The Construction and Configuration of Anasazi Pebble-Mulch Gardens in theNorthern Rio Grande

Dale R. Lightfoot; Frank W. Eddy

American Antiquity, Vol. 60, No. 3. (Jul., 1995), pp. 459-470.

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THE CONSTRUCTION AND CONFIGURATION OF ANASAZI PEBBLE- MULCH GARDENS IN THE NORTHERN RIO GRANDE

Dale R. Lightfoot and Frank W. Eddy

Rio Grande Anasazi in thr,fi)urteenth andfr'fternth centurie.~ A.D. mulched hundred.^ ofgarden-sized plots with pebb1e.s to increase soil moislurr, reduce erosion, extend the growing season, and increase crop yir1d.c. This paper reports on the con- struction and configuralion ofpebble-mulch gardens in New Mexico, ,fi~cucingparticularIv on ~hosr in the Galisteo Basin. Surfaces adjacent to these gardens wrrr scraped and pits were excavu~ed to collect gravel, which was placed over garden surface.^ in layers 5 lo I I cm thick. Gardens averaged 15 x 23 m in size, although both size and shape wrrr highly vari- able, and they ccollrclive~v covered an area qf41.000 m2. Although this unique agricullural strategy has hrrn shown to be rffictive, construction was limited to sites with natural gravel depo.sils, pebbled surfacrs inhibited the recycling o f crop wastes, and such gardens never hecume us widely used as more traditionulfield,fi,rms.

Duruntr 10s siglos catorce y quince de nucstru era, ugricultores Anasazi drl Rio Grande cuhrirron cientos de huertas con sedimentos pedregosos para incremmtar la humedad del suelo, rrducir la erosirin, extender la ~emporada dr crrcimien- to, e incrementar la cosechu. Eslr urticulo reporta la construccirin y configuracirin de las huertas pedregosas en Nurvo MGxico, en/ocando purticukarmente lus huertas de la Cuencu Gulisteo. Las suprrficies adyacenles a eelas huertas,furron raspadas ).sr excavuron hurcos para recolectar 10s sedimmtos prdregosos que se pusieron sobre la suprrficir dr la huer- la, en cupus dr 5 a I I cm de espesor. Las hurrtus promediaron entre 15 y 23 m de lamaiio, aunque tanto su tumuiio como su/brmafitrron allamente variables; en total, estas huertas cubrirron un drru dr 41.000 m2. Aunque esta estrategia agri- cola unicu demostro ser efecliva, la conslruccirin sr limitri u sitios con depd~itos pedregoso~ nalura1c.s. Adrmds, las super- ,ficirs prdregosas inhihieron el reciclado de de.sechos dr cosrcha, por lo cuul rstus huertas nunca se utilizaron tan ampli- amente como aquillus dr,forma mas traditional.

Pebble-mulching, a relatively rare form of mulch gardens are found only in the Chama- agriculture, was employed by the Rio Ojo Caliente valleys, north of Santa Fe, and in Grande Anasazi in the fourteenth and fif- the Galisteo Basin, south of Santa Fe (Figure 1).

teenth centuries A.D. Contemporary agricultural Their use in this region appears confined to a experiments and field tests of prehistoric century and a half, spanning the period A.D. mulched gardens have shown that this strategy 1350-1 500 (Clary 1987; Gish 1984; Lightfoot conserves soil moisture, reduces soil erosion, 1993a; Maxwell and Anschuetz 1992). extends the growing season, and increases crop Crops no longer grow on the former pebble- biomass and crop yield. This allowed the mulch fields, but the water-conserving pebble Anasazi to expand arable land into areas previ- mulch is still in place, stimulating more vigor- ously considered less suitable for agriculture and ous plant growth among the native short grasses improved the productivity of existing croplands. growing on the gardens when compared to

Despite the benefits, these pebbled plots grasses growing off the gardens on nonmulched remained confined in their use, both spatially soil. This difference in plant density and vigor is and temporally. Rio Grande Anasazi pebble- visible in color infrared photographs of the

Dale R. Lightfoot. Department of Geography, Oklahoma State University, Stillwater, OK 74078 Frank W. Eddy. Department of Anthropology, University of Colorado, Boulder, CO 80309

American Antiquity, 60(3), 1995, pp. 459 -470. Copyright O 1995 by the Society for American Archaeology

AMERICAN ANTIQUITY [Vol. 60, No. 3, 19951

t

ALBUQUERQUE

Figure 1. Northern Rio Grande P-IV pueblos and areas of pebble-mulch gardens.

Galisteo Basin taken by the National Aeronautics and Space Agency's Stennis Space Center in September 1987. Photo interpretation of these images resulted in the discovery of more than 100 pebble-mulch gardens in the Galisteo Basin. During the period 1988-1 990, detailed maps were made of these gardens and empirical data collect- ed through fieldwork in order to better understand the agricultural effectiveness of pebble mulch (Lightfoot and Eddy 1994), the landscape context of these gardens (Lightfoot 1993b), how and when they were used (Lightfoot 1993a), and the construction and configuration of pebble-mulch gardens. This paper reports on the latter of these topics, specifically addressing ( I ) pebble-mulch garden size, (2 ) the orientation of the gardens

with respect to slope, (3) soil-gravel loads on and off gardens, and (4) sources of gravel and con- struction techniques.

