ABSTRACT

One of the few examples of high mountain prehistorical archaeo -logical sites in northern Italy has been discovered within the socalled Plan di Modzon area, at 2300 m a.s.l., in the Aosta Valley(Western Alps). This area shows interesting geological features andmorphological evidence which have been recently linked to a wideDeep-Seated Gravitational Slope Deformation (DSGSD). A probablefilled lake and the presence of buried landforms have been alsohypno tized on the basis of geological observations, resulting in theneed for new devoted surveys to confirm these hypotheses. A betterand comprehensive understanding of the geological evolution of thearea is indeed necessary to allow a precise reconstruction of theenvironments in which man has inhabited these territories.

Combined Electric Resistivity Tomographies (ERT) and geologicalsurveys have been therefore used to better define the occurrence andnature of morphological evidence along the western slope of the filledlake, in which one of the archaeological sites has been discovered andexcavated. The outcomes of the integration of the two surveys is a morerefined definition of the geological forms, in respect to the one previ-ously assumed on the basis of surface surveys only. Indeed, ERTrevealed buried zones with low resistivity associable to the presence ofsliding surfaces and also showed the geometry and thickness of the sedi-ments within the filled lake. Geological surveys also evidenced a DSGSDevolution in the area constrained to Lateglacial, because of the lack ofparticularly evident gravitative forms, i.e. remodeled by glacial abrasion.

INTRODUCTION

This research relates to the geological characteriza-tion of a mountain sector, at approximately 2300 m ofelevation, located in the Aosta Valley (Western Alps)within the so called Plan di Modzon area. This area(asterisk in fig. 1) corresponds to the hanging thresholdof a tributary glacial valley (Verrogne Valley) and hasbeen studied in the last years for its special context con-cerning both the archaeological findings and the geologi-cal features (FORNO et alii, 2013).

The Plan di Modzon surroundings is one of the fewknown examples of prehistoric archaeological high moun-tain site. Nine sites (MF 1-MF 9 in FORNO et alii, 2013; seewhite square symbols in fig. 4) have been discovered atthe time, two of them (MF 1-MF 3) have already beenexcavated by archaeologists. Their remains, consisting ofprevalently hyaline quartz artefacts and some man-madestructures, including fireplaces, are referred to theMesolithic, the Middle Neolithic and the Copper Age(RAITERI et alii, 2008).

Geological studies aimed in establishing a link betweengeological contest and archaeological remains are welldocumented in literature (e.g. GOLBERG et alii, 2001;GARRISON, 2003). This link is however very complex inthe investigated area due to the non-trivial interactionbetween glacial and gravitational processes. Theseprocesses created, beside the surficial landforms (visible),the possible occurrence of buried landforms (supposed)that require an accurate geological reconstruction. Thepresent work is aimed at discovering and investigatingthese buried landforms with the use of geophysical sur-veys, and related geological interpretations, with the gen-eral aim of correctly read the framework in which manhas inhabited the sites. This work allows, therefore, toconnect the prehistoric settlement in high mountain tothe occurrence of suitable morphological conditions,resulting from the geological evolution.

Several visible geological evidence such as open frac-tures, minor scarps and trenches are clearly evident in thePlan di Modzon. These have been recently related to awide Deep-Seated Gravitational Slope Deformation (PointeLeysser DSGSD - FORNO et alii, 2013), not reported inprevious geological literature (ELTER, 1987; MORTARA &SORZANA, 1987). The DSGSD deformation involves aglacial shaped valley floor, with subglacial rounded ridgesand glacial margin moraines. Moreover, a wide, almostflat, surface in the valley floor has been observed, formedby shallow peat deposits and likely related to a filled lake.In addition to the outcropping surficial landforms, someof them strongly modified by glacial processes, the arealikely presents also some subsurface evidence related tothe DSGSD (buried sliding surfaces) which have not beenalready studied in detail.

To investigate the occurrence of these subsurface evi-dence more specific surveys are needed. Geophysical sur-veys are the most cost-effective and rapid means ofobtaining subsurface information, especially distributedover wide areas. Electrical resistivity methods have been

(*) Dipartimento di Scienze della Terra, Università di Torino,Via Valperga Caluso, 35 - 10125 Torino, Italy. Tel: +39 011 6705173,fax: +39 011 6705317; e-mail: cesare.comina@unito.it

(**) NatRisk Interdipartimental Centre, Università di Torino,Via Leonardo da Vinci, 44 - 0095 Grugliasco.

