 5also was pedogenically modified. The pres-ence of nodular carbonate at the 4/5 contactor within the upper 0·5 m of layer 5 suggestsprecipitation near the top of the water table(Tucker & Wright, 1990), which may havebeen stable for some time near the top oflayer 5 when the paleosol was formingimmediately above.

Each in situ artefact and fossil bonerecovered from layer 4 was assigned to a

sedimentary unit based on grain size andcolor of the surrounding sediments. Forexample, the prominent light brown silt-stone that comprises much of the upper partof the layer 4 was designated 4LF (light,fine), while dark grey silty fine sandstonewas designated 4DC (dark, coarse). In thisreport, we control for depositional andtaphonomic equivalence of the assemblagesfrom different trenches within UM1p bylimiting our analysis to artefacts and fossilsfound in pedogenically altered units of thefinest grain size (4LF).

0 1.0 km

2.0

1.0

0 m

AD1–1

Actual plan view

0 1.0 km

N

80

C91/01

C93/01

25

102170D91/04

D91/04 AD1–1 170 150 102 12A 15 25 C91/01 C93/01 60 80

NE lavaoutcrop

Lavahump

HyenaHill

Site museum

West Northeast

Trachyte Upper Mb. 1 Paleosol

Basalt

E/C marker

Main marker ash

Horizontal reference line

Figure 3. Lateral profile of Member 1 and lower Member 2 from Locality D (west) through Locality A toLocality C (Site Museum; northeast), showing the boundaries of the UM1p (layer 4) in relation to theoverlying Evaporite/Carbonate Marker unit (E/C Marker) and the underlying Main Marker Ash, over adistance of 3·68 km. The thin, laterally extensive E/C Marker represents an evaporitic deposit in shallowlacustrine strata and is used as the horizontal reference for the profile. Individual points on the profile arespaced as shown in the Plan View (inset) and projected on to a two-dimensional plane, with 135� verticalexaggeration. Actual gradients are very low, e.g., 0·8 m in 200 m for a slope of 0·4% between the ‘‘high’’at the hyena den site (Site 102) and the base of the UM1p at the elephant excavation (Site 15).

Member 7Outcrop exposures of Member 7 cover anarea of �5·5 km2 in localities A–D. Theseexposures have been most thoroughlystudied in the Site Museum area (LocalityC), where they occupy a strip approxi-mately 800�125 m (Figure 1). In thispaper we confine our analysis to the SiteMuseum area in order to explore the spatialpattern of hominid activities near the dense

758 . ET AL.

Figure 4. Lateral profile of Member 1 and lower Member 2 in Locality A, showing the major units inupper Member 1, including UM1p, in relation to the E/C Marker and the Main Marker Ash over adistance of 0·9 km. Individual points (trenches and archeological sites) on the profile are spaced as shownin the Plan View and projected on to a two-dimensional plane, with 36� vertical exaggeration. Theprofile shows the paleotopography of UM1p from Site 170 to Site 25, with the most prominent featurebeing a channel between Site 10 and Site 305. Orientations of channel features are indicated by thecurrent direction symbols and degree measurements. Artefacts and bones occur in varying concentrationsin UM1p throughout the lateral extent of this profile (see text). The heavy bar at the top of the profileindicates the position of the fence diagram in Figure 6.

concentrations of bifaces documented inthis area. In addition to five main exca-vations by Leakey and Isaac (Isaac, 1977),we have opened 14 others in Locality C thatsample the three stratigraphic units ofM6/7s, LM7ds, and UM7p.

Members 4–8 have a relatively consistentand recognizable sequence of lithologiesthroughout Locality C. The total thicknessof these members is on the order of 10 m,with Members 6/7 consisting of 2–3 m ofprimarily diatomaceous silts and volcani-clastic sands within this total. A widespreadsheet of medium-grained, greenish graysand with reworked diatomite, obsidian, andoccasional pumices forms Member 6 in thestratigraphic nomenclature of Shackletonand Isaac. This unit overlies yellowish-white, relatively pure diatomites of Member5 with little evidence of significant erosion orchannel cutting except in the vicinity of

excavations C7–11, C7–13, H9, and DE/89.Our work indicates that these sands arecontinuous with more complex channeldeposits within the base of Member 7, andwe thus refer to the composite unit asM6/7s. Above the sandy deposits in someareas are diatomaceous silts with minorpedogenesis (LM7ds), followed by lacus-trine diatomites with relict horizontal bed-ding, which grade up into more massive,pedogenically modified diatomite cappedby a distinctive, greenish-brown, clay-richpaleosol with locally abundant CaCO3 nod-ules. This paleosol (UM7p) can be tracedthroughout Locality C and also westward inLocality A. It clearly represents a significantperiod of landscape stability and soil for-mation. Above the paleosol are reworked,massive diatomites of Member 8, whichcontain abundant CaCO3 nodules and roottraces indicating pedogenesis but without

759

any significant concentration of clay. Abovethis poorly developed paleosol are distinc-tive, red-stained, irregularly bedded silts andvolcanic ash beds. The contact betweenthe lowermost paleosol of Member 8 andan overlying grey-brown ash is used as ahorizontal reference plane for plottingpaleotopography in Member 7 (Figure 5)because in most places it represents a con-formable contact between a soil surface andthe transgressive lacustrine deposits of lowerMember 8.

Figure 5. Lateral profile of the Member 7 target intervals in Locality C, showing the major units (Members4–lower 8) plotted in relation to the contact between a poorly developed soil and an overlying volcanic ashin Member 8, over a distance of 1·1 km (note that this is a south–north profile, in contrast to the west–eastprofile in Member 1). Individual points (trenches and archeological sites) on the profile are spaced asshown in the Plan View and projected on to a two-dimensional plane, with 36� vertical exaggeration.The profile shows the paleotopography of archeological levels in Member 7 from Geological ReferencesS10 to S4, with several channels represented by topographic lows and thickening of the Member 6/7 sand.Excavation sites are indicated below the profile in their approximate positions relative to the 2-Dprojection. ‘‘X’’ indicates archeological occurrences within LM6/7, which are more localized than inUM1p (Figure 3 and text). The heavy bar at the top of the profile indicates the position of the fencediagram in Figure 7.

Lava outcrops and lithic sourcesPaleolandscape research typically focuses onlowland strata, where bone remains andartefacts were susceptible to sedimentaryburial and preservation. In addition, thistype of research may identify importantenvironmental features that existed onthe periphery of a basin during periods of

hominid occupation. The most prominentexample at Olorgesailie involves the volcanicrocks of Mount Olorgesailie and nearbyhighlands, which were used by toolmakersas sources of lithic raw material. The sedi-mentary strata of the Olorgesailie Formationare somewhat unusual in that they do notinclude channel deposits with lava cobblesthat could have served as raw material, inspite of proximity to the volcanic uplands.To date, we have identified 14 distinctiverock types used as sources and have mappedat least 35 separate outcrops of these rockson the mountain and ridges surroundingand within the basin. Although nottreated in detail here, knowledge of thesesources draws attention to the attraction oftoolmakers to the Olorgesailie highlands.

Figure 1 shows the lowest foothills ofMount Olorgesailie and two prominent lavaoutcrops that project into the lowlands.

760 . ET AL.

The easternmost outcrop (near the SiteMuseum, Locality C) has been termed LavaPeninsula [after ‘‘peninsula’’ or ‘‘lava ridge’’of Isaac (1997)]. Although this name accu-rately describes the present exposure, we usethe term Northeast Lava Outcrop to identifyits probable exposure during Member 1 as abroad plain of volcanic rock northeast of theSite Museum area. The second lava outcropis called Lava Hump (separating LocalitiesA and D), presently exposed as a projectionfrom Mount Olorgesailie (Isaac, 1977; seeFigure 1). This southern outcrop, calledLava Hump (S), was largely buried byMember 2 sediments, while a northernoutcrop—Lava Hump (N), largely buriedtoday but aligned with the present-dayprojection—was a relatively prominent fea-ture during deposition of Members 1–9 (seeFigure 8 and later discussion).

A rare volcanic rock, termed LH trachyte,is found on the northeastern slope of LavaHump (S) where it meets the lake basinsediments of Member 1. The LH trachyteoutcrops are immediately overlain by theevaporite carbonate (E/C Marker) of lowerMember 2, and here an in situ quarry site(AD1-1) equivalent in time to UM1p wasdiscovered and excavated in 1997. Artefactsof two other rare lithic materials, obsidianand quartzite, are found in Members 1 and7. These materials were introduced fromdistant sources, a minimum of 26 and45 km, respectively, from the Olorgesailiebasin (Isaac, 1977).

Statistical methodsIn this paper we focus our comparisons onthe density (degree of concentration) ofstone artefacts and animal bones. The firstobjective is to demonstrate the extent towhich the behavioral and taphonomicprocesses of deposition, scattering, andaggregation resulted in random, even, orclustered distributions of artefacts andbones. These distributions are then exam-ined in relation to well delimited features

(e.g., channels, lava outcrops), patches (e.g.,grassy plains, marshes), topographic gradi-ents, and other characteristics of the time-averaged paleolandscape in order to assesshow environmental factors and hominidactivities were related.

Four statistical tests are employed:1. Test of random spatial distribution

(Poisson distribution): Equal probabilitythat an artefact or bone will fall in any cell(or excavation square) of a paleolandscapearea is described by a Poisson distribution,where the mean � for artefact or bone den-sity equals the variance �2 (Hayek & Buzas,1997). Thus �2/�=1·0 means that objectsare randomly distributed over the sample ofexcavations (i.e., there is no preference forwhere artefacts or bones were dropped orended up in the target stratum). There aretwo types of nonrandom distribution (i.e.,deviations from the Poisson distribution):(a) an even distribution, where �2/�<1·0,and (b) an aggregated or clustered distri-bution, where �2/�>1·0. There is no level ofsignificance attached to this statistic; rather,it provides a relative measure of the devia-tion from randomness—the degree to whicha comparison of two distributions shows oneto be more even or more clustered than theother.

2. Chi-square test. This familiar testmeasures the degree to which a distributionof artefact or bone densities matches a ran-dom distribution of densities. The densitycontinuum is divided into four cells: 0–1objects, 1–10 objects, 10–20 objects, and>20 objects per 0·1 m3; and the number ofexcavation sites in each cell is counted. Theobserved distribution is then comparedagainst the ‘‘expected’’ distribution of anequal number of sites in all four cells. Thechi-square complements the Poisson distri-bution test by providing a significance levelat which the null hypothesis is violated.

Two other tests are applied when compar-ing different paleolandscape distributions(e.g., UM1p vs. M6/7s):

761

3. Kolmogorov–Smirnov test; and4. Mann–Whitney U test.

These tests are nonparametric, as the distri-butions of artefact and bone densities aredemonstrably nonnormal. Both tests calcu-late whether two independent samples (inthis case, the density of artefacts or bones intwo different strata) have been drawn frompopulations with the same distribution. TheK–S test is concerned with the agreementbetween two cumulative distributions, whilethe U test is the usual, and most powerful,nonparametric alternative to the t-test(Siegel, 1956). Systat 6.0 was used tocalculate the tests and levels of significance.