Pebble-Mulch Gardens and Borrow Pits

The Rio Grande Anasazi mulched many of their garden plots with available gravel and pebbles, often creating rock borders around these plots with fist-sized or larger stones. Pebbles were accumulated for this purpose by scraping adjacent surfaces and by excavating borrow pits (Figure 2). Although the excavation and spreading of a peb- ble mulch requires a considerable investment in time and labor, pebble-mulching offers many advantages over a nonmulched cropping sur-face advantages that are particularly relevant in

REPORTS 461

0 Borrow Pit

0 '-\ Scraped Area I /// Gravel Relocation

Figure 2. Sources of gravel for the construction of a pebble-mulch garden.

marginal, stony to gravelly areas, during periods of drought, and in cooler uplands with a short growing season.

Pebble mulch will generally increase crop bio- mass and crop yield by acting as a one-way water valve to increase the amount of moisture available to plants growing on the mulched plots by both increasing infiltration capacity and reducing evaporative losses of water from wind and sun (Corey and Kemper 1968; Fairbourn 1970, 1973; Lamb and Chapman 1943). Furthermore, pebble mulch can help to control weeds and increase soil temperature to aid the germination, growth, and

maturation of crops (Lamb and Chapman 1943). Mulching also protects the soil surface from runoff to reduce erosion on agricultural plots.

A pebble mulch increases surface roughness, generating more turbulent air flow over the gar- den surface. This air flow moderates the diurnal soil temperatures as the hottest daytime tempera- tures are lowered and the lowest nighttime tem- peratures are increased (Chang 1968). The increase in wind turbulence also lowers the veloc- ity of wind over the mulched plot, thereby reduc- ing wind erosion of the soil. The application of a pebble mulch also inhibits salinization because

AMERICAN ANTIQUITY [Vol. 60, No. 3, 19951

Contour interval 1W It N c z Pueblo Room Blocks

Figure 3. Pueblo San Marcus and pebble-mulch gardens.

the evaporation of salt-laden water at the soil sur- face is dramatically reduced (Unger 1971). The net effect of these benefits is an increase in crop biomass and crop yield (Bleak and Keller 1974; Fairbourn 1970, 1973; Hakimi and Kachru 1978; Lightfoot and Eddy 1994; Othieno and Ahn 1980; Rodale 1949; Walker 1969). However, a perma- nent pebble mulch on the surface discourages the return of crop wastes following harvest and there- by disrupts nutrient cycles and could eventually lead to a depletion of some essential soil nutrients (Tsiang 1948).

A large number of borrow pits or growing pits are commonly found in association with Anasazi pebble-mulch gardens. These features appear as very shallow to moderately deep depressions or hollows in the land surface. They are most com- monly about 1 to 3 m in diameter but occasional-

ly reach 10 m or more. They m.iy appear to be clearly constructed by humans (even encircled with cobbles) or be almost indistinguishable from naturally eroded hollows at the head of gullies or small arroyos.

There is no real consensus on the intended use or practicality of these pits. Some researchers believe they are growing pits, or depressions intentionally hollowed out of the landscape so that they will retain water, especially if lined with clay, and thereby encourage the growth of crops plant- ed in them (Bugc 198 1;Lang 1979, 1980, 198 1 ). Because they are so often found in association with pebble-mulch gardens, and often sited on gravelly soil that would not hold much water after a rainstorm, these features are more likely borrow pits excavated into the gravelly soil in order to "mine" the pebbles needed to construct the peb-

REPORTS 463

ble-mulch gardens (Lightfoot 1993a; Lueben 1953). Furthermore, as this paper will demon- strate, through gravel particle-size analysis, peb- bles on gardens did, in fact, come from nearby borrow pits.

It is possible all but the smallest of the pits, especially if lined with clay, could have been used as growing pits after their initial excavation had been achieved. Once pebble-mulch gardeners noticed that borrow pits held moisture, they could have used the pits for growing crops, and may have even constructed a few depressions away from pebble-mulch gardens specifically for growing crops.

In parts of northern New Mexico prehistoric bordered gardens are sometimes found together with pebble-mulch gardens (Figure 1). Cordell (1984) reports that as much as 67,000 m2 may be covered by pebble-mulched fields throughout the lower Chama and Ojo Caliente valleys. Yet, the number of separate garden areas in the Chama- Ojo Caliente region remains unknown. In fact, the total number of individual Chama-Ojo Caliente pebble-mulch gardens examined through archaeological research amounts to fewer than 20 (Anschuetz and Maxwell 1986; Buge 198 1, 1984; Clary 1987; Ellis 1970; Fiero 1978; Gish 1980, 1984; Lang 1979, 1980, 198 1 ; Maxwell and Anschuetz 1992).