(***) Regione Autonoma Valle d’Aosta, Soprintendenza per ibeni e le attività culturali, Piazza Roncas, 12 - 11100 Aosta.

(****) Dipartimento di Ingegneria dell’Ambiente, del Territorio edelle Infrastrutture, Politecnico di Torino, Corso Duca degli Abruzzi, 24 -10128 Torino, Italy.

ERT geophysical surveys contributing to the reconstruction of the geologicallandscape in high mountain prehistorical archaeological sites

(Plan di Modzon, Aosta Valley, Italy)

CESARE COMINA (*), MARIA GABRIELLA FORNO (*), (**), MARCO GATTIGLIO (*),FRANCO GIANOTTI (*), LUCA RAITERI (***) & LUIGI SAMBUELLI (****)

applied in this context since they are particularly suitablein locating contrast between shallow sediments andbedrock (like is the case for paleo-channels or paleo-lakes shaped in the bedrock and filled by soft sediments,eventually with relevant peat content) or to delineatefractured or weathered zones within the bedrock in tec-tonically or gravitational active areas (like in a DSGSDcontext). Particularly Electric Resistivity Tomography(ERT) has been used in similar situations, e.g. to con-strain the geometry of a paleo-polje developed during aglacial period (ROMEY et alii, 2012) or to characterizeglacial and postglacial successions including peat over-lapping outwash sediments (COMAS et alii, 2004; PEL-LICER & GIBSON, 2009). For structural geology appli -cations, ERT has also been used to document thesubsurface occurrence and geometry of a fault affectingan archaeological site (SIMILOX-TOHON et alii, 2006) orin other different tectonic contexts (e.g. CAPUTO et alii,2007; ŠTEPANCÍKOVÁA et alii, 2011).

In the Plan di Modzon area the ERT surveys haveessentially two aims. The first one is to understand theburied geometry of some morphological scarps, a counter-slope depression and a small flat sector along the slope onwhich one (MF 3) of the excavated archaeological siteoccurs, since these features seem essentially linked to theDSGSD evolution. The second one is to delineate thegeometry and the geoelectrical characteristics of the layersbeneath the wide closed depression in the valley floor,probably related to a filled lake.

An advancement of the knowledge in respect to amore refined definition of the geological forms and

their link with prehistoric settlement is thereforeobtained through an interdisciplinary approach apply-ing various methods: geological mapping, morphologi-cal and structural observations and geophysical surveys.Given the not so favourable logistical condition of highmountain sites (i.e. difficulties in performing directsoundings) the adopted methodology is showing a valu-able approach also for the study of others similararchaeological settings.

GEOLOGICAL AND ARCHAEOLOGICAL SETTINGS

The Plan di Modzon area is placed in the left side ofthe middle Dora Baltea Valley, on the southern slope ofthe Mont Fallère immediately WNW of the Becca Francepeak (fig. 1). This area consists in a gently sloping sur-face (5%) that occurs into the floor of the NNW-SSEtrending upper Verrogne Valley tributary, suspended bythe Verdjouan scarp above the main Dora Baltea Valley(approximately 600 m a.s.l. - fig. 2). A well-articulatedhydrographical network, drained by three main water-courses (named western, intermediate and eastern Ver-rogne Torrents) characterizes the area. These mainwatercourses, separated by low rocky ridges, are feeblyincised at the head and deeply entrenched downstream,near and across the Verdjouan scarp, were they flow inrocky gorges (fig. 2 and fig. 4). Several tributaries can bealso recognized in the surroundings.

The Plan di Modzon area is located in the axial sectorof the alpine chain where a portion of the nappe pile is

Fig. 1 - Shaded relief map of the left, northernside of the Dora Baltea Valley, where the P. Leysser DSGSD develops (dashed line). Thelower sector of the DSGSD shows a convexshape, prominent towards the main valley (a).The white square refers to the relief map offig. 4, within the Plan di Modzon area (asterisk).Shaded relief map is derived from a 2 m cellsize LIDAR elevation model (courtesy of theRegione Autonoma Valle d’Aosta).

exposed. In detail, it occurs along the contact between theGran San Bernardo Nappe (Middle Penninic) and theoverlying upper units of the Piedmont Zone (fig. 3). TheGran San Bernardo Nappe consists of a Paleozoic base-ment (micaschist, gneiss and small bodies of greenschist)and discontinuous Mesozoic cover (white/gray carbon-atic-dolomitic marble). The Piedmont Zone is composedby carbonatic calcschist alternating with marble referredto the Upper tectono-metamorphic unit (Upper TMU) asdefined by FORNO et alii (2012).