Analysis of Upper Member 1

Figure 6. Fence diagram showing details of the paleotopography and lateral facies relationships for UM1pbetween Geological Trench 87-I and Site 22 in Locality A. Stratigraphic sections are shown in theircorrect plan view relationships, not projected on to a two-dimensional plane as in Figures 3 and 4.Irregular vertical lines on the profiles indicate pedogenesis. Positions of channel boundaries (dashed lines)are based on facies changes and paleo-current indicators.

Paleolandscape reconstructionThe UM1p paleosol is remarkable for thelateral continuity of its internal features over4 km, the generally flat paleotopography,

and the lack of channeling other than aslight depression represented by a partiallyfilled, shallow channel eroded into layer 5between Sites 10 and 305 (Figures 4, 6).The terrain overall was a homogeneousplain crossed by one west- or southwest-directed water course. The plain laybetween two raised areas of volcanic rock,the Lava Hump and the Northeast (NE)Lava Outcrop (Figures 3, 8). West of theLava Hump in Locality D, the plain con-tinued at least another 0·5 km, but theunderlying volcanic basement in this part ofthe basin was closer to the surface andUM1p is superimposed on reddish brownsilts mixed with diatomite rather than therelatively pure diatomites of Localities A andC. In Locality C, to the northeast, thereare three pedogenically modified units, thelower one correlated with UM1p and theupper two in layer 3. UM1p (layer 4) is lessdistinctly separated from the underlying

762 . ET AL.

diatomite (layer 5) except locally (e.g., Site60) in Locality C, implying that the paleo-topography sloped gently up toward the NELava Outcrop and the soil formed directlyon the underlying diatomite with only trivialadditions of fluvial silts (in contrast toLocality A). Excavations at the edge of theNE Lava Outcrop indicate that a Member 1paleosol (partially equivalent to UM1p)formed directly on the irregular, low-lyinglava surface. Thus this area records a localedge of the lacustrine basin at the time.Layers 2 and 3 of Member 1 and the over-lying diatomites of Member 2 covered thislava northeast of Locality C, and we hypoth-esize that it formed a relatively flat, slightlyelevated plain in Members 1–2 times ratherthan a fault-bounded ridge as in the present.Thus, the Lava Peninsula (Figures 1 and 8)that is reconstructed as a prominent paleo-geographic feature above Member 5 wassubdued or nonexistent as a ridge earlier inthe sequence.

Immediately above UM1p adjacent to theNE Lava Outcrop, lithofacies features inlayers 3 and 2 indicate swampy or lacustrineconditions whereas these layers have morefluvial characteristics in Locality A. Basedon its chemical composition (A. Deino,personal communication), a volcanic ashdirectly links this layer 2 facies in the twolocalities; it occurs in fluvial sands in Local-ity A but as an airfall unit in diatomites inLocality C. Although the cross-section plot-ted relative to the E/C Marker (Figure 3)shows that UM1p slopes down toward thenortheast, this could indicate increased sub-sidence after deposition of layer 4/UM1prather than an original slope on the paleosolsurface. Increased subsidence near the NELava Outcrop would also account for athicker layer 3 and more lacustrine facies inlayer 2 compared with more fluvial faciesin these layers in Locality A.

Preserved current indicators (cross-stratification) in the channels are rare, butwhere these are found they consistently

indicate north- and west-directed channelsin layers 2–4, implying that the paleolakelay toward the northwest through the timeencompassed by the deposits of upperMember 1. These westward drainage direc-tions are at odds with a gradient towardlacustrine conditions in the northeast inlayers 2–3. This anomaly could be resolvedif the channels represent distributaries on analluvial lobe prograding into a lake thatextended from the north around the lobe tothe west and possibly also to the southwestinto the Koora Graben (Figure 8). Thisscenario requires a relatively fixed drainagepattern directed from east to west, whichwould be generally parallel to the present-day drainage of the Ol Keju Nyiro River(Figure 1). The NE Lava Outcrop and theLava Hump may have influenced this drain-age by forcing it south, then north as shownin Figure 8.

It is remarkable that even with the prox-imity of lava outcrops with several meters oflocal relief during Member 1 time, there isso little coarse sediment (pebbles, cobbles)present in the channel fill, other than thearcheological materials. This implies thatthe active drainage as well as local tributariesand overbank flow lacked the energy totransport this material even over distances ofa few hundred meters. Moreover, the lavaexposures may have been heavily vegetatedand covered with a stable soil that inhibitederosion and fluvial dispersal of pieces of thevolcanic basement.

It is not possible to safely infer the exactposition of the lake shoreline during the timeof hominid activity on the UM1p land sur-face, but the paleogeographic reconstructionbased on available evidence indicates that itlay to the west or northwest (Figure 8). It isvery likely that lacustrine or swampy areaswere within a few kilometers of the UM1pareas sampled in this study, but it is notclear whether Locality C was closer to thewet areas than Locality A at the time ofland surface occupation. Based only on the

763

features of UM1p, rather than an overlyingstrata, it appears that Locality C may havebeen slightly more elevated than Locality A.It is also possible, however, that subsidenceproximal to the NE Lava Outcrop createdlocal ponds or an embayment of the laketoward the end of the period of UM1pformation.

The overall geological history of upperMember 1 can be reconstructed as follows:( 1) Following regression, the land surface

was stable for some time and initialpedogenesis on the emergent diatom-ite created the soil features in layer 5and began the development of layer 4.A 60-m wide, west-directed channelsystem cut across the emergent lakeplain and overbank flow carried tuffa-ceous silts and fine sands onto theadjacent land surface. It is possiblethat the hyena den at Site 102 wasexcavated during this time, when thewater table was relatively low andcrack systems formed in the emergentdiatomite.

( 2) The water table rose and stabilizedtemporarily, and the nodular carbon-ate layers were precipitated underlayer 4, while the upper part of thechannel system in Locality A and adja-cent floodplain surfaces aggraded withfine-grained sediment (4LF), andpedogenesis continued across the landsurface. The hyena den collapsed andwas filled in with layer 4 sedimentsduring this interval.

( 3) Subsidence near the NE Lava Outcropcreated ponds or a local embayment ofthe lake, and pedogenesis occurred onthe resulting diatomites during periodsof emergence (layer 3). (This impliesthat UM1p was buried near the NELava Outcrop while it was still activeas a landsurface in Locality A.)

( 4) Another brief regressive phase led toincision by a channel through UM1pwest of Hyena Hill (Trench 87-IV)

and other minor channeling into the topof layer 4 (Figure 4). UM1p was theninundated and saturated as the lake roseonce again, hippo trackways formed,and the diatomites of layer 3 weredeposited in Localities A, C, and D.

( 5) Another regression led to channelinginto layer 3 and fluvial deposition ofthe volcanic silts and sands of layer 2.

( 6) A major lake transgression inundatedthe clastic deposits of layer 2 andbegan the diatomite deposition ofMember 2. Aside from occasional siltylaminae in the lower diatomites of thisMember, the only visible break inlacustrine deposition within LowerMember 2 is the E/C Marker.

It is possible that the period indicated by(2) above would have been the most favor-able for both hominid occupation and thepreservation of bones and artefacts becauseof a relatively high water table and associ-ated plant growth plus the addition to theland surface of fertile, volcaniclastic over-bank sediments. The channel at Site 15 wascut and partly filled during phase (1), andthen formed an abandoned swale that wasinfilled with fine sediment during phase (2).Pedogenesis proceeded at this site duringinfilling, and produced three soil-modifiedstrata at the locus where an elephant diedand butchery took place. Sediments of thelower two strata were mixed at the elephantskeleton, indicating that butchery occurredafter these two periods of pedogenesis;the activities at this site were followed bycontinuing fine sedimentation and a thirdperiod of pedogenesis. The butchery activityat Site 15 thus occurred within the timeinterval represented by other artefacts andfossils encased in the fine sediments (layer4LF) of UM1p.

Estimated amount of time for UM1pJudgments concerning the amount of timerepresented by the bone and stone accumu-lations are critical to the interpretation of

764 . ET AL.

hominid behavior. Estimation of time-averaging represented by single layers ofsediment in the geological record usually isbased on two lines of evidence: (1) calcu-lation of average rates between two datedpoints within the same stratigraphicsequence, and (2) inferences about probabledepositional rates of particular types of sedi-ments based on modern analogues. We usean approach that incorporates both of theselines of evidence as well as depositionalfeatures that suggest relatively long or shortintervals of erosion, land surface stability,or sediment build-up. The comparativeapproach is particularly appropriate forassessing differences in the relative amountsof time in the UM1p and Member 6/7artefact-bearing deposits, even if it is notpossible to provide a definitive measureof the absolute time span represented ineach target interval (see Paleolandscapecomparisons below).

There are 16·5 m of sediment betweenthe two 40Ar/39Ar age estimates thatbracket UM1p (0·992�0·039 and 0·974�0·010 Ma). The difference between the iso-chron ages is 18 Kyr; taking into account theerror range, the time span could be as littleas �0 to as much as 67 Kyr. A directcalculation of sediment rate based on an18 Kyr time interval gives 0·92 m per 1000years. The thickness of layer 4LF is10–70 cm, suggesting a maximum time spanfor accumulation of about 760 years. At itsthickest point within the channel fill at Site15, there are three distinct pedogenic zoneswithin the fine silts, implying interruption byfluvial aggradation; each soil unit would thusrepresent about 250 years of accumulation.

The 16·5 m of strata consist mainly ofdiatomites (11 m), but also include the maintarget paleosol UM1p, two periods of minorsoil formation in layer 3 and one at the topof layer 5; at least 3 separate phases ofchannel cutting and filling associated withregression–transgression cycles of the lake(layers 2–4); an evaporite layer (the E/C

marker) �15 cm thick; and 4 m of volcanicsands and gravels (Member 4). Rates ofHolocene lacustrine diatomite deposition(Isaac, 1977:22) range from about 500 yearsper meter to 1200 years per meter, with theslower rate probably most appropriate forthe shallow-water diatoms of paleolakeOlorgesailie. By this reasoning, a total of13,200 years is represented by the 11 m ofdiatomites and diatomaceous silts, leaving�4800 years for the periods of pedogenesis,fluvial erosion and deposition, transgressionand regression, and evaporite formation.The channels in layers 2 and 3 are narrowand relatively steep-sided, suggesting lesstime for development of these channelsthan the wider one cut into layer 5. Of thepaleosols in upper Member 1, the periodrepresented by UM1p was the longest, andincluded pedogenesis of diatomites, concur-rent channel filling and associated overbankdeposition, and pedogenesis of thesedeposits. Taking all of this evidence intoconsideration, we estimate that the period ofsedimentation and pedogenesis reflectedby UM1p, particularly the fine-graineddeposits (4LF) that contain the archeo-logical materials examined in this report,represents <1 Kyr. This is consistent withthe immaturity of the UM1p soil based onthe lack of identifiable clay mineral specieswithin the paleosol (Cooke, n.d.).

Distribution of stone artefacts and fossilanimal bonesTable 1 lists the sites, areas of excavation,sediment volumes, and densities of artefactsand fossils (number of specimens per0·10 m3) recovered from the rootmarked silt-stone of UM1p. Our analysis is limited toremains found in fine sediments of the layer 4paleosol (4LF). This is done to assure thetaphonomic comparability of excavatedsamples of artefacts and fossils. A paleosolmay preserve objects originally deposited byoverbank flow or runoff/sheetwash and thenburied by sediments prior to pedogenesis. It

765

may also preserve material incorporated intothe same stratum during the period of pedo-genesis, for example, by bioturbation andaccretion of fine-grained sediment. By elimi-nating the sandier pockets and lenses, ourobjective is to limit the potential biasesrelated to water flow that introduced coarsersediments prior to pedogenic alteration.These coarser units appear to have preservedmixed assemblages of artefacts and bones,including objects that were moved and buriedduring initial sedimentation of layer 4 in someareas, as indicated by slight to moderaterounding of edges, as well as objects buriedduring subsequent pedogenic phases.