Accurate figures and maps regarding the num- ber, size, and extent of pebble-mulch gardens exist only for those in the Galisteo Basin. Ninety- six pebble-mulch walk-out gardens, collectively covering an area of 4 1,000 m2, have been mapped and analyzed around the pueblo ruin of San Marcos (Figure 3). The color infrared pho-tographs, from which these gardens were origi- nally identified, reveal only six mulched gardens near Pueblo San Lazaro (south of San Marcos) and no pebble-mulch gardens around two other Galisteo pueblos surveyed.

Methodology

A series of maps were produced at a scale of 1:1000, depicting 89 of the 96 San Marcos (Galisteo) pebble-mulch gardens (along with bor- row pits) by surveying each of these features in the field. These maps, which show the full range of garden sizes, allow for the calculation of aver-

age and extreme garden sizes, and the total area covered by pebble-mulch gardens. These maps were also used for determining the regional dis- tribution of gardens and assessing variations in garden design, for plotting onto a large-scale topographic map and for overlaying with soil and geology maps.

A large-scale (1 :2700) topographic map with a contour interval of 1.5 m was commercially produced from the I :13,000-scale infrared aerial photos on which the Galisteo mulched plots were discovered. The mapped area includes the pueblo of San Marcos (roughly centered) and the sur- rounding landscape within a 1.6-km radius of the pueblo. All prehistoric cultural features, includ- ing pueblo room blocks, pebble-mulch gardens, and other prehistoric agricultural remains were placed on this map.

Additional information on the Galisteo peb- ble-mulch gardens was collected by visually inspecting, photographing, and trenching mulched plots and adjacent nonmulched soils. Three "average" mulched gardens were selected for trenching on the basis of (1) their being rep- resentative of the various pebble-mulch garden styles (heavy mulch, moderate mulch, light mulch; border stones and no borders; adjacent scraped soil or natural-looking surface; borrow pits nearby or far away), and (2) the accessibility of the plots and permission given by landowners to dig on their land.

Each trench (one for each of the three gardens selected) was 2 m long by .5 m wide, for a total surface area of I m2. Trenches were laid out per- pendicular to the edge of a mulched plot so that the boundary between the pebble-mulch garden and adjacent nonmulched soil divided the trench precisely into two halves. Trench fill was removed at 5-cm increments to a depth of 15 cm (5, 10, and 15 cm). At each increment, both on and off the gardens, fill material was sieved through a graduated series of screens designed to sort fill gravel by particle size.

Gravel size intervals are (1) greater than one inch (2.54 cm+), (2) one-half inch to one inch (1.27 to 2.54 cm), (3) one-quarter inch to one- half inch (.64 to 1.27 cm), and (4) less than one- quarter inch (less than .64 cm). The weight of each bag of gravel-sorted according to particle

AMERICAN ANTIQUITY [Vol. 60, No. 3, 19951

Figure 4. Pehhle-mulch gardens and borrow pits south of Pueblo San Marcos.

size, depth in soil, and position on or off the peb- ble-mulch garden--was then recorded for each of the three gardens. These data were analyzed to examine similarities or differences in the quantity of different-sized gravel at various depths on and off mulched gardens. This helped to determine if the composition of gravel varied by depth in soil, between gardens and nonmulched soil, or between gardens associated with scraped soil ver- sus borrow pits.

Additional empirical data collected during this

research include sketches of trenches, which record the soil profile and the concentration of pebbles within the mulched and nonmulched soils. Information gleaned from ancillary litera- ture on the prehistoric pebble-mulch gardens of the Chama region also provides supporting evi- dence for the construction of these features.

Garden Construction and Configuration

The size and shape of Anasazi agricultural fea- tures tcnd to be highly variable. Likewise, specif-

I

REPORTS 465

ic methods used to construct fields and gardens often varied between and within pueblo sites. Pebble-mulch gardens also vary widely in appear- ance and design. Analysis of these differences allows insight into the construction and function of these unique features. This section outlines the size and orientation of Anasazi pebble-mulch gar- dens, and then discusses details concerning their construction and construction materials.

Garden Size and Orientation

A total 4 1,000 m2 (4.1 ha) of surface area is cov- ered by pebble-mulch gardens around Pueblo San Marcos. Garden plots are irregularly shaped (though generally rectilinear) and vary widely in size (Figure 4). The smallest pebble-mulched plot, measuring 4 x 8 m, covers only 32 m2. The largest mulched garden measures 30 x 105 m and covers an area of 3,150 m2.

The average garden size is 441 m2 (.04 ha), with average dimensions measuring 15 x 23 m. This provides a size easily tended by a single family, and a pebble-mulched field (cluster of gardens) that could be cared for by an extended family group. However, the area covered by pebble-mulch gar- dens, although far greater here than elsewhere in the Galisteo Basin, is not sufficient to have con- tributed a large portion of the total agricultural yield at San Marcos. However, during periods of drought when the yields from other field types would have dramatically declined, the moisture- retaining pebble-mulch gardens would have pro- duced more consistently (Lightfoot 1993a), and their relative contribution would have increased. It also appears likely that these features became sig- nificant only to those households that experiment- ed with or applied this agricultural strategy.