The whole area is now completely free of glaciers, butit was largely shaped by the Pleistocene Verrogne Glacier(fig. 1). The valley floor, where Plan di Modzon occurs, isformed by some rounded rocky ridges separated bytroughs, parallel to the trend of the Verrogne Valley,linked to subglacial erosion (FORNO et alii, 2013).Trenches and minor scarps reshaped by the glaciers areshown in fig. 4. Subglacial deposits are discontinuouslypreserved on these ridges (fig. 3), comprising sporadic, atmost decimetric and partially rounded, Gran SanBernardo gneiss and micaschist erratic boulders. Shortand low lateral moraines, underlined by sub angularboulders, indicate the presence of glacial margin depositsat the eastern side of the valley floor. The distribution ofthe glacial sediments and landforms suggests an age fromthe LGM to Lateglacial. The same glacial activity isresponsible for the formation of relatively wide over-deepened depressions, likely hosting some lakes nowfilled by lacustrine or palustrine sediments.

THE POINTE LEYSSER DEEP-SEATED GRAVITATIONAL SLOPEDEFORMATION

The investigated area is comprised into the PointeLeysser Deep-Seated Gravitational Slope Deformation(DSGSD) (fig. 1). This gravitational phenomenon extendsmore than 20 km2 and is well marked by a set of preva-lently WNW-ESE scarps. The DSGSD shows typical evi-dence as a) the noticeable doubled ridges, evident in thespan of watershed between the Pointe Leysser and the

Fig. 2 - Morphological features of the Plan di Modzon area (observedby ESE).

Fig. 3 - Geological map of the Plan di Modzon area. The location of the geophysical cross sections is also reported. The numbers from 1 to 9indicate the morphological elements showed in fig. 6.

Becca France (FORNO et alii, 2012); b) the convex plantshape, in the lower sector prominent towards the mainvalley (a in fig. 1); c) several varying size gravitationallandforms including elongated depressions parallel andtransverse to the contour lines (trenches), very steepsectors (minor scarps) and counter-slope scarps; d) thehighly loosened bedrock, with open fracture systemswhich have WNW-ESE, NW-SE, SW-NE and N-S trends.

Well preserved gravitational landforms can be easilyconnected to the Pointe Leysser DSGSD (FORNO et alii,2013). On the other hand, some gravitational evidence,such as minor scarps and trenches reshaped by the sub-glacial abrasion, are more difficult to be recognized(fig. 4).

In the DSGSD most elongated depressions sub-verti-cal walls shaped in the bedrock, several tens of meterslong and a few meters deep, are clearly visible. These

depressions are also often hosting watercourses. Eventhough these landforms may be confused with torrentialincisions, their narrow and straight shape suggests thatthey are gravitational trenches, which have been onlysubsequently used by the watercourses. These trenchesdevelop both transversally (NE-SW) and longitudinally(NW-SE and WNW-ESE), i.e. respectively orthogonal andparallel to the DSGSD movement. Along some fractures,downward movements of the loosened rocky massinvolved in DSGSD also occur. Often, antithetic exten-sional structures with opposite dipping with respect tothe main fractures were also observed (fig. 5).

THE ARCHAEOLOGICAL SITE MF 3

In the Plan di Modzon and surroundings numerousarchaeological sites have been found; these are spreadover a relatively wide area (approximately 0.2 km2, fig. 4)and referred to a broad range of time (MEZZENA & PER-RINI, 1999; GUERRESCHI et alii, 2010; FORNO et alii, 2013).The artifacts collected in these sites document the hunt-ing activity between the Mesolithic to the Copper Age, asalso confirmed by radiocarbon dating on coal fragments.