The finer rootmarked silts, on the otherhand, represent pedogenic alteration ofdiatomites and fine-grained volcaniclasticsduring a period of relative land surfacestability, when only a minor amount ofallochthonous sediment was added to thesoil. Samples from this component of thepaleosol (4LF) thus reflect greater consist-ency from a depositional and taphonomicperspective. Artefacts and bones from thislithology were moved and deposited almostexclusively due to behavioral or ecologicalfactors. Moreover, the fossilized bones werealmost always fragmented and weathered,indicating their exposure to subaerialdecomposition and mechanical breakage ona relatively stable land surface prior toburial. In this analysis, bone and artefactdensities are calculated using numbers ofexcavated specimens rather than the recon-structed number of bone elements or thetotal weight or volume of stone material.

Statistical tests applied to the UM1p data-set demonstrate that the artefact and bonedistributions are nonrandom and, in fact,highly aggregated (Table 3). For example,values of the Poisson distribution test forstone artefact and bone densities are 7·73and 13·55, respectively (artefacts are over 7times more aggregated than a random distri-bution value of 1·00), and the chi-squarevalues show a significant difference from

randomness (P<0·001, based on an equalprobability of any discarded artefact endingup in one of four density groupings (seeStatistical methods, above). The paleoland-scape sampling area is sufficiently large,however, to show that the degree of cluster-ing is scale-dependent. For example, the 22excavations at Hyena Hill, which are northand east of the dense concentrations on theperiphery of the hill, show random distri-butions of both artefact and bone densities(Poisson test values: 0·996 and 0·914).

The raw data (Table 1) indicate that bothdense concentrations and diffuse scatters arerepresented in UM1p. There is a break inthe artefact and fossil bone distributions atabout ten objects per 0·10 km3. With thedenser clusters removed, the backgrounddensity sites exhibit statistical means of�2·3 bone specimens per 0·10 m3 and�3·0 artefacts per 0·10 m3. Consideringonly these diffuse scatters, both the artefactand bone densities exhibit relatively randomdistributions, with Poisson values of 1·36(artefacts) and 1·38 (fossil bones). Animalbones were found, it should be noted, in allexcavations except one (Site 140; 4·0 m2

area), and stone tools in all but three (Sites12b, 17a, and 80), each of which happens torepresent a narrow trench dug primarily toexpose strata and thus provides an unusuallysmall sample area.

Excavations of denser clusters includeSites 1, 2 (plus extensions), 7 (stones only),15 Main (plus extensions), 102 Hyenas, 102Extension, and Isaac’s I3 site (see Table 1).The mean densities for this subsample are15·7 and 19·3 objects per 0·10 m3 for arte-facts and bones, respectively. According tothese calculations, the higher density sitesin UM1p contain approximately 5·2 timesmore artefacts and 8·5 times more fossilbones than the diffuse occurrences, on aver-age. This is the first direct calculation of thedifference commonly made between high-and low-density sites based on excavation ofa Pleistocene locality.

766 . ET AL.

Tab

le1(

a)E

xcav

atio

nd

ata

for

up

per

Mem

ber

1p

aleo

sol

(UM

1p),

incl

ud

ing

area

and

dep

thof

exca

vati

on,

sed

imen

tvo

lum

e,d

ensi

tyof

foss

ilb

ones

and

ston

ear

tefa

cts

(per

0·10

m3),

and

nu

mb

erof

bif

aces

Exc

avat

ion

site

Are

aD

epth

Vol

ume

Fos

sil

dens

ity

Art

efac

tde

nsit

yN

umbe

rof

bifa

ces

Exc

avat

ion

site

Are

aD

epth

Vol

ume

Fos

sil

dens

ity

Art

efac

tde

nsit

yN

umbe

rof

bifa

ces

170

4·8

0·1

0·48

0·62

4·17

04

3·6

0·17

0·61

0·98

1·48

016

59·

00·

05a

,I3·

00·

30·

93·

113·

00

160

5·9

0·1

0·59

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54·

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767

Unusual concentrations and their associationwith environmental featuresSeveral excavations in UM1p yielded un-usual aggregations of stone tools and/oranimal bones. We briefly describe these siteshere and note their association with paleo-landscape gradients, habitat patches, orother environmental features.

Table 1(b) Data for the Hyena Hill excavations in Upper Member 1 paleosol (UM1p)

Excavationsite Area Depth Volume

Fossildensity

Artefactdensity

Number ofbifaces

541/725 9·0 0·15 0·9 8·0 6·67 0536/704–8 12·0 0·4 4·8 3·02 2·12 0532/691–5 12·0 0·15 1·8 3·44 4·11 0526/729 9·0 0·2 1·8 2·39 2·78 0540/731 9·0 0·3 2·7 1·44 1·52 0523/711 9·0 0·1 0·9 2·56 5·22 0530/722 9·0 0·25 2·25 1·38 2·36 0522/689 9·0 0·25 2·25 5·82 4·0 0520/697 9·0 0·25 2·25 7·29 5·73 0531/705 9·0 0·4 3·6 3·53 5·03 0546/707 9·0 0·3 2·7 2·63 2·15 0529/714 9·0 0·2 1·8 2·22 4·28 0528/736 9·0 0·2 1·8 1·72 2·67 0534/730 9·0 0·35 3·15 2·06 1·68 0543/737 9·0 0·6 5·4 1·52 1·3 0527/699 9·0 0·25 2·25 5·16 8·09 0518/704 9·0 0·2 1·8 4·5 5·5 0517/694 9·0 0·2 1·8 4·56 5·33 0539/711 9·0 0·25 2·25 3·47 3·24 0516–7/694 3·0 0·25 0·75 5·6 1·47 0524/704 9·0 0·2 1·8 2·33 3·83 0539–40/731 3·0 0·3 0·9 2·89 5·44 0

Site 102-Hyenas and Site 2 (plus extensions).The excavation at 102-Hyenas, located onthe southwest edge of Hyena Hill (Figure 1),uncovered a series of sediment-filled bur-rows and depressions. These representeda complex burrow system which originatedin the target paleosol and penetrated theunderlying diatomaceous sediments to adepth of 30–120 cm (Potts, 1989a, 1994).The burrows were filled in with clasts of theUM1p paleosol and diatomite of layer 5. Anexpanded chamber at the base of the burrowsystem contained four complete adult andsubadult skeletons of Crocuta sp. A zone of

disturbed silts derived from the paleosol,from which two branches of the burrowsystem diverged downward, was buried andpreserved at the top of UM1p; this zoneappears to have represented an originalentrance to a den. A few bone fragments ofother animals were found mixed with thehyena skeletons, and gnawed specimenswere found in a separate chamber within thesame burrow system. The large number ofhyena skeletal parts is responsible for thehigh bone density (Table 1), although 53stone artefacts found in the disturbed sedi-ments throughout the den suggest thathominids had deposited artefacts on theancient landscape prior to burrow forma-tion, which then were apparently movedbelow ground during the time of hyenaactivity.

Located 12 m southeast of 102-Hyenas,Site 2 yielded a dense concentration ofstone artefacts and broken animal bones,which included cut- and percussion-marked

768 . ET AL.

specimens and no evidence of involvementby any bone collector other than hominids.The artefact assemblage consisted of sharplava flakes, scrapers, and simply flakedpieces of diverse raw material. More than 15individuals of diverse large mammal specieswere represented in the fauna (Potts, 1994).The bone and artefact clusters, which coin-cide exactly, were found in an area approxi-mately 14·5 m by 4·5 m, which includes thewestern boundary of the concentration atSite 1. The area of high bone and stoneconcentration passes into low, backgrounddensities of both artefacts and bones imme-diately beyond this area to the north andeast, as indicated by data from the HH(Hyena Hill) excavations. (Exposure on theedge of the hill precluded similar informa-tion on the concentration’s edge to the westand south.) The evidence overall indicatesthat hominids visited Site 2 repeatedly andwere primarily responsible for the concen-tration of diverse animal bones and artefactsthere.

As noted previously, this area was slightlyhigher in elevation (�0·8 m) than terrainimmediately to the east (Figures 3 and 4).The diatomaceous sediments beneathUM1p were penetrated by large, diatomitebreccia-filled cracks, suggestive of a rela-tively dry zone well above the water tablecompared to surrounding areas. Thesecracks could easily have formed the initialsetting for the extensive burrow system at102-Hyenas. The difference in elevationbetween UM1p at Hyena Hill and Site 15 tothe east was probably not enough in itself toexplain the crack systems, and it is likely thatthese formed during one or more periods oflower water table (see Paleolandscape recon-struction, above). Fine root traces and stableisotope values from Sites 2, 102-Hyenas,and nearby sites in Locality A indicate thepresence of a fairly homogeneous grassland(Sikes et al., 1999). The non-hyena faunawithin 102-Hyenas, Site 2, and other HyenaHill excavations, is dominated by equids,

white rhino, and grazing bovids, also indi-cating the local dominance of grass (Potts,1994). Besides the slight elevation differencefrom the surrounding area and its relativedryness, the most notable aspect of Site 2is that the cluster of remains occurs withinan unremarkable portion of the paleoland-scape, lacking any preserved paleoen-vironmental feature that might have beenresponsible for attracting hominids on arepeated basis to this specific location.

Site 15 and extensions. This excavationuncovered most of a skeleton of the extinctelephant Elephas recki, the bones of whichcoincided exactly with an accumulation of2322 stone artefacts over a sediment thick-ness of �40 cm. Cut marks made by stonetools have been found on a rib within theanatomically-arranged series of thoracic ele-ments. Artefacts occur within and aroundthe confines of the carcass in the centralexcavation area of 23 m2, though elephantbone fragments were found over the con-tinuous area of 64 m3 exposed by excava-tion. The artefacts consist mainly of sharplava flakes, often with obvious edge damage;159 flaked pieces (cores/tools) were found,mostly confined to the main carcass and anarea of stone flaking immediately adjacent toit (Extension 1). Two bifaces were found,and these were in the main carcass area. Thepresence of biface trimming flakes and offaceted platforms and dorsal surfaces meansthat hominids had flaked the edges of largebifaces and discoidal cores; at least some ofthe flakes were then used in butchery.Hominids introduced rocks to this placefrom at least 17 different lithic sources, 14 ofwhich were available in the volcanic high-lands from the southwest to the southeastof Site 15, at distances ranging from 300 mup to 2·5 km away. According to thestratigraphic relationships and topographicreconstruction in Figures 3, 4, 6, and 8, Site15 was situated in a topographic low of thepaleolandscape. The sediments, reed-type

769

root traces, and animal footprints originallymade in wet substrate all suggest that theelephant died and was butchered in an aban-doned channel swale. Typically low back-ground densities of artefacts and bonesoccur immediately beyond Site 15 and ex-tensions. It thus appears that the presence ofthe elephant was the main or sole factor thatattracted hominid toolmakers to this place.