Lang (1 979) has suggested that Anasazi culti- vators constructed their pebble-mulch gardens with long axes oriented perpendicularly to the slope in order to further reduce soil erosion from water coursing the length of the mulched plot. Eighty-five percent of the mulched gardens he examined in the Chama-Ojo Caliente valleys were oriented in this fashion. However, Galisteo (San Marcos) pebble-mulch gardens were not so con- sistently oriented. Only 57 percent of the 96 San Marcos mulched gardens were laid out with their long axes perpendicular to the slope, while 43

Ralsed Pebble Mulch

I Natural Gravelly Soil

Ralsed Pebble Mulch

Scraped Surface

Natural Gravelly Soll

Figure 5. Profiles of a prehistoric pebble-mulch garden from A.D. 1425 (bottom), with a layer of pebble mulch on top of a gravelly soil, and a newly constructed pebble- mulch garden from A.D. 1990 (top). The pebble mulch of the earlier garden has settled, becoming filled with aeo- lian sediment, and appears as a very gravelly soil.

percent were either oriented diagonally or parallel to the slope.

Garden Construction and Gravel Sources

The surface application of a pebble mulch is far superior to the incorporation of mulch into the soil. If pebbles are incorporated, rather than placed over the surface of the soil, evaporative losses of water from the soil are even higher than from an untreated soil. Only by placing pebbles on the sur- face will there be an increase in soil-water storage (Alderfer and Merkle 1943; Unger 1971). Furthermore, a smaller percentage of incoming precipitation will become part of the soil-water system if gravel is mixed into the soil. For a con- stant percent of pebble mulch, surface sealing and crusting are lowest, and thus infiltration is highest, when pebbles are placed on the surface and not pushed into the soil (Poesen 1985).

Surely the Anasazi were aware of this impor- tant engineering consideration, yet most of the prehistoric pebble-mulched plots we see today are only slightly raised above the surface, and much of the gravel seems to be mixed with soil to an average depth of 5 to 15 cm (average depth of the soil-pebble matrix is 8 cm). However, this is like- ly the result of wind-blown sediment becoming incorporated into the long-abandoned mulch layer, which has settled and now appears to be a very gravelly soil (or soil-filled gravel) rather

466 AMERICAN ANTIQUITY [Vol. 60, No. 3, 19951

than the pure gravel layer, which must have origi- nally covered the surface of the prehistoric peb- ble-mulch gardens (Figure 5). While sampling for niaize pollen on pebble-mulch gardcns in the Ojo Caliente valley, Fiero (1978) noted that maize pollen is mixed with surfacc pollen, suggesting that the gardcns arc erodedideflatcd and that the gravels may not have been as compacted when these features were in use.

Pebbles nccdcd for mulching arc found natu- rally within the soil on the flat ridges whcrc pcb- blc-mulch gardcns arc found. Some of these grav- el fields may also have been exploited for agri- cultural purposes, without altering the natural pebbly surfacc. However, in order to construct a true "niulched plot the Anasazi gardener nccdcd to accumulatc additional gravel. The pebbles nccdcd for building a mulch layer were gathered by scraping the gravelly soil adjaccnt to dcsignat- ed plots and/or by "mining" pebbles in nearby borrow pits. Many borrow pits are very close to pebble-mulch gardens and niay even be found within the borders of a gardcn (Figurc 4). Where the soil has been scraped around gardens, the absence of gravel and the difference in soil color (compared to gardcns) are striking.

Cobbles taken from scraped soil and borrow pits were sometimes used to create a border around pebble-mulch gardcns, or to create intcr- nal dividers as well (Figurc 2). Thcse borders and dividers contribute further to the ability of a peb- ble-mulch gardcn to retain soil (inhibit erosion), absorb solar energy to raise the soil teniperaturc, and decrease surface wind velocity to help niod- erate diurnal extremes in soil teniperaturc. Thcse internal subdivisions niay also have served a boundary role.

Many pebble-mulch gardens look very similar to bordered gardcns except for the addition of a raised pebble mulch within the borders. It is not known whether these mulched gardcns were built "from scratch," with borders and dividcrs added after the mulch had bccn applicd or whether pcb- ble-mulch gardens were made by mulching exist- ing bordcrcd gardens. With an abundance of bor- dcrcd gardcns at most late Rio Grandc Anasazi sites, it is likely that at least some of the pcbble- mulch gardcns were incrementally constructed by adding a mulch layer to existing bordcrcd gardens.

Because the scraped soil adjacent to mulched gardens appears so distinct from the pebbly gar- den soil, it is natural to assume that a scraped area contains less gravel and a pebble-mulched surface contains more gravel. Quantified analysis of grav- el fill removed froni trenches verifies the inten- tional loading of gravel onto niulched gardens. Furthermore, analysis of the coniposition of grav- el by particle size sheds sonic light on how pcb- blc-mulch gardcns were constructed, confirming that some were mulched with scraped pebbles and some with borrow pit pebbles.