Only one site, named Mont Fallère 3 (MF 3), is insidethe investigated area. This site is located into a weakcounter-slope depression (less than one meter deep, 3 infig. 3 and fig. 6), stretched in the N-S direction andperched about 10 m on the wide filled lake plain (8 infig. 3 and fig. 6). In this site glacial boulders lie on thebedrock and are covered by yellowish-brown silty-sandycolluvial sediments, coming from the reworking of theglacial sediments. The colluvial body, partially filling thedescribed depression, contains several artifacts of rockcrystals (hyaline quartz) referred to the Middle Neolithic.

MORPHOLOGICAL FEATURES

The surveys in the Plan di Modzon area (fig. 3),showed an articulated morphology consisting of elon-gated rocky ridges, scarps and elongated depressions.

Fig. 4 - The main evidence of the gravitationalphenomenon reported in the Plan di Modzonarea: a) trenches; b) minor scarps; c) trenchesand minor scarps reshaped by the glaciers. Thefilled paleo-lake (d), the Verdjouan scarp (e)and the nine archaeological sites (white squaresymbols) are also indicated.

Fig. 5 - NW-SE trending extensional structures dipping in the antitheticway with respect to the main NW-SE longitudinal fractures of theDSGSD rocky mass.

These landforms, showing WNW-ESE, NW-SE and NE-SWtrends, develop according to the fracture systems interestingthe bedrock. A wide closed depression in the valley floorhas been also observed.

The main longed ridge (1 in fig. 3 and fig. 6) in frac-tured micaschist (with a rounded shape due to the sub-glacial erosion) is covered by some erratic boulders(subglacial sediments). Its eastern slope has a very typi-cal morphology consisting of three scarps (respectively2, 5 and 7 in fig. 3 and fig. 6) joined together downwardby a weak counter-slope depression, a very low ridgeand a small flat sector (respectively 3, 4 and 6 in fig. 3and fig. 6). A possible reading of the slope provides three gravitational minor scarps dipping towards east(fig. 3) connected to the DSGSD. However, also morecomplex models are conceivable since the interpretationis highly dependent on the possible occurrence of buriedstructures.

The wide close depression in the valley floor (8 and 9in fig. 3 and fig. 6) shows essentially a planar morphologysuggesting the occurrence of a lake, subsequently filled bylacustrine and palustrine sediments. The lacustrine bodyis not outcropping, but it likely consists of laminated siltysands and subordinate gravelly sands with planar bed-ding. The palustrine cover (8 in fig. 3 and fig. 6) is insteadcomposed of black silty sediments rich in organic matterand plant remains (peat). Elsewhere, in the northern(proximal) sector of the valley floor, the morphology isslightly dipping towards SE and SW. This evidence corre-sponds to two alluvial fans (9 in fig. 3 and fig. 6), relatedto the intermediate and eastern Verrogne torrents formedby torrential gravel sediments consisting of subangularclasts mixed to a sand-silty matrix.

GEOPHYSICAL SURVEYS

The preliminary researches resulting from the geo -logical surveys, exposed in the previous paragraphs, haveallowed to collect some morphological and geologicaldata regarding the surface. This surficial picture is notcomplete enough to delineate the buried structuresrequiring, therefore, the use of geophysical methods.

Electric tomographies were acquired in two differentspreads deployed approximately along W-E and N-Sdirections (see ERT1 and ERT2 in fig. 3 and fig. 6). Theacquisitions were performed by mean of a Syscal-Protomograph (IRIS Instruments) with 72 measuring elec-trodes at 2 m spacing. In order to extend both profiles fora length useful to the aims of the surveys, a roll along pro-

cedure has been used combining two different acquisi-tions with 48 electrodes superposition; in this way, thelength of each profiles was 190 m. A Wenner-Schlum-berger measuring sequence was used in order to obtain agood compromise between both vertical and lateral reso-lution. Acquired data were processed by the commercialinversion code Res2Dinv® (LOKE & BARKER, 1996). Agood convergence of the results has been obtained in boththe inversions with RMS error below 3.5%. On the recon-structed resistivity sections an indication of model sensi-tivity is also reported. The hatched boundary evidencesthe zones of the sections with sensitivity lower than 0.5,therefore not particularly reliable in the reconstruction(figs. 7 and 8).