Site I3. This excavation, undertaken in1961–62, unearthed a dense concentrationof diverse and fragmented animal bones andstone artefacts (Isaac, 1977). The site isadjacent to the NE Lava Outcrop (i.e.,present-day Lava Peninsula), which duringUM1p times may have been exposed as alow-lying volcanic plain. Of particular noteis the presence of handaxes and otherbifaces, which are more abundant at I3(n=37) than any other place in UM1p (n=5for all other sites combined). Bifaces com-prise 17% of the flaked pieces, comparedwith 1% at Site 15 (Table 5). A thin paleosolon and between the lava boulders of the NELava Outcrop contains in situ artefacts andfaunal remains, and excavations in 1997revealed dense concentrations of thesematerials deposited during middle andupper Member 1 times. This confirmedIsaac’s idea that hominids were attracted tothis lava outcrop, and that I3 is probably asample of a much more extensive concen-tration of materials over the southern endand southwestern side of this area ofexposed lava.

Attraction of hominid toolmakers to theexposed volcanic areas is further suggestedby the 1997 excavation of Site AD1-1 on thenortheast side of the Lava Hump (notincluded in this analysis but worth com-ment; see Figure 3). Over 1700 numberedartefacts and many thousand waste flakesand angular fragments (mostly under 5 mmmaximum dimension) were recovered at thissite. No animal bones were found, while thedensity of stone artefacts is minimally 97 per

0·10 m3, making it the richest artefact con-centration in upper Member 1. The flakedstone is all of one type of material, and theartefacts were uncovered as a buried screeon top of an outcrop of this same material,Lava Hump trachyte. The characteristicsof the artefacts further indicate thathominid toolmakers were attracted to thisplace in order to quarry and test rocks fortoolmaking purposes.

The quarry site (AD1-1) and the hyenaden (102-Hyenas) are two examples inwhich the densities of fossils and artefactsdo not track each other. In the sample ofall other sites, however, the correlationbetween fossils and artefacts is very strong(Spearman’s rs=0·739, P<0·001 for allother sites; rs=0·626, P<0·01 for sites withdensities <10·0 objects per m3). Thus thespatial distributions of bones and tools arenot independent of one another.

Although variation in the clumping ofartefacts and fossil bones is apparent, thehigh density sites occur against a continuousscatter of objects. In other words, hominidsleft artefacts virtually everywhere over thepaleolandscape, and fossil bones were alsodeposited consistently over the same area.Several lines of evidence suggest, moreover,that the paleolandscape supported a rela-tively homogeneous habitat. The topo-graphic gradients were quite gradual (exceptnext to the lava outcrops); the large mammalfauna from each locality was consistentlydominated by equids and other grazers(Potts, 1994); and isotopic values (Sikeset al., 1999) correspond to an opengrassland–wooded grassland through thestudy area. Given the sedimentological/depositional equivalence among the sitesand the relatively uniform ecological picture,neither taphonomic nor environmental fac-tors appear to explain the strong correlationbetween artefacts and bones. At Sites 2,15, and I3, hominids were very active andwere apparently the main agent of stone-and-bone accumulation. The correlation

770 . ET AL.

Table 2(a) Excavation data for lower Member 7 sands (M6/7s), including area and depth of excavation,sediment volume, density of fossil bones and stone artefacts (per 0·10 m3), and number ofbifaces

Excavationsite Area Depth Volume

Fossildensity

Artefactdensity

Number ofbifaces

C7–1,I/III 10·8 0·22 2·38 0·0 0·0 0C7–10 3·0 0·2 0·6 0·0 0·0 0C7–11 4·0 0·2 0·8 0·0 0·0 0C7–12 4·0 0·4 1·6 0·0 0·12 0C7–13 4·4 1·05 4·62 0·11 0·48 0C7–14 6·67 0·3 2·0 0·45 1·5 0C7–15 9·0 0·2 1·8 0·0 0·0 0C7–3 7·0 0·15 1·05 0·38 0·86 0C7–4 1·0 0·1 0·1 0·0 0·0 0C7–6 7·0 0·3 2·1 0·0 0·0 0C7–8 9·0 0·2 1·8 0·06 0·0 0C7–8,II 8·0 0·2 1·6 0·0 0·0 0C7–9 2·8 0·25 0·7 0·0 0·0 0DE89A/B 260·0 0·1 26·0 >35·5 18·31 602DE89peak 1·0 0·1 0·1 >250·0 80·0H9 96·0 0·2 19·2 11·36 31·93 72Meng 0·3 32·4 98Mid 27·0 0·2 5·4 No data 6·48 90

Data for Meng based on Isaac (1977:68).

Table 2(b) Excavation data for lower Member 7 diatomaceous silts (LM7ds)

Excavationsite Area Depth Volume

Fossildensity

Artefactdensity

Number ofbifaces

C7–1 peak 3·6 0·2 0·72 13·19 40·69 0C7–1,I/III 9·4 0·4 3·76 3·67 10·82 3C7–1,II 3·0 0·2 0·6 1·83 9·5 1C7–10 3·0 0·25 0·75 0·0 0·0 0C7–11 4·0 0·2 0·8 0·0 0·0 0C7–12 4·0 0·4 1·6 0·0 0·0 0C7–13 4·4 0·2 0·88 0·0 0·0 0C7–14 6·67 0·4 2·67 0·26 0·75 3C7–15 9·0 0·3 2·7 0·0 0·0 0C7–4 1·0 0·4 0·4 0·0 1·5 0C7–5 1·6 0·4 0·64 0·0 0·47 0C7–6 6·0 0·2 1·2 0·75 6·42 2C7–8,I 9·0 0·3 2·7 0·48 0·59 0C7–8,II 9·0 0·3 2·7 0·0 0·0 0C7–9 2·8 0·5 1·4 0·0 0·0 0P10 (Della) 4·2 0·2 0·84 No data 35·71 1

between the number of bones and stonescontinues to be very strong after eliminationof these sites. It thus seems possible thathominid behavior was a factor linking thedistributions of bones and stones over theentire paleolandscape, a point that requires

testing with further taphonomic analysis.The strong correlation also suggests, how-ever, that hominids and other animals weresimilarly attracted to environmental fea-tures and the gradient of resources foundthroughout the area.

771

Small habitat patches (e.g., low, wetterrain at Site 15) and distinctive features(e.g., Site I3’s location on the periphery ofthe NE Lava Outcrop) have been identifiedwhere stone tools were preferentially dis-carded, indicating a concentration of homi-nid activity. These areas and sites reflect,however, focal points in the continuum ofhominid movement through the lake marginfloodplain. Essentially no fish or otheraquatic remains were found in the exca-vations of UM1p; the faunal assemblagesconsisted almost entirely of large mammalbones. This suggests that hominids inter-acted with animals that were also attractedto the lowland plain, but they did not visitthis area primarily to obtain lake resourcesother than perhaps water. The abundance ofstone artefacts in UM1p implies that homi-nid attraction to the lowland plain entailedthe significant transfer of rocks from thelocal highlands over distances of 300 m to5 km.

Analysis of Lower Member 7

Table 2(c) Excavation data for upper Member 7 paleosol (UM7p)

Excavationsite Area Depth Volume

Fossildensity

Artefactdensity

Number ofbifaces

C7–1,I/III 9·4 0·6 5·64 0·02 0·09 1C7–10 4·0 0·35 1·4 0·0 0·0 0C7–11 4·0 0·5 2·0 0·05 0·01 0C7–12 4·0 0·6 2·4 0·04 0·12 0C7–13 4·4 0·5 2·2 0·0 0·45 0C7–14 6·67 1·4 9·34 0·11 0·31 2C7–15 9·0 0·4 3·6 0·0 0·0 0C7–4 1·0 0·3 0·3 0·0 0·0 0C7–5 1·6 0·1 0·16 0·62 0·0 0C7–6 2·0 0·4 0·8 0·0 0·25 0C7–8,II] 9·0 0·3 2·7 0·0 0·07 0

Paleolandscape reconstructionThe two target intervals in lower Member 7,which are the grey sands of M6/7s and thediatomaceous silts of LM7ds, have pro-duced significant archeological assemblages(Table 2). Although these two units have a

complex lateral association, throughout thestudy area (Locality C) they are distinctand represent successive paleolandscapes.Within any given stratigraphic profile, thereis usually a sequence of lower sandy sedi-ments and overlying diatomaceous silts withminor pedogenic modification. Artefactsand bones occur in discrete patches withineither of these two sedimentary units.

The M6/7s is a remarkably continuous,homogeneous fine- to medium-grained greysand with a notable paucity of larger-gradeclastic material (e.g., pebbles and cobbles)except for artefacts and bones. The sand isgenerally only 20–40 cm thick except whereit passes laterally into channel deposits. Insuch instances, the typical M6/7s extendsinto a thicker package (up to 1 m) of distinctlayers of interbedded medium to fine sand,reworked diatomite, and diatomaceous silts,which converge back to a single continu-ous sand on the other side of the channel.Examples of such places are the DE/89channel, C7-11, C7-13 and M10, whichappears to link up to the east (i.e., upstream)with channel deposits at C96/01-02 (Figures5, 7, and 8). In some places the sheet sandappears to be an overbank deposit thatcovered the interfluves between channelsthat were part of a larger channel complex(probably a braided stream) (e.g., nearDE/89). In others, it lines the bottom of

772 . ET AL.

N

S12

South NorthArcheological sites

1 m

0100 m0

C96/07C91/02

C96/03

S9C96/06

C96/01

C93/05 C7–14

C7–10

S7

S8

C7–7,9

M4

Cla

y

S13 C96/09

C7–1

C96/05DE/89H9 "Mid"

M10P10

C7–3,4,5,6

M5 M9/7s

M7

LM8

C7–12C7–8,11

UM7pLM7ds

Sil

tS

and

Hypothesized profile of small-scale topographyacross channel system in M6/7s.

250°

290°

300°

Figure 7. Fence diagram showing details of the paleotopography and lateral facies relationships forMember 6/7 between Geological Reference S12 and Site C7-12. Approximate positions of otherexcavation sites are indicated by the letter–number designations. Irregular vertical lines on the profilesindicate pedogenesis. Positions of channel boundaries (dashed lines) are based on facies changes andpaleo-current indicators. Members are indicated by M4 (=Member 4), etc., with LM8=lower Member 8;see text for other stratigraphic abbreviations.

topographically low areas (e.g., C96/06,C96/01; Figures 5 and 7) and was a channeldeposit. The channels themselves are shal-low and broad, with low angle cross-stratification suggesting lateral accretiondeposits, and they do not cut more thanabout 0·5 m into the top of Member 5. Insome areas, an angular breccia of greenishclay and reworked diatomite occurs at thebottom of the channels, evidence for localdisruption of the top of Member 5 withoutsignificant transport and rounding of thefragile diatomite clasts. The combined evi-dence indicates that the landscape develop-ment of the M6/7s interval following theformation of a paleosol on top of Member 5was dominated by fluvial aggradation ratherthan erosion and resulted in a widespreadsandy substrate crossed by a series ofshallow channels (Figure 8).