On all three pebble-mulch gardcns that wcrc trenched, the quantity of gravcl is greater on the mulched plots conipared to the quantity of gravel from a like volume of nonmulched soil. At the 5- cm lcvcl (gravcl removed from the surface to a depth of 5 em), the pebble-mulch gardcn gravel weighed froni 2.4 to 4.2 timcs morc than the grav- el collected froni the adjaccnt unmulched half of the trench. At the 10-cni level (gravel removed bctwccn the 5-10 cm depth), the gardcn gravel weighed 1.3 to 1.9 timcs morc than the gravel taken froni adjaccnt bare soil areas at the same depth. Bctwccn the 10 and 15 cm levels there was only an average 30 percent increase in gravcl within the niulched plots.

Because the mulch layer was no more than 5 to I I cm deep, and originally applied on the surface of the soil, it is not surprising that the greatest dif- fcrcnccs in gravel are found in the upper layers, and especially in the near-surface layer (0-5 em). Soil morphological changes associated with prc- historic agricultural features are commonly limit- ed to the surface or near-surface soil horizons (Sandor et al. 1986). Such regular and quantifiably distinct differences in the gravel content of pcbble- mulched plots and adjacent nonniulchcd soil can only be explained as an intentional loading of gravcl onto one surfacc while an adjacent surface was scraped of gravcl or, at best, left unaltered.

The depth of the mulch on gardens around San Marcos varies from 5 to 1 1 em, and rises 2 to 10 cm above surrounding surfaces. Pebble- mulch gardcns in the C'hama-Ojo Calicntc region commonly rise 3 to 5 cm above adjaccnt surfaces (and as high as 10-20 cm) and have mulch layers averaging 10 cm in depth (Anschuctz and Maxwell 1986). These data arc comparable with

REPORTS

Table I . Composit~on of Gravel by Particle Size.

Garden 1 Garden 2 Garden 3

OffPMG OnPMG OffPMG OnPMG OffPMG OnPMG

Increase in gravel I x 2 . 4 ~

Percent of gravel in each gravel size cohort

> I " 37 37

!4"l" 3 3 36

%" i.i" 30 27

Notes- PMG = pebble-mulch garden. Gardens I and 2 were mull zhed with scraped gravel. Garden 3 was mulched with gravel from borrow pits.

observations of Chinese pebble-mulch gardens where the pebble mulch, mixed with sand, was spread in a layer between 10 and 15 cm thick (Tsiang 1948).

Analysis of gravel by particle size provides additional clues to the construction of Anasazi pebble-mulch gardens. An examination of gravel weights for each pebble-size cohort in the upper 5 cm of soil, on and off the three trenched pebble- mulch gardens, reveals two distinct phenomena (Table 1): ( 1 ) for Gardens 1 and 2, whose adja- cent surfaces appeared scraped the composition of gravel by particle size is essentially the same on and off the gardens, although absolute quanti- ty is higher on the gardens (2.4 and 2.8 times, respectively), and (2) for Garden 3, which was nearest to borrow pits and had adjacent surfaces that appeared natural (not scraped), the composi- tion of gravel is dissimilar on and off the garden, and the garden contained 4.2 times more gravel than the adjacent soil.

When a garden is mulched by scraping adja- cent soil and loading the scraped pebbles onto the garden surface, there will be more gravel on the garden but the composition of gravel both on and off the garden will be about the same; grav- el removed from one surface is simply added to an adjacent surface. Only the smallest particle size cohort (less than .64 cm or '12 inch) appears to have greatly changed between the mulched and nonmulched sides, with fewer of these tiny pebbles on the mulched plots (as a percentage of all pebble sizes). This may be the result of the smallest particles being lost and/or intentionally sifted out of the scraped gravel prior to dumping the gravel on gardens, or many of the smallest

particles settling out of the mulch layer over time. When this smallest particle cohort is omit- ted from the data, the composition of garden gravel and adjacent gravel is even more alike for the two scraped gardens (Gardens 1 and 2), while the composition of gravel on and off the borrow pit garden (Garden 3) remains distinct (Table 1).

When the soil around a garden is not scraped to obtain gravel, the pebbles for a mulched plot would have to come from some distance away, most likely from a borrow pit. Because the mulch on such a garden did not come from adjacent soil, the composition of pebbles on the garden should instead reflect the character of nearby borrow pit gravel. The third garden listed in Table 1 was apparently mulched using pebbles from nearby borrow pits because differences in gravel on and off this garden are quite dissimilar, yet the gravel sampled on this garden is almost identical to gravel later collected from the rim of the nearest borrow pit.

It is unlikely that the entire surface of a peb- ble-mulch garden, especially a very large one, would be mulched only with scraped gravel or only with borrow pit gravel. The two gardens associated here with scraped gravel are not far from borrow pits, and the garden here mulched with borrow pit gravel lies adjacent to a scraped surface along another edge, which was not trenched. These facts, along with maps showing the arrangement of pebble-mulch gardens and borrow pits, suggest that Anasazi farmers collect- ed pebbles from both sources and simultaneously used scraped gravel and borrow pit gravel to mulch different parts of each garden.