Profile ERT1 (fig. 7A) has been mainly used to inves-tigate the geological setting along the slope, on which thearchaeological site MF 3 occurs, and the lacustrine suc-cession of the ancient lake. According to the geologicalreconstruction, this profile develops along two main sec-tors: the WSW one (1-7 in fig. 7B) located on the bedrockmodelled by the subglacial erosion and the ENE one (8 infig. 7B) located on the sediments that filled the ancientlake. Profile ERT2 (fig. 8A) is essentially located on thelacustrine sediments, cutting the whole basin along itsmain axis (8 and 9 in fig. 8B). This last profile has beentherefore devoted to delineate the geometry and thicknessof the sediments that filled the ancient lake.

The results of the two tomographies are respectivelyreported in fig. 7A and fig. 8A. Along ERT1 (fig. 7A) highresistivity values (above 5000 Ohm.m) can be observed,particularly in the WSW sector, and a decreasing trendis noticeable with depth (values between 2000 and 5000Ohm.m). In the same tomography lateral continuity isinterrupted by different medium-low resistivity anomalies;the main one with values between 50 and 150 Ohm.mcan be clearly noted between progressive 60 and 80 m.The ENE sector of the same tomography is instead char-acterized by increasing resistivities (from 300 to 5000Ohm.m) with depth. A further resistivity reduction isalso noted but in a low sensitivity zone. A marked jumpin resistivity is observed in the shallower part around 5 m depth. Same trend and same resistivity values can beobserved in the overlapping segment of ERT2 (fig. 8A).In the NNW sector of this last tomography a shallowportion with higher resistivities (between 1000 and 2000Ohm.m) can be also noted together with a wider zonewith depth (to about 15 m) interested by medium-lowresistivity values (between 300 and 1000 Ohm.m). Againlateral continuity of the resistivity values is locally inter-rupted by sub-vertical low resistivity anomalies.

Fig. 6 - Panoramic view of the studied area,focused near the archeological site MF3 (observed by NW). Along the western slope ofthe Plan di Modzon ancient lake three scarpsoccur (2, 5, 7) joined together by a weak coun-terslope depression (3), a very low ridge (4)and a small flat sector (6). The planar and flatmorphology (8) of the valley floor testifies theoccurrence of the lake, now partially filled byalluvial fans (9). The location of geophysicalcross sections (ERT 1 and ERT 2) is also reported.

The results of the two independent surveys are highlycoherent as can be observed in the composed 3D viewreported in fig. 9. At their respective intersection similardepths and resistivities can be observed in the imagedbodies confirming the high data quality.

DISCUSSIONS

GEOLOGICAL INTERPRETATION OF ERT RESULTS

A full reconstruction of geological setting was then per-formed integrating the geological evidence with geophysi-

cal results. Along ERT1 (fig. 7A) the presence of the emerg-ing bedrock at both ends of profile is confirmed (electricalresistivity higher than 5000 Ohm.m). In the WSW sectorthe same bedrock shows a reduction in resistivity (valuesbetween 2000 and 5000 Ohm.m) with increasing depth,evidencing the probable transition from dry rocks, on thetop, to saturated rocks, in the bottom. In the same sectorthe bedrock also shows two low resistivity zones (the mainone showing resistivity between 50 and 150 Ohm.m) westand east dipping (40°-50°), likely connected to very frac-tured and loosened rocks. Some structures having similarfeatures and the same dip, observed near the Plan de Mod-

Fig. 7 - The transversal geophysical cross sectionERT 1 (A) testifies the local occurrence ofloosened rocks (B) suggesting that the slope(1-7) is involved in open fractures and minorscarps; a subglacial, lacustrine and palustrinesuccession can also be assumed in the filledancient lake (8). The hatched boundary evidencesthe zones of the sections with low sensitivity.Numbers indicate the morphological elementsshowed in fig. 6 and fig. 3.

Fig. 8 - The longitudinal geophysical crosssection ERT 2 (A) suggests the occurrence ofburied subglacial and lacustrine sediments (B)below the surficial palustrine (8) and torrential(9) cover; the local occurrence of loosenedrocks testifies also some buried trenches at thebottom of the filled lake. The hatched boundaryevidences the zones of the sections with lowsensitivity. Numbers indicate the morphologicalelements showed in fig. 6 and fig. 3.

Fig. 9 - Composed 3D view of the two ERTcross sections seen from the same point ofview of fig. 6. Numbers refer to the samemorphological features reported in fig. 6 andfig. 3.

zon area, are related to opposite gravitational minor scarps(fig. 5). The reported low resistivity zones may be, there-fore, related to a bedrock dissected by zones of loosenedrocks connected to the DSGSD.