Deposition of finer-grained sedimentabove M6/7s formed the diatomaceous siltsof LM7ds that contain part of the archeo-logical record in Member 7. Some of thesedeposits have relict lamination and appear to

be primary lacustrine diatomites, implyingthat the lake periodically inundated theland surface formed by the basal sands ofMember 6/7. This is particularly true towardthe southern end of Locality C. However,over much of the study area LM7ds consistsof reworked diatomite mixed with dispersedsands and silts and bears evidence of sub-aerial pedogenesis in the form of root tracesand extensive bioturbation. Thus it appearsthat there was an interval of fluctuatinglacustrine/wetlands and subaerial conditionsimmediately following (and perhaps partlycontemporaneous with) the deposition ofM6/7s. The shoreline was probably near thesouthern end of Locality C, and rises inwater level occasionally inundated the low-gradient plain to the north (most of LocalityC), which nevertheless was exposed period-ically and subject to fluvial overbank sedi-mentation as well as pedogenesis (Figure 8).Lacustrine conditions appear to have con-tributed to fine-grained fill of a lower part ofthe channel complex at C96/06 and C91/02.This implies a relatively rapid ‘‘drowning’’ of

773

the drainage system by rising water level, asotherwise the low areas of the channel wouldhave been filled with coarser sediment aschannel flow gradually decreased.

Relatively pure, well bedded diatomitesoverlie LM7ds and record a transgressionfollowed by a prolonged period of lacustrinesedimentation. Over the entire extent of theavailable outcrop in Locality C, Member 7 isthinner in the north and thicker toward thesouth (Figure 5), suggesting more rapidsubsidence near Mount Olorgesailie. Thedominantly westward fluvial channel direc-tions are at odds with a drainage sump beinglocated toward the south; it is possible thatareas to the west were subsiding even morerapidly and the channels were part of aprograding sediment lobe that was rimmedby lake both toward the west and south(Figure 8). Evidence for such subsidence isprovided by syn-sedimentary faulting atthe eastern margin of the Lava Humpwhere there is localized thickening ofMembers 1–7.

Following deposition of the lower Mem-ber 7 lacustrine diatomites, a major phase ofemergence and land surface developmentformed the paleosol in upper Member 7(UM7p). This is a very clay-rich, dark greenpaleosol with abundant prismatic andgranular ped structures, clay cutans, slicken-sides, root traces and casts, CaCO3 nodules(often with MnO staining), and angular,fragmented diatomite blocks and clasts. Insome sections (e.g., C96/01), the paleosolhas distinct layers and appears to be accre-tional, but for the most part it is a single unitbetween 0·3 and 0·8 m thick, with some roottraces and nodules occurring deeper in theunderlying diatomites. All evidence indi-cates a substantial period of land surfacedevelopment and pedogenesis.

The deposition of Members 6/7 can bereconstructed as follows:( 1) Regression of the lake that deposited

the Member 5 diatomites and forma-tion of a paleosol on these diatomites.

( 2) Shallow channeling in several areasspanned by Locality C outcrops, witha major, west-directed, braided chan-nel complex approximately 250 m inwidth in the main excavation area(C96/07 to S8 on Figure 7) and asmaller one to the north in the vicinityof C7-13 (Figure 5).

( 3) Deposition of fine- to medium-grainedvolcaniclastic sands and reworked di-atomite in the channels (M6/7s), alongwith the artefact concentrations in theSite Museum area. It is likely thatdifferent parts of the major channelcomplex were active at different times,and some floodplain silts (which areone component of LM7ds) may havebeen deposited lateral to and above thesands in some areas while channel flowcontinued in others.

( 4) Fluctuating lacustrine and fluvialdeposition, with pedogenic modifi-cation of diatomaceous silts (LM7ds)during subaerial intervals, followed bytransgression of the lake and diatomitedeposition that buried the artefact-bearing LM7ds deposits.

( 5) Lake regression and the formationof a stable land surface on the 1·5–2·0 m of diatomite, with substantialpedogenesis in the upper meter of thisdeposit (UM7p).

( 6) Transgression and diatomite deposi-tion, with substantial reworking andoccasional small channels indicatingfluctuating subaerial and subaqueousconditions (base of Member 8).

( 7) Temporary regression and stabiliz-ation of the land surface, with somepedogenesis affecting the lowermostdeposits of Member 8. Deep (up to1 m), ash-filled mudcracks in lowerMember 8, occasionally penetratinginto upper Member 7, indicate thatMember 8 sediments were not well-lithified during ash deposition, and itis likely that relatively little time is

774 . ET AL.

represented in the basal paleosol ofMember 8.

Artefacts and bones are associated pri-marily with the sands and overlying diatom-aceous silts of lower Member 7, wheredeposition was probably relatively rapid(i.e., during phases (3) and (4) above. Thereis little preserved material in the upperMember 7 paleosol, even though this clearlyrepresents a substantial period of landscapestability and soil formation. Although thereis a paleosol developed on the Member 5diatomites, few artefacts have been found inassociation with this land surface. Instead,the concentrations in lower Member 7 occurin channels, or in silts with weak pedogenicmodification of floodplain deposits associ-ated with these channels or temporarilyemergent areas of the diatomaceous lakebed. Based on lithofacies trends towardthicker, more lacustrine deposits to thesouth, it is likely that wetlands and possiblyan arm of the lake were within a fewhundred meters of Locality C during thedeposition of lower Member 7 (Figure 8).

Estimated amount of timeA combination of channel and interfluvesands comprise the target stratigraphicinterval M6/7s. Overlying M6/7s are thediatomaceous silts of LM7ds, topped bythe well developed paleosol of UM7p. Allthree stratigraphic intervals are bracketedby a pumice age of 0·974 Ma in Member 5and the Brunhes–Matuyama boundary at0·78 Ma in lower Member 8 (Figure 2).These ages, which indicate a span of�200 Kyr, do not constrain very preciselythe amount of time represented by the threeintervals. Lacustrine diatomites make upapproximately 3 m of this stratigraphicinterval in Locality C, amounting to 1·5–3·6 Kyr based on rates of Holocene diatom-ite deposition (Isaac, 1977:22). The periodof channel incision and sedimentation at thebase of Member 7 probably does not repre-sent more than a few thousand years,

possibly on the order of a few hundredyears since cutting produced very shallowchannels and sediments were not affectedby pedogenesis. The diatomaceous silts(LM7ds) preserve bone remains at SiteC7-1 that were highly weathered and frag-mented, suggesting a period of surface expo-sure of 15 years or more (Behrensmeyer,1978). To the SE of the main artefact-boneaccumulation at this site, a steeply-bankedchannel, interpreted as an ancient gametrail, has an upper boundary in LM7ds. Thechannel is 0·8 m deep and 1·4 m across andprobably took several decades to be cut byrecurrent animal trampling. This indicatesthe minimum amount of time representedby LM7ds, which overall probablyrepresents at most several hundred years.

There are at least four periods of pedo-genesis between the two radiometric dates:the top of Member 5, LM7ds, UM7p, anda poorly developed soil at the bottom ofMember 8. The latter soil could representon the order of 1 Kyr, and the top of Mem-ber 5 slightly longer at 5 Kyr. Thus, if weattribute 1·5 Kyr to diatomite deposition,1 Kyr to M6/7s, 500 years to LM7ds(including deposition and pedogenesis),5 Kyr to the soil in Member 5, and 1 Kyr tothe lowermost soil in Member 8, thisamounts to only 9 Kyr, leaving �190 Kyrfor the soil in upper Member 7 (UM7p).This is probably an overestimate, but it islikely that a large proportion of the elapsedtime is represented by the soils at the top ofMember 5 and upper Member 7 rather thanin the sediments that preserve the archeo-logical record. The above time-averagingestimates are consistent with those for fossilassemblages in similar sedimentary environ-ments found throughout the geologicalrecord (Kidwell & Behrensmeyer, 1993).

Distribution of stone artefacts and fossil bonesTable 2 lists the sites and excavation data forM6/7s, LM7ds, and UM7p and shows thatthere are a number of excavations in which

775

Table 3 Statistical tests (Poisson distribution and chi-square) of the spatial distribution of stoneartefact and fossil bone densities, over four target intervals in the Olorgesailie Formation

Targetinterval

Number ofsites

Meandensity Variance

Poissondistribution test Chi-square

Chi-squareprobability

UM1p Stone artefact density 78 5·4 41·75 7·73 121·32 P<0·001Fossil bone density 78 5·41 73·29 13·55 77·05 P<0·001

M6/7s Stone artefact density 18 9·56 423·81 44·33 27·5 P<0·001Fossil bone density 16 4·87 127·36 26·18 17·1 P<0·001

LM7ds Stone artefact density 16 6·65 164·97 24·8 12·5 P<0·01Fossil bone density 15 1·35 11·75 8·74 24·7 P<0·001

UM7p Stone artefact density 11 0·118 0·023 0·19 33·0 P<0·001Fossil bone density 11 0·076 0·034 0·45 33·0 P<0·001

Poisson distribution test: (variance/mean=1·0) indicates random distribution; (<1·0) indicates even distribution(>1·0) indicates clumped (clustered) distribution. See Statistical methods in text.

no fossils or artefacts were found. This resultpertains particularly to both M6/7s andLM7ds and was unexpected given that theseexcavations sampled areas lateral to denseaccumulations in the same stratigraphicintervals (Figures 5, 7). The Poisson distri-bution and chi-square tests confirm thehighly clumped distribution of materialsalong both of these paleolandscape transects(Table 3). In general, a given excavationyielded either no (or very few) remains orextreme concentrations of artefacts and fos-sil bones. As a result, the mean density of insitu objects within low density sites of M6/7swas 0·08 artefacts and 0·23 bone specimensper 0·10 m3, compared with 23·43 artefactsand 22·28 bones in the high density sites.Thus the latter sites had artefact and fossildensities 293 times and 97 times greaterthan the negligible background scatter.There were three sites in LM7ds thatyielded dense stone artefact clusters,approximately 52 times the mean concen-tration of the background scatter. Althoughonly one of these sites (C7-1) preserved adense accumulation of fossil bones, the peakdensity there was also 52 times largerthan the background, a contrast nearly anorder of magnitude greater than in upperMember 1.

Unusual concentrations and their associationwith environmental featuresAn important goal of the Isaac and Leakeyexcavations was to uncover assemblages ofAcheulean bifaces, and they excavated sev-eral dense concentrations of handaxes,cleavers, and other classic Acheulean arte-facts in Member 7 (Isaac, 1977). These sitesinclude DE/89, H9, Mid, and Meng. Eachoccurrence is situated in a complex of fluvialsands and sand-gravel lenses deposited by achannel system through what is now the SiteMuseum compound. At DE/89 an extraor-dinary bone concentration of Theropithecusoswaldi was uncovered, associated with therich handaxe assemblage. Due to the fluvialcontext of these sites, water flow wasdeemed an important agent of redistributingand concentrating the artefacts and boneremains (Isaac, 1977). Prior to this study ofM6/7s in 1996–97, we also assumed thatchannel flow and flooding had picked upand moved objects within or near the chan-nel beds and then focused their burial andpreservation within the sandy fill (e.g., Potts,1989a). This view was consistent withIsaac’s emphasis on stream transport [thekinematic wave effect (Isaac, 1977)] andBinford’s critique of Isaac’s behavioralinterpretations (Binford, 1977).

776 . ET AL.