AMERICAN ANTIQUITY [Vol. 60, No. 3, 19951

Discussion

In order to be most effective, pebble-mulch gar- dens were intentionally sited with respect to req- uisite resources, including arable cropland on rel- atively flat sites, and surface or near-surface deposits of gravel. The pebbles needed to create a mulch layer were gathered by scraping adjacent gravelly soil surfaces and piling the accumulated gravel onto gardens, or by excavating shallow borrow pits and piling these quarried pebbles onto a designated garden plot.

In the dry environment of the Galisteo Basin the need to water crops periodically during much of the growing season required that garden sites not be too far from reliable springs or a perennial stream. None of the San Marcos pebble-mulch gardens is farther than 2 km from the springs adjacent to the pueblo, a walking distance of less than 45 minutes.

The San Marcos environment, out to a distance of 5 km, provided about 400 ha of arable land, of which about 280 ha would have been used on a regular basis (Lightfoot 1993a). The total surface area covered by pebble-mulch gardens in the Galisteo Basin is 4.1 ha. The average garden is .04 ha in area and 15 x 23 m in size. The smallest mulched garden measures only 4 x 8 m, while the largest measures 30 x 105 m. These sizes place pebble-mulch gardens within a range that could easily be tended by a single household or an extended family walking out from the pueblo.

The San Marcos (Galisteo) pebble-mulch gar- dens were not consistently oriented to the direc- tion of the slope, unlike those in the Chama-Ojo Caliente valleys, which were mostly laid out per- pendicularly to the slope. This may be because those near San Marcos were constructed on flat sites where slope orientation would not have been a major consideration.

The construction and maintenance of pebblc- mulch gardens was not likely undertaken as a sin- gle event of rapid agricultural change but as an accretionary or incremental process spanning sev- eral years. As population growth encouraged the farming of more marginal, gravelly uplands, or after each season that rainfall was low and crops were poor, pebble-mulch gardens would have been built, or existing bordered gardens mulched.

Individual decisions would be made to invest the additional time and labor needed to apply a peb- ble mulch to the surface of gardens in order to expand arable land into new niches or to arrest declining yields (Lightfoot 1993a).

Anasazi pebble-mulch gardens could not have been used for longer than a few decades because ceramic data and maize pollen analyses suggest a fairly short-term period of use spanning only about 100 years in the Chama region (about A.D. 1350-1450) and about 100 years for the Galisteo pebbled plots (A.D. 1400-1 500) (Clary 1987; Gish 1984; Lightfoot 1993a; Maxwell and Anschuetz 1992). The Galisteo pebble-mulch gar- dens likely diffused from the Chama-Ojo Caliente valleys, where these features were already present before they were constructed in the Galisteo Basin.

The restricted use of pebbled gardens, at least those in the Galisteo Basin, may be due, in part, to the variable nature of droughts, which appear correlated with the construction and abandon- ment of Galisteo mulched gardens (Lightfoot 1993a). The moisture-retaining benefits offered by a pebble mulch are accentuated during drought, and continuity in the diffusion process could have been disrupted every time the rains returned and nonmulched (less labor-intensive) plots produced a sufficient crop. Whether drought induced or simply used to expand cropland into more marginal (gravelly) areas, they may have been used for such a brief period because a peb- ble mulch inhibits the return of crop wastes and the recycling of nutrients. Pebble-mulched plots begin to play out in 20 to 30 years, and after 50 to 80 years become "old" and less productive than unmulched fields (Tsiang 1948).

Natural resources and social stimuli were never quite adequate to encourage an entire pueblo community to mulch its gardens. Most of the area farmed around pueblos, even around those pueblos where pebble-mulch gardens are found, is not suitable for pebble mulching because of a lack of natural gravel deposits (Lightfoot 1993b). Many of the areas that were ideally suit- ed were never completely mulched because pueblo inhabitants were, during good years, pro- ducing enough food from traditional agricultural methods and, during drought years, continuing to

REPORTS 469

rely on established food stress coping mecha- nisms such as increased foraging, intra-pueblo reciprocity and ritual food exchange, inter-pueblo exchange, and trade with Plains Indians (Lightfoot 199321).

The basic technology and construction of peb- ble-mulch gardens are much the same. There are, however, a number of variations on the basic theme: cobble borders or no borders; a thick, raised gravel mulch or very little gravel; internal dividers and terraces or a uniformly pebbled sur- face; rectangular or irregular in shape. In some places the Anasazi apparently used existing natur- al gravel surfaces, without adding an additional mulch, although this cannot be considered a peb- ble-"mulched" garden.

Some of these design variations could have resulted from the absence of a central authority capable of standardizing construction techniques; separate individuals, families or clans instead constructed their own mulched gardens without engineering instructions. Perhaps variations in pebble-mulch garden design are instead related to the quantity of gravel locally available for spread- ing as a mulch. But if any gravel was available at all, it could have been accumulated to create a mulched layer of any desired depth.

