Deep-seated gravitational slope deformations as possible suitable locations for prehistoric human settlements: an example from the Italian Western Alps

The Plan di Modzon is a mountain area (2300 m) located in the Verrogne Valley, NW of the Aosta town (Western Alps). It occurs along the contact between the Middle Penninic (micaschist and gneiss from the Gran San Bernardo Nappe) and the overlying upper units of the Piedmont Zone (carbonate calcschist alternating with marble). This area, largely shaped by Pleistocene glaciers, was involved in a wide deep-seated gravitational slope deformation (P. Leysser DSGSD) on the western extension of the Becca France doubled ridges. In detail, several ridges that were affected by glacial erosion, discontinuously covered by glacial sediments, are diffused throughout the area. The extremely fractured rocks and various gravitational forms (minor scarps and trenches) instead mark the DSGSD. Several archaeological sites (MF1÷MF9) have recently been found between 2242 and 2292 m in altitude. They have returned artifacts of rock crystal (hyaline quartz) referred to an ancient stage of the Mesolithic period (Sauveterrian). An ensemble of other archaeological evidence is referred to the Copper Age. The investigation in progress specifically concerns the