The new excavations reported here pro-vide lateral context for the earlier ones,and the difference between the two sets ofexcavations is dramatic. It is apparent thatbifaces are not widely distributed, butinstead occur only in very dense concen-trations in M6/7s (Tables 2 and 5). Theseries of fluvial channels, separated laterallyby sandy interfluves, represents the mostprominent environmental characteristic ofthe M6/7s paleolandscape (Figure 8). Thedearth of artefacts outside the channels isstriking, suggesting that hominid tool userswere largely inactive in the sandy interfluvescompared with the channels. The high den-sity sites (e.g., DE/89) were preserved inshallow channels, which exhibited lateralaccretion stratification suggesting flowdepths of 50 cm or less. There is little evi-dence of scouring of underlying sedimentswithin the sequence of channel fills, andthere are many angular diatomite clasts andpoorly sorted sediment lenses indicating lackof strong and persistent flow. The absence ofnaturally occurring lava cobbles and pebblesthroughout the channel complex, in an areawhere lava outcrops were no more than600 m away, further attests to the lowgradient, aggradational nature of the localfluvial system. These observations stronglysuggest that flow was not competent tomove the handaxes significant distances andsecondarily concentrate them along withother stone artefacts and animal bones. Inother words, we believe it highly unlikelythat the artefact concentrations were createdby the fluvial ‘‘harvesting’’ of widely dis-persed materials and their subsequent depo-sition in hydraulic jumbles due to thekinematic wave effect proposed by Isaac(1977).

The geological evidence does indicate,however, that patches of artefacts and bonesleft by hominids in the channels were subse-quently rearranged and clumped withinthese patches by fluvial flow, therebydestroying any original small-scale relation-

ships of artefacts and bone. Large numbersof artefacts deposited by hominids on barsurfaces would have affected local hydraulicconditions, creating turbulence duringbank-full flow that caused increased erosionin some areas of the same bars and deposi-tion in others. This is our working hypoth-esis for the stratigraphic context of theartefacts and bones at DE/89, which occurin successive beds that appear to fill lowareas adjacent to a bar surface where theartefacts could have been concentrated byhominid activity.

Within the channels, the distribution ofhominid tools is also extremely patchy. Cer-tain excavations, such as C7-14, sampledpart of the same channel system as SiteDE/89 yet produced very little material fromequivalent lower M7 channel sands. Accord-ing to Isaac (1977:45, 51), extensions of theDE/89 excavations along the paleochannelbed also revealed a dramatic dropoff inartefact and fossil concentrations. The sandsof two other minor channels along M6/7swere sampled at Sites C7-11 and C7-13,and these excavations also uncovered verylittle or no material (Table 2a). The local-ized hydraulic rearrangement and burial ofartefacts and bones described above initiallywould have blurred the edges of discretepatches but left them essentially intact.However, if the fluvial system remainedactive for a long period of time, channelreworking would eventually have led tomore widespread distribution of suchmaterials throughout the fluvial deposits. Ifthis had occurred, some components of theoriginal assemblages would be encounteredwherever the deposits were present. This isclearly not the case for the excavatedsamples in M6/7s.

The correlation between the densities ofanimal fossils and stone artefacts in M6/7s isstrong (Spearman’s rs=0·900, P<0·001),which reflects the highly patchy distributionof both kinds of remains and their sharpspatial coincidence. The strong tool–bone

777

Paleolandscape comparisons

A characteristic of the three paleolandscapesamples in the Olorgesailie basin (UM1p,M6/7s, LM7ds) is that all exhibit a laterallyheterogeneous distribution of hominid tool-related activities. Tools were found in aggre-gates, which indicates the varied use ofspace by hominids. Animal bones are alsoclumped, and their distribution tracks thatof stone tools. However, there are two dif-ferent patterns of debris represented inMembers 1 and 7. Table 4 presents statisti-cal tests (Kolmogorov–Smirnov and Mann–Whitney U) of paired comparisons of the

association is most specific at Site DE/89(archeological levels A and B) with its denseconcentrations of handaxes and bones ofTheropithecus oswaldi [MNI�43 (Isaac,1977; Shipman et al., 1981; Koch, 1986)].As noted above, Isaac’s and our own exca-vations saw a reduction of bone and artefactmaterial to almost nil beyond the 260-m2

area of the cluster, both within the DE/89channel and in the sands outside the chan-nel. In addition, channel sands laterallyequivalent to the channel at DE/89 (i.e., ina parallel channel) are preserved �70 msouth, in Isaac’s grid square M10 (Figure 7).Although erosion in the square has pre-cluded excavation, the surface material lyingin, on, and around the remaining channelsands includes no Theropithecus bones, butrather a fauna dominated by Equus, bovids,suids, and Elephas (1997 survey).

In the context of these lateralobservations—within the DE/89 channelbed, in the tabular sands outside the chan-nel, and in nearby channels of M6/7s—thespatial overlap between the handaxe concen-tration and the bones of Theropithecusoswaldi at DE/89 is amazingly precise. Thisprecise distribution, without any similar ma-terials in laterally equivalent areas, coupledwith the lack of evidence for sufficient waterpower to transport the artefacts, argueagainst fluvial processes as an explanation ofthe unusual DE/89 concentration. Althoughwinnowing of the assemblage and roundingof bone edges occurred, upstream or flood-plain entrainment and concentration of dis-persed materials by flowing water appears tohave had at most a minor role in creatingDE/89. We see no alternative but to con-clude that ecological and behavioral pro-cesses were mainly responsible for accumu-lating the Theropithecus remains and thehandaxes. The circumstances that broughtthe monkeys and hominids to this placewere unlike ones that operated elsewherein M6/7s and were inextricably related,whether they involved butchery or shared

ecological attraction to a very specific fea-ture within the DE/89 channel. AlthoughShipman et al. (1981) detected distinctivepatterns of bone damage, the lack of defini-tive tool cut marks on the Theropithecusbones so far (Koch, 1986:234) means thatthe butchery interpretation is by no meanscertain.

The strata immediately above M6/7sshow that lacustrine deposition helped to fillsome of channels of the Site Museum area,implying a rapid initial lake transgression.LM7ds is a mixed diatomite and fluvial siltunit that shows evidence of pedogenesisrepresenting emergence as a shallow-gradient lake-margin plain during temporaryregression of the lake. The LM7ds paleo-landscape seems to have lacked distinctivefeatures besides slight topographic depres-sions related to the underlying channel com-plex (Figures 7, 8). Site H9 occurs in one ofthese, and no artefacts have been reportedother than those in M6/7s (Isaac, 1977).The main artefact-and-bone concentrationin LM7ds at C7-1 is adjacent to anotherchannel-like feature, which we interpret as apreserved game trail. Two other artefactconcentrations in LM7ds—P10 (Della) andC7-6—are not associated with any particu-lar topographic or obvious environmentalfeature within LM7ds.

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779

Table 4 Statistical tests (Kolmogorov–Smirnov and Mann–Whitney U) of difference between pairs ofpaleolandscape target intervals, for stone artefact and fossil bone density distributions

Paleolandscapecomparison

Kolmogorov–Smirnov

K–Sprobability

Mann–WhitneyU

Uprobability

UM1p:M6/7s Stone artefact density 0·547 P<0·0005 664·5 P=0·008Fossil bone density 0·69 P<0·0005 662·0 P<0·0005

UM1p:LM7ds Stone artefact density 0·503 P=0·002 616·0 P=0·023Fossil bone density 0·592 P<0·0005 692·5 P<0·0005

M6/7s:LM7ds Stone artefact density 0·12 P=0·999 151·0 P=0·799Fossil bone density 0·147 P=0·991 117·5 P=0·912

three paleolandscapes. According to thesetests, UM1p and the two intervals in Mem-ber 7 differ significantly (P=0·002 toP<0·0005) in both fossil and artefact den-sity. Neither of these tests, on the otherhand, detects a difference between the twoMember 7 intervals (M6/7s and LM7ds) inthe spatial distributions of either artefact orfossil densities.

Although significantly clumped, stonetools and animal bones are distributed con-tinuously in UM1p—i.e., some remainswere found in virtually every excavatedsample. By contrast, neither interval inMember 7 exhibits a continuous distribu-tion of artefacts or bones. This distinction—continuous vs. highly patchy (gaps in thespatial distribution)—is largely responsiblefor the strong statistical difference betweenthe two members and for the similaritybetween M6/7s and LM7ds.

A number of factors could be responsiblefor this difference between upper Member 1and Member 7. Variation in the amount oftime represented by two paleolandscapescould affect the distributions of materialsthat developed in each. For example, UM1pand LM7ds both represent floodplain landsurfaces near lakes, and these two intervalsshould be ‘‘isotaphonomic’’ (Behrensmeyer& Hook, 1992) in many respects, but UM1prepresents a longer period of land surfacestability and soil formation than doesLM7ds. It is likely that a continuous back-ground scatter of artefacts and bones takes

more time to develop than is necessary tocreate a few high density concentrations. Asa null hypothesis, therefore, one could pro-pose that the pattern of artefact and bonedistribution in UM1p simply was the resultof a longer period of time-averaging of thesame behavior patterns that created theshort-term, patchy sample in LM7ds. It isalso possible that preservational differencessecondarily created a patchy record in Mem-ber 7 from a more continuous original dis-tribution (i.e., artefacts and bones werepreserved only in discrete places wherepreservational processes were optimal). Thepatchier distribution in LM7ds (with gapsthat lack artefacts or fossils) could alsoreflect lower intensity use of this landscapeby hominids, with the toolmakers traversingit less frequently, or perhaps passing throughas frequently but discarding fewer artefacts.Thus there are at least five variables thatcould have affected the pattern of artefactand bone distributions: time-averaging,preservational patchiness, intensity of land-scape use, travel frequency, and artefactdiscard rate.

Comparison among stratigraphic intervalsprovides insights regarding these issues.First, the estimated time represented byeach of the three stratigraphic intervals iswithin an order of magnitude (i.e., fromhundreds up to 1000 years). Within thatrange, we estimate that UM1p representsthe longest period, M6/7s a shorter span,and LM7ds the least amount of time based

780 . ET AL.

on their sedimentological features and depo-sitional rates. According to the null hypoth-esis stated above, the behaviors responsiblefor the discontinuous artefact distribution inLM7ds would require approximately twicethe amount of time to yield the continuousartefact distribution seen in UM1p (e.g.,500 years extrapolated to 1000 years). How-ever, Table 3 shows that the mean density ofartefacts in LM7ds had already reached thelevel seen in UM1p, suggesting that timealone is not sufficient to explain the differ-ence between the two intervals, particularlythe much larger variance in the artefactdistribution of LM7ds. Member 7 providesus with additional comparative informationfrom a fourth paleolandscape, UM7p, thepaleosol complex in upper Member 7 aboveLM7ds (Figure 7). The pedogenic featuresof UM7p indicate that its maturity as a soilexceeds that of UM1p, and this combinedwith the estimated time between radiometri-cally dated horizons (see above) make itlikely that UM7p represents a much longerinterval. Although artefacts and fossils werefound in very low levels in UM7p, theirdistribution conforms to the discontinuousspatial pattern observed in the otherMember 7 paleolandscapes. Table 2(c)shows that four out of 11 excavations un-covered no stone artefacts, while six of thesame excavations exhibited no evidence ofbone. If artefact density and distribution areprimarily a function of the time interval ofland surface development, then we shouldsee more abundant and spatially continuousarcheological materials in UM7p thanrecorded in UM1p. Therefore the UM7pcomparison is further evidence that differ-ences in the spatial patterning of artefactsand fossils in Members 1 and 7 are notsimply a function of time-averaging.