It is likely that differences in pebble-mulch garden construction were, to some degree, inten- tional. Changes in the percent cover, depth, peb- ble size, and even the color of the pebble mulch can significantly alter the moisture content and temperature of a soil beneath the mulch (Lamb and Chapman 1943; Mannering and Meyer 1963; Smith 1975; Unger 1983; Walker 1969). The amount of moisture in the soil will affect the qual- ity and yield of a maize crop, especially at the early seedling and later silking stages of growth. Walker (1969) further demonstrates that even a one degree (centigrade) difference in soil temper- ature during the early growth of maize (Zeu muys L.). which is easily affected by the depth and per- cent cover of a pebble mulch, will result in up to a 20 percent change in the dry weight biomass of maize seedlings.

Perhaps the Galisteo and Chama valley inhab- itants learned enough about pebble-mulch gar- dening to intentionally vary the amount of mulch they applied to the plots, in order to vary the reg-

imen of maize growth and further diversify their subsistence strategies. If this were the case, it would suggest that pebble-mulch gardens were more than experimental plots, and had evolved to fill a naturally selected niche in a variable semi- arid environment.

References Cited

Alderfer, R. B., and F. G. Merkle 1943 The Comparative Effects o f Surface Application

versus Incorporation o f Various Mulching Materials on Structure, Permeability, Runoff, and Other Soil Properties. Soil Science Societ?j ofAnmerica Proceedings 8:79-86.

Anschuetr, K. F., and T. D. Maxwell 1986 The Multidisciplinary Investigation o f Prehistoric

Puebloan Gardens in the Lower Rio Chama Valley, New Mexico. Paper presented at the 9th Annual Ethnobiology Conference, Society o f Ethnobiology, Albuquerque.

Bleak, A. T., and W. Keller 1974 Crested Wheatgrass Yields as Influenced by Water

Conservation Practices. Agrononmj~ Journcrl66:326-328.

Buge, D. E. 1981 Prehistoric Subsistence Strategies in the Chama

Region, Northern New Mexico. Paper presented at the 40th Annual Meeting o f the Society o f American Archaeology, San Diego.

I984 Prehistoric Subsistence Strategies in the Ojo Caliente Valley, New Mexico. In Prehistoric Agricultural Strategies in the Southwest, edited by S. K. Fish and P R. Fish, pp. 27-34. Anthropological Research Paper No. 33, Arizona State University, Tempe.

Chang, Y . 1968 Clinmcrte and Agriculture: An Ecologictrl Survey.

Aldine Press, Chicago. Clary, K . H .

1987 Pollen Evidence.f;)r the Agricultural Utilization of Ltrte Cltrssic Period (A.D. 1350-1500) Puehlotm Gravel Mulch Terrace Gardens, the Rio Chtrrntr, in the Vicinit~. r?f'Mendanales, New Me.rico. Castetter Laboratory for Ethnobotanical Studies Technical Series Report No. 198. University o f New Mexico, Albuquerque.

Cordell, L. S. 1984 Prehistory ($the Southwest. Academic Press, New

York City. Corey, A. T., and W. D. Kemper

1968 Conservation of Soil Water t)y Gravel Mulches. Colorado State University Hydrology Papers No. 30. Colorado State University, Ft. Collins.

Ellis, F. H. 1970 Irrigation and Water Works in the Rio Grande.

Paper presented at the 43rd Annual Pecos Conference, Santa Fe.

Fairbourn, M. L. 1970 Gravel Mulches Improve Dryland Tomato Yields.

Colortrdo Rancher and Farmer 24(3):64-65. 1973 Effect o f Gravel Mulch on Crop Yields. Agwnonmj~

./ourntrl 65925-928. Fiero, K.

1978 Prehistoric Garden Plots Along the Lower Rio (,'hamcr Vu1le.v: Archaeologiccrl Investigtrtions crt Sites LA

AMERICAN ANTIQUITY [Vol. 60, No. 3, 19951

11830, LA 11831, and LA 11832, Rio Arrihtr County, New Mexico. Laboratory o f Anthropology Note IIlc. Museum o f New Mexico, Santa Fe.

Gish, J . W . 1980 Pollen Results from Three Sites in Rio Arriba

County, New Mexico. In Archarologiccrl 1nvc.stigation.s a f tr Puehlo Agric~ulfurul Sifc, undArc,htric trnd Puehlotm Enc~umpmcnt.~on the Rio Ojo Calientr, Rio .4rrihu Countj: New Mexico, edited by R. W . Lang, pp. 264-277. Contract Archaeology Division Report No. 007. School o f American Research, Santa 1%.

1984 Pollt~n Re.rultr /ion7 cr Prehi.storic Puehlo Garden Plot crt Ojo Culientc 7. I<io Arrihtr C 'ounfy, New Mrrico. Manuscript on file, Quaternary Palynology Research, Littleton, Colorado.

Hakimi, A. H.. and R. I? Kachru 1978 Silage Corn Responses to Different Mulch Tillage

Treatments under Arid and Semiarid Climatic Conditions. Journal of Agronomy trnd C'roy Sciencv~ 147:15-23.

Lamb, J.. Jr., and J . E. Chapman 1943 Effect o f Surhce Stones on Erosion, Ebaporation.