Using the geophysical results, the geological recon-struction of the described scarps along the slopebecomes more complex than previously proposed (fig. 3)and implies the occurrence of some minor sliding surfaces dipping toward east and some opposite slidingsurfaces dipping toward west (fig. 7B). These slidingsurfaces, with opposite dipping, may be related to themajor and antithetic fractures connected to the DSGSD.As mentioned, similar features have been also observedin some outcrops (fig. 5). In detail, the site MF 3 istherefore located in the weak counter-slope depressiondeveloped at the intersection of these sliding surfaces (3 infig. 7B).

Three different bodies hosted in a glacial over-deep-ened depression of the bedrock can be instead recognizedin the ENE sector of the same tomography. These bodiesmay consist of subglacial deposits (values between 500 and1000 Ohm.m), lacustrine sands (values between 300 and500 Ohm.m) and locally palustrine organic deposits (peatand gyttja with values between 150 and 300 Ohm.m). Anaverage overall thickness of about 5 m is observed forthese shallower deposits.

To define the resistivity ranges of these bodies thewater resistivity of the area has been also measured in astream that flows over the supposed lake plain. A value ofabout 70 ohm.m, coherent with a mountain water withlow percentage of dissolved salts, has been observed.Given this resistivity value for the saturating water, theaverage resistivity of the ancient lake sediments (310Ohm.m) could be therefore associated to fine sedimentswith high porosity (sands and silts, locally peats). The evi-dence of peat is indeed observed at the surface and simi-lar investigations in peat basin environments, supportedby direct soundings, report comparable values (COMAS etalii, 2004). Even if peat deposits generally show lowerresistivity values, the relatively high resistivity of the peatis attributed by COMAS et alii, 2004 to the low ionic con-centration of the peat pore water, as is the case for thepresent case history.

The found thickness and the presence of superim-posed sedimentary bodies shown along the ENE sectorof ERT1 appear consistent with the ones observed in pro-file ERT2 (fig. 8A). Three shallow low resistivity layers ofsediments (likely subglacial, lacustrine and palustrine)overlay an higher resistivity bedrock (fig. 8B). The thick-ness of these sediments increases from 5 m to about 15 mfrom the edges of the filled lake to its inner sector (fromprogressive 30 to 130 m). It is worthy to note that thedepocentre of the filled lake is not in correspondence ofthe geometrical centre of the basin but is located in itsNNW sector. Here, these three sedimentary bodiesappear also covered by higher resistivity material (valuesbetween 1000 and 2000 Ohm.m), in accordance with anoverlaying coarse alluvial fan (9 in fig. 8B). This result isin agreement with the surficial observation related to theless flat morphology of the northern sector of the filleddepression, where gravel torrential sediments occur. The buried bedrock shows some low resistivity zonesroughly dipping toward SSE (40-60°). These low resisti -vity zones too may be related to a bedrock dissected by zones of loosened rocks connected to the DSGSD,

according to the geological surficial data collected in thewhole studied area.

RECONSTRUCTION OF GEOLOGICAL EVOLUTION

The gravitational evidence recognizable in the investi-gated area consists of some scarps inclined toward ENE(2, 5 and 7 in fig. 6), a weak counterslope depression anda small planar sector (3 and 6 in fig. 6). In previousresearch (FORNO et alii, 2013) these elements have beeninterpreted as related to the occurrence of some minorscarps dipping towards ENE. On the contrary, the geo-electric surveys performed within this study showed somezones of loosened rocks dipping toward ENE and WSW,suggesting the presence of sliding surfaces dipping in theopposite side.

With the aim to evaluate the morphology of the inves-tigated sector during the archaeological settlement (testi-fied by the site MF 3), the reported data has allowed todraw a series of time-sketches related to the differentperiods (fig. 10), although different interpretations remainpossible.

In the Last Glacial Maximum, in agreement with theclimatic evolution of other sectors in the Alps (IVY-OCHSet alii, 2008), it can be assumed that thick glaciers werepresent both in the main valley (Dora Baltea Glacier) andin the Verrogne Valley (Verrogne Glacier), suggesting thatthe DSGSD was not evolving substantially. The describedslope and the wide valley floor were, then, progressivelyinvolved in the subglacial erosion, that also deleted theeventual previous gravitational elements (step 1 in fig. 10).