A second point arises from the compari-son of M6/7s and LM7ds. These two unitsexhibit a similar spatial pattern of artefactdiscard by hominids despite obvious differ-ences in paleoenvironmental setting and

taphonomic processes—channel incisionand aggradation, on the one hand, and alake margin plain, on the other. This sug-gests that the artefact-discard pattern repre-sented in Member 7 is robust with respect todepositional setting. The decoupling of theartefact-discard pattern from depositionalsetting also argues against the possibility thatartefacts and bones were only preserved indiscrete, taphonomically optimal patches inMember 7. Moreover, we see no evidencefor such preservational biases, i.e., no corre-lation of fine-scale sedimentary differenceswith bone–stone presence or absence.

The difference between UM1p and thepaleolandscapes of Member 7 is evident,therefore, despite variation in paleoen-vironments, taphonomic modes, and time-averaging. We thus conclude that thecontinuous vs. patchy distributions of homi-nid artefacts, so strongly indicated by thestatistical tests, reflect two distinct spatialpatterns of hominid interaction withenvironments.

Comparison between UM1p and M6/7ssuggests one possible reason for the differ-ence. The continuous distribution of arte-facts and bones in UM1p is associated witha grassland plain of low topographic relief(see Sikes et al., 1999). Around 990,000years ago in upper Member 1, the tool-makers at Olorgesailie encountered a rela-tively uniform plains habitat, at least in thearea where sampling has been possible sofar. While hominids created clusters ofobjects in certain places, the presence of acontinuous background scatter is consistentwith a lack of barriers or boundaries on arelatively homogeneous land surface. Bycontrast, the smaller area sampled of M6/7swas a sandy area crossed by shallow chan-nels; the overall impression is that of a moreuneven or patchy habitat than in UM1p(Figure 8). The distributions of artefactsand animal remains are consistent with thepresence of environmental boundaries thatguided the hominids’ behavior, especially

Lava Hump site: see text.

781

discontinuities associated with the channelsversus overbank areas. Given the dearth ofartefacts in the interfluve sands just beyondthe channels, it is possible that ephemeralchannels afforded some attraction for homi-nid activity, such as comfortable substrates,pathways through a dense (perhaps thorny)floodplain vegetation, or ambush sites forother animals that used the channels foraccess to and from the lake.

There is no clear evidence, on the otherhand, that the floodplain habitat of LM7dsexhibited such discontinuities (Figure 8).Either we have failed to find evidence forenvironmental patchiness that once existed,or the behavior of hominids was character-ized by a more discontinuous spatial patternof land use regardless of habitat patchiness.In this regard, it is pertinent that while thefaunal remains in LM7ds were distributednonrandomly, discarded artefacts were evenless random (more clustered) by a factor ofnearly three (Table 3, Poisson distributiontest). Comparing the two target intervals ofMember 7, therefore, suggests that the tool-related activities of the hominids were con-fined to more discrete patches than thetaphonomic processes that preserved arecord of other large mammals.

The best estimates of absolute ages forMembers 1 and 7 indicate that UM1p andthe two Member 7 samples were separatedin time by about 90 Kyr. Within Member 7,M6/7s and LM7ds are close to the same age,

Percentage bifaces of all flaked pieces from selected sites in Members 1 and 7

Stratum Excavation siteNumber of

flaked/shaped piecesPercentage

bifaces

UM1p 15 159 1UM1p I3 219 17M6/7s DE89A/B 932 65M6/7s H9 241 30M6/7s Mid 112 80M6/7s Meng 147 67Member 7 Lava Hump site 22 14

Table 5

and probably were deposited from withina few hundred to at most a few thousandyears of each other. Whether the differencesbetween Members 1 and 7 represents anevolutionary change in land use behavior ora response to environmental differences canonly be decided by expanding the sample ofAcheulean landscape-scale studies. As thestrongly patchy distribution of artefacts isrepresented in varied environmental andtaphonomic settings of Member 7 (M6/7sand LM7ds, with hints of it also in UM7p),our comparison seems to favor the evol-utionary interpretation, at least within theOlorgesailie Basin.

The most striking aspect of the artefactdistribution at Olorgesailie is the highlydelimited concentration of specific types oftools, i.e., handaxes and other bifaces. Thispatchy distribution is apparent in UM1pand especially in M6/7s (see Tables 1 and 2,last column; Table 5). Concentrations ofbifaces appear to be correlated spatially withtwo factors: the proximity of lava outcropsand, to a lesser extent, the presence ofchannels. Test excavations and detailed sur-face survey through the basin indicate thatthe Site Museum area is highly unusual inhaving large aggregates of bifaces. In M6/7s,there is an overwhelming dominance ofbifaces compared with other flaked pieces inthis area, and the biface sites are foundexclusively in stream channel contexts. Theone site in UM1p that exhibits a strong

782 . ET AL.

percentage of bifaces (Site I3) is not associ-ated with a channel but is located in the SiteMuseum area. The dominant environmentalfeature of this area in both Members 1 and 7times was the NE Lava Outcrop. While it isa large ridge today (Lava Peninsula), it wasmanifested in UM1p times as a gently risingarea of volcanic rock, possibly a low-lyingplateau, and during Member 7 times as aseries of lava outcrops in the Site Museumarea leading toward the Mount Olorgesailiefoothills (Figures 1, 8). The biface-dominated sites of M6/7s were less than1 km from the lava outcrops. Site I3 inUM1p lapped on to the edge of the NE LavaOutcrop. Furthermore, Isaac excavated oneother site in Member 7, the Lava Hump Site(LHS), that contained a fair representationof bifaces (14% of flaked pieces). This site islocated about 200 m away from a contem-poraneous exposure of the Lava Hump tothe north [Lava Hump (N) in Figure 8; seealso Figure 3]. While the sample of artefactsfrom LHS is small, it further suggests thatthe one commonality of biface depositionover time and space involved proximity toan area of locally higher relief, i.e., a lavaoutcrop or promontory.

Vegetation, including shade, may havebeen one factor that attracted hominid tool-makers to the lava exposures. Isotopic studyof carbonates formed in the shallow soils ofthe NE Lava Outcrop adjacent to Site I3 inUM1p yields a slightly higher C3 vegetationsignal than in the adjacent paleosol acrossthe basin (Sikes et al., 1999).

The location of the lava outcrops near theinterface between the highlands and the lakebasin may have been another factor. Thehighlands offered lithic sources for toolmak-ing, and abundant stone tools still litterthe surface of Mount Olorgesailie and thevolcanic highlands, though these tools arehighly weathered and barely protected bythin soils formed in crevices and on top ofthe volcanic rock. The great abundance ofthese artefacts suggests that some hominid

activity was focused in the highlands. Thelowland lake margin and floodplains, on theother hand, offered no lithic source outcropsor conglomerates, but did offer foodresources. Since the NE Lava Outcrop andthe Lava Hump were situated near the junc-tions between the highlands and the lakebasin, it is possible that biface deposition atpoints of transition between the two zonesreflected a strategy related to hominid stonetransport. Bifaces served as effective toolsand as convenient pieces of raw material.Useful lithic raw materials occurred onMount Olorgesailie and its foothills, andhominids introduced bifaces to the lakebasin from these sources. It appears thathominids, on the return trips, left bifacesbehind at points where they entered backinto the highlands, where stone was alreadyvery abundant. These entry points were nearor at the margins of the low-lying lava out-crops. If this was so, it may be that hominidtoolmakers lived primarily in the highlands,entering the lowland basin to forage beforereturning to the local highlands. This ‘‘high-land hypothesis’’ may be evaluated throughmore systematic study of the artefacts onMount Olorgesailie and nearby volcanicridges, especially by mapping the dispersionof tools of diverse raw material across thehighland zones.

Finally, there is a strong correlation inboth Members 1 and 7 between the distri-bution of animal bones and stone tools. Insome cases (e.g., Sites 15 and 2 in UM1p),hominids were attracted to animal carcassesand were involved in the aggregation ofbones. In other cases (e.g., the backgroundscatter in UM1p and the concentrations inM6/7s), the lateral comparison of excavatedsites shows that hominids and the skeletalremains of other animals were present in thesame places, yet no direct evidence ofhominid–animal interaction is apparent. Inthese cases, the toolmakers and large mam-mals such as Equus and Theropithecus mayhave been attracted and/or limited to the

783

Comparisons of Plio-Pleistocenebasins

The paleolandscape approach establishes abasis for extrapolating inferences abouthominid activities between different spatialscales. Just as a single excavation cannotrepresent the nature of artefact and bonedistributions at the landscape scale, so toothe pattern of paleolandscape use by homi-nids at one stratigraphic level need not rep-resent the patterns at other intervals, even ifthese intervals are relatively closely spacedin time. At a broader scale, significant vari-ations in the pattern of hominid land usemay be found in different depositionalbasins, and these may be related to variantenvironments and physical geographicalsettings. Such comparisons, however, couldalso help to define persistent patterns ofstone tool distribution and environmentalassociation that occurs in a variety of geo-graphical settings. These patterns mayreflect important evolutionary develop-ments. A brief comparison with two otherPlio-Pleistocene basins—Turkana andOlduvai—illustrates how evolutionary impli-cations may be drawn from the study ofhominid behavior at a landscape scale.

Substantial changes in land use havebeen documented in the Turkana basinbetween 2·3 and 1·6 Ma by Rogers et al.(1994) based on the paleoenvironmentalreconstructions of Feibel (1988) and Brown& Feibel (1991). Late Pliocene artefact

occurrences were distributed along the eastand west sides of the proto-Omo River,limited specifically to the inter-channelzones of alluvial fans near their confluencewith the axial drainage. Only low densityclusters of stone artefacts are known at thistime (�2·3 Ma), and these are located closeto where marginal drainages conveyed lithicraw materials to the major river axis. Homi-nids found suitable rocks for toolmaking atthese intersections of the marginal and axialdrainages, and apparently did not relocate ordeposit stone tools in areas far beyond theconfluences. At a later interval, 1·9 to1·8 Ma, stone artefacts are known for thefirst time from lake margin environments(e.g., sites in the KBS Member of the KoobiFora Fm.) and within the axial drainageitself, where hominids appear to havecollected cobbles from the proto-OmoRiver. Although not as spatially restricted asbefore, hominids still relied on locally avail-able rock sources and transported rocks overshort distances. By 1·6 Ma, hominids wereengaged in making new, distinctive tooltypes; artefact concentrations were morevaried (from very high to low density); andarcheological sites were located in virtuallyall areas of the shifting fluvial systems(Rogers et al., 1994; Isaac & Behrensmeyer,1997). Although the size of available streamcobbles was a limiting factor toward thebasin center (Toth, 1985), hominids werecapable of transporting artefacts from thestone sources over longer distances com-pared to earlier times (Isaac, 1997; Bunn,1994; Rogers et al., 1994).

Significant change in the paleohydrologi-cal setting of the Turkana basin took placebetween 2·3 and 1·6 Ma, with the formationof a temporary lake near the basin axis wherethe proto-Omo River formerly flowed, fol-lowed by the return to fluvial deposition ataround 1·8 Ma. Thus it is uncertain whetherchange in the spatial relationship betweenstone tools and environmental settingsdescribed above represented an evolutionary

same areas. The common amenities ofwater, food, other lowland resources, andpossibly the use of seasonally dry channelsas pathways through the lower Member 7habitat may be responsible for the strongartefact–bone correlation on a paleoland-scape scale. If this is true, it implies thatthese tool-bearing hominids were not un-usual, compared with other animals, in theways that environmental features influencedthe distribution of their activities.

784 . ET AL.

transition or a localized response by hominidtoolmakers to different conditions. The casefor an evolutionary transition is madestronger, though, by comparison with thearcheological record of Olduvai, in whichsimilar alterations in the pattern of hominidtool activity are manifested.

In contrast with the largely fluvial-dominated setting of Turkana, a lacustrineenvironment was present in the Olduvaibasin throughout the period of Beds I andII, between �1·9 and 1·4 Ma (Hay, 1976;Leakey, 1971). The Olduvai area was sub-ject to both moist and arid climatic con-ditions, but a lake of fluctuating sizeremained the centerpiece of the basin overthis period. Stone material was availableto hominid toolmakers in four types of lo-cation: (1) the volcanic highland complexesto the south and east of the basin, (2) streamconglomerates that entered the basin fromthe volcanic highlands, (3) inselbergs nearthe lake (e.g., quartzite at Naibor Soit,gneiss at Kelogi) and isolated volcanic cones(e.g., phonolite at Engelosin) within thebasin, and (4) the basal lava beneath Bed I,mainly of vesicular basalt that was accessibleespecially during early Bed I times (Hay,1976). During Bed I (�1·85–1·78 Ma), thedistribution of archeological sites is confinedto the lake margin lithofacies, which repre-sents a zone approximately 0·5 to 5 km wideof periodic lake flooding. Neither Leakey(1971) nor Hay (1976) found stone arte-facts in the fluvial facies to the east of thelake margin. The spatial distribution of sitessuggests that stone-tool-assisted activitieswere confined to the lakeside zone, wherehominids had access to quartzite and certaintypes of volcanic rock suitable for flaking.

According to Leakey’s excavations,archeological accumulations in Bed I areoften found in superjacent stratigraphiclevels; this unusual vertical stacking ofartefact-rich sites at certain localities (e.g.,through 1·5 m at FLK-N) seems unique toOlduvai among known Plio-Pleistocene

open-air sites (Potts, 1994). The discardingof stone artefacts by the toolmakers was byno means limited to these dense clusters.Yet the stratigraphic stacking of remainsseems to imply a tethering of tool-assistedactivities to specific places within the lakemargin zone over time. Olduvai hominidsevidently practiced a way of using stone thatinvolved the movement of unmodifiedrocks, or manuports, over considerable dis-tances. Most of the major clusters and minorassemblages of in situ artefacts includeabundant manuports, and in this respectthe archeological record of Olduvai differsfrom that of Turkana or other late Plio-Pleistocene basins. Manuports typicallymake up 20–60% of the stones recoveredfrom M. D. Leakey’s excavations in Beds Iand II, in contrast with 0–6% of the stonesfrom sites in the Turkana basin (Isaac &Harris, 1997:275; Leakey, 1971).

In Bed II Olduvai (1·78 to at least1·4 Ma), the vertical stacking of remainscontinued to occur (e.g., MNK Main site),but hominid tool activities became distrib-uted over a wider area, both within the lakemargin zone and, by �1·6 Ma, outside of it.The toolmakers exploited new stonesources, such as Engelosin phonolite, thatlay beyond the lake floodplain and alsofauna (e.g., Equus oldowayensis) that prob-ably ranged well beyond the lakeside. Stonetransport distances exceeded 10 km, thoughmore local rock sources (within a 2–3 kmrange of known archeological sites) con-tinued to be used. Hominids depositedstone tools in a greater diversity of depo-sitional settings than in Bed I, includingchannel, alluvial floodplain, and lake margincontexts (Hay, 1976).

Furthermore, as Hay (1976:113) haspointed out, Bed II sites with abundantAcheulean bifaces occur more than 1 kmfrom the reconstructed lake margin, whilecontemporaneous biface-poor sites (Devel-oped Oldowan assemblages) occur less than1 km from the lake. As we have seen at

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Olorgesailie, therefore, early Acheuleanbehavior involved the repeated discarding ofcertain artefact types at particular places onthe paleolandscape. This distribution ofhandaxes and other bifaces at Olduvai cor-relates with distance from the lake and thenearby presence of stream channels, while atOlorgesailie the clustering of Acheuleanbifaces was associated with lava exposuresnear the interface between the lowland lakebasin and the volcanic highlands.

This brief summary of the Turkana andOlduvai records indicates that, over time,hominids engaged in longer distances ofstone transport. In the Turkana basin at2·3 Ma, hominids are known to have carriedrocks over a kilometer or less from theclosest available sources (e.g., Kibunjia,1994). Transport distances of about 10 kmare known within the lake margin zone ofBed I Olduvai, at 1·8 Ma (Hay, 1976).Although over 95% of the artefacts at Olor-gesailie were made of locally available rocktypes, some lithic material was brought intothe basin from at least 45 km away in theperiod between 992 and 780 ka (Isaac,1977). This increase in maximum transportdistance over time is evidence that hominidsbecame less dependent on local sources ofraw materials, perhaps due to a combinationof greater home range and increased abilityto utilize widely separated lithic and foodresources. This change is echoed by thegrowing disassociation of stone tool distri-butions from lake margin (Olduvai), axial-marginal confluences (Turkana), or otherspecific environments. At a landscape scale,the distribution of sites in a wider diversityof depositional settings also representsincreasing independence of hominid tool-makers from environmental constraints, andalso perhaps changes in the deliberate use ofparticular places for specific activities. Thedistributional patterns of bifaces at Olorge-sailie and Olduvai show that hominids con-tinued to locate some of their tool activitiesin relation to particular environmental

features. Nonetheless, a fundamental evol-utionary change in hominid land use isindicated by the interbasin comparison—namely, a limited set of tool activities thathad once been tethered to specific resourcesand places ultimately were replaced bydiverse behaviors that reflected more flexibleadaptive responses to different landscapes.

Conclusion

Paleolandscape research at Olorgesailie hasdocumented the presence of environmen-tal gradients, patches, and boundaries inthree stratigraphic intervals within upperMember 1 (�990 ka) and lower Member 7(�900 ka). Each stratigraphic intervalexhibits a unique pattern of taphonomicfeatures and paleoenvironmental variation.The spatial distribution of hominid artefactsfits neither a random nor uniform patternover comparable lateral distances in any ofthe three target intervals. Significant differ-ences in artefact-fossil distributions occur,however, between Members 1 and 7. Sincethese differences cannot be explained bytaphonomic or depositional factors, behav-ioral and ecological factors appear to beresponsible. In contrast with the continuous(though clustered) distribution of materialsin upper Member 1, the dense accumu-lations of artefacts in lower Member 7 con-trast with a negligible background scatter,which included gaps where no artefacts werefound. Fossil animal remains in lower Mem-ber 7 follow the same spatial pattern, whichsuggests that the tool-related activities ofhominids and the behavior of other animalswere focused on the same distinct placesacross the paleolandscapes of Member 7.During upper Member 1, by contrast, themore continuous spread of stone tools andanimal remains is consistent with evidenceof a uniform habitat with few boundaries inthe sampled area.

A distinctive aspect of the archeologicalrecord of Olorgesailie is the presence of

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dense concentrations of stone handaxes andother bifaces. Our work and earlier studies(Isaac, 1977) have established the highlylocalized spatial distribution of Acheuleanbifaces, especially in lower Member 7. Evi-dence from both Members 1 and 7 suggeststhat proximity to lava exposures, which formraised areas in the lowland basin, stronglyinfluenced the pattern of hominid stonetransport and the discarding of bifaces. Inlower Member 7, the largest concentrationsof bifaces were made in shallow ephemeralstream channels that passed near areas ofexposed volcanic rock on the eastern edge ofthe Legemunge Plain. As a working hypoth-esis, we suggest that hominids were active inhighland zones, where stone raw materialwas plentiful; they took advantage ofresources found in the lowland plains anddeposited bifaces near the lava exposures(the lowland–highland interface) duringthese forays, perhaps most commonly on thereturn trip.

The paleolandscape approach dependson comparison of excavation sites andstratigraphic intervals rather than oninterpretation of a single excavation area orstratum. Comparison aids the assessmentof time-averaging and helps to establishthe relative influence of other taphonomicfactors that may have affected the in situdistribution of hominid artefacts and fossilanimal remains. At a broader scale, thecomparative approach allows evolutionaryissues to be addressed by testing for cor-relations between patterns of hominidactivity and environmental variations overtime. A persistent positive correlationshould indicate continuing dependenceof hominid tool-related behavior on par-ticular environmental features or con-ditions such as substrate, vegetation, sourceareas for raw materials, specific animalfoods, or water sources. A decreasing cor-relation on the other hand, should denotegreater independence from prior spatialconstraints.

Although synthetic analysis of diversepaleolandscapes remains an importantfuture goal, initial landscape-scale compari-son among the Olorgesailie, Olduvai, andTurkana basins shows that, from latePliocene through early Pleistocene, the dis-tribution of hominid stone artefacts is lessand less tied to specific depositional facies.Tool-related behavior varied more freelywith respect to environmental and spatialconstraints. This increasing independenceis consistent with wider ranging distancesover time, including the transport of stonefrom more distant sources. At the sametime, however, the spatial distribution ofimportant kinds of artefacts—i.e., bifaces—becomes strongly place-specific. Thisintense spatial focusing of particular artefactforms has yet to be documented in earlierOldowan times; it also appears to beindependent of paleoenvironment and basindifferences, and thus may be correlated withthe development of Acheulean technology.Finally, as we have shown here, the denseconcentrations of Acheulean bifaces inMember 7 Olorgesailie are part of an overallspatial pattern of paleolandscape use thatdiffers significantly from that of Member 1.The strongly focused placement of bifacesbecame coupled with a very uneven andnonrandom discarding of artefacts in gen-eral. Since the difference with Member 1seems to be independent of paleoenviron-mental or taphonomic variation, at leastwithin the Olorgesailie basin, we suggestthat it signifies an important change inhominid interaction with environments on alandscape scale between one million and900,000 years ago.

Acknowledgements

This paper is dedicated to Dr Mary D.Leakey, who originally inspired our involve-ment with Olorgesailie and with whom weenjoyed many wry and frank discussionsabout the research. The Olorgesailie project

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is conducted in collaboration with theNational Museums of Kenya (NMK), andwe thank M. G. Leakey, M. Isahakia, G.Abungu, and the NMK PalaeontologyDivision for their support. We also thankR. E. Leakey for support and permission toconduct research during the first years of theproject. We gratefully acknowledge theOlorgesailie field crew led by J. M. Numeand B. Kanunga, and also the contributionsof A. Deino, T. F. Jorstad, J. B. Clark,W. G. Melson, N. E. Sikes, T. W. Plummer,M. Noll, W. F. Keyser, and other scientists,technical assistants, and students who haveparticipated in the research. We also thankJennifer Clark for her assistance withproject logistics and manuscript prep-aration; Ralph Chapman for statisticaladvice; and Mike Noll, Alison Brooks, andBernard Wood for discussion of ideas inthis paper. This project is funded by theSmithsonian’s Human Origins Program,Scholarly Studies Program, and theNational Museum of Natural History. Thisis a publication of the Smithsonian’s HumanOrigins Program.

� 1999 US Government

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