Mannering, .I. V, and 1 . D. Meyer 1963 The Effects o f Various Rates o f Surface Mulch on

lnf~l trat ion and Erosion. Soil Sc,icncc, Societj' of America Procrrdingc 27:84-86.

Maxwell. T. D., and K. F. Anschuctz 1992 The Southwestern Ethnographic Record and

Prehistoric Agricultural Dibersity. In Garden. of Prehictorj,: The .4rchtrrologr of Settlement Agriculfure in Greater Me.<oumericu, edited by T. W Killion, pp. 35 08. Unibersity o f Alabama Press, Tuscaloosa.

Othieno. C . 0..and I? M. Ahn 1980 Effects o f Mulches on Soil Temperature and Growth

o f Teaplants in Kenya. Exprrirnmtal Agriculture 10:287 294.

Pocsen. J . 1985 Surface Sealing on Loose Sediments: The Role o f

Texture. Slope and Position o f Stones in the Top Layer. In As.re.,mmf o f Soil Sur/acr Srtrling trnd ('rusting. edited by I < Callebaut. D. Gabriels and M. deBoodt, pp. 354 302. Proceedings o f the International Symposium on the Assessment o f Soil Surface Sealing and Crusting. Ghent, Belgium.

Rodalc. J . 1Soil Tem erature, and Soil Moisture. ~ g r o n o m , ~ " ~ " ~ " ~ 35:567-f78.

Lang, R. W. 1979 An Arc,hacologicul S u r v q Necrr the Con/lurnc,c, (!/

fhr ('htrrnaand Olio Ctrliente river.^, Rio Arriha Counly, New Mexico. Contract Archaeology Division Report No. 065. School o f American Research. Santa Fe.

1980 Arc~hueologic~crl 1nvrtigation.r at tr Puehlo Agricultural S i f r , and Archaic and Puchlocrn Enctrrnpment.r on the Rio Ojo Culientc. Rio .4rrihrr Counfj: Neb{, Meric,o. Contract Archaeology Dibihlon Report No. 007. School o f American Research. Santa Fe.

198 1 A Prehi.storic, Puehlo Garden Plot on the I<io 010 ('uliente, Rio Arriha Count.v, New Mexic,o- Ojo Calientc Si fe 7. Ft~atures 1 2 . Contract Archaeology Dibision Report No. 065. School o f American Research, Santa Fe .

Lightfoot, D. R. 1993a The Cultural Ecology o f Puebloan Pebble Mulch.

Humcrn Ec,olo>gy 2 1 : 1 1 5-1 43. 1993b The Landscape Context o f Anasazi Pebble-

Mulched Fields in the Galihteo Basin. Northern New Mexico. Geoarchtreolov 8:349-370.

1994 Morphology and Ecology o f Lithic Mulch Agr~culture. Geogrul~hic~crl re vie^' 84: 1 7 2 185.

Lightfoot. D. R.. and F. W. Eddy 1994 The Agricultural Utility o f Lith~c-Mulch Gardens:

Past and Present. Geojournul34:425 437. Lueben, R. A.

1953 Leaf Water Site. In Strlvage Archucologr in the Chamcr Erll~:~. N ~ M ,M~xic.o, edited by F. Wendorf, pp. 9-33. Monographs o f t h e School o f American Research No. 17. School o f American Research, Santa 1:e.

1949 Stone Mulc,hing in the Garden. Rodale Press, Emmaus. Pennsylvania.

Sandor. .I. A.. I? L. Gersper, and J . W . Ilawley 1980 Soils at Prehistoric Agricultural Terracing Sites in

New Mexico: ( I ) Site Placement, Soil Morphology and Classification: ( 1 1 ) Organic Matter and Bulk Density ('hanges: ( I l l ) Phosphorus. Selected Micronutrients, and pll. Soil So,ic~nc.e Socirtx of Alncric.u .Iournul 50( 1 ): 166-1 80.

Smith, L. P 1975 ,llefhod,v in Agric~ulturul Meteorolofl.. Elsevier

Sclentlfic Publishing. New York City. Ts~ang,7'. C'

1948 So11 ('onsrrvtrtion. An Internutiontrl Sfudv. FAO Agricultural Studies No. 4 . United Nations Food and Agriculture Organization (1JNI:AO). Washington, D.C.

Unger. Paul W. 1971 Soil Profile Gravel Layers: ( I ) Effect on Water

Storage, Distribution. and Evaporation; ( 1 1 ) Effect on Growth and Water Use by a Hybrid Forage Sorghum. Soil Sc,ienc,e Society o f An7eric.a Proceedings 35:031 634. 980-983.

IJnger. P W 1983 Water ('onservation: Southern Great Plains. In

1)r:vland Agric~ulturc~. edited by 11. E. Dregne and W 0 . Willis. pp. 35 SO. Soil Science Society o f America. Madison, Wihconhin.

Walker, J . M. 1969 One-degree Increments in Soil Temperatures

Af fect Maize Seedling Behavior. Soil Scienc,e Societ.v of .3mrrrc.u proceeding.^ 33(5):729-736

Rrtr~vc~d I99 3 Jlmr 4 1995l k v cmhrt I at cq~ t rd