In the subsequent stadial A of the Lateglacial, for cli-matic reasons, the Dora Baltea Glacier and the VerrogneGlacier gradually became thinner. The deglaciation pro-moted, therefore, the release of the rock tensions. It isthen possible to hypothesize (step 2 in fig. 10) that thefirst preserved gravitational evidence, consisting of openfractures (dipping towards ENE and WSW) and the firstsliding surface (dipping towards WSW) began to formalong the slope. The occurrence of a first sliding surfaceis in agreement with the developing, likely for a longtime, of a wide zone of loosened rocks testified by thelowest resistivity values observed in ERT1 in the samelocation (progressives from 60 to 80 m in fig. 7A). Thepossible morphological evidence of these structures (assome incipient scarps or depressions) would have beenprogressively deleted by the glacial erosion.

In the stadial B of the Lateglacial the Verrogne glacierbecomes further thinner (step 3 in fig. 10). This evolutionfavored a further release of the rock tensions, allowingthe previous open fractures to evolve into other slidingsurfaces. This interpretation can be proved by the scarpspreserved along the slope, by the zones of loosened rocksreported by the geophysical cross section (fig. 7A) and bydirect evidence of similar deformation zones in outcrop(fig. 5). The scarps were lately progressively eroded by theglacial abrasion, which produced their present roundedform. At the junction of these sliding surfaces dipping inopposite direction, a weak counter-slope depression (3), avery low ridge (4) and a small flat sector (6) have set.Later, the depression and the small flat sector were filledby glacial sediments, becoming buried landforms.

Finally, at the end of the Lateglacial and during theHolocene, after the glaciers permanently left the area, a further evolution of the DSGSD occurred, variously

developed in different areas. In detail, along the profile,there are no evidence of significant gravitational evolution(step 4 in fig. 10). The ridge (1) and the scarps along theslope (2, 5 and 7) were further remodeled by the surfacerunoff and the depression (3) and the small flat sector (6)were covered by thin colluvial sediments. In the valleyfloor, instead, in the closed depression shaped by the glac-ier and covered by discontinuous subglacial sediments, anancient lake occurred, progressively filled up by lacustrinedeposits. The archaeological site MF 3, with artifactsreferred to the Middle Neolithic, is related to this step.

The current features are linked to gradual emptyingof the lake and its further filling by palustrine deposits(observed in surface and in the geophysical cross sec-tions). This hypothesized emptying is likely linked to theopening of some N-S trending fractures, evident in thewhole area, also connected to the DSGSD. During andafter the filling of the lacustrine basin, the tributary torrents (intermediate and eastern Verrogne torrents)built the alluvial fans, consisting of the gravel sedimentsobserved in the marginal sectors of the basin and showedalso by the geophysical cross sections.

CONCLUSIONS

The Electric Resistivity Tomography surveys per-formed in the present study have contributed to a betterunderstanding of the geological evolution of the Plan diModzon area. They show, along the counter-slope depres-sion visible on the surface, a buried zone with lowresistivi ty, associable to the presence of a main slidingsurface inclined in the opposite side compared to thewestern slope of the lake basin. This slope is character-ized by zones of very loosened rocks along minor scarpsdirected both upstream and downstream. Although thegravitational interpretation is confirmed, the geologicalcontext is more complex of that hypothesized in previousworks on the basis of surface surveys only. Indeed thelacks of morphological evidence of this structure can bereferable to the glacial abrasion and is indicative of asinglacial (e.g. Lateglacial) evolution of the gravitationaldeformation. Furthermore, the geometry of the lakedepression and the nature and thickness of the lacustrinesediments were clarified evidencing also the thickness ofa coarse fan cover over them.

All of the reported results are valuable for a firstdepiction of the geological setting of the studied sectorand for the reconstruction of the environments in whichman has inhabited these territories further evidencing thepossible presence of a lake very close to the prehistoricalsite MF 3. Further tests, including direct sampling andcore dating, are planned in order to confirm the results.These tests will make use of the relevant finding of thispaper for selecting the best sampling sites.

ACKNOWLEDGEMENTS

Authors are indebted with Soprintendenza per i Beni e leAttività culturali of Regione Autonoma Valle d’Aosta for partiallyfunding the geological and geophysical surveys and with DiegoFranco of the Poli tecnico di Torino for his contribution to geo-physical data acquisition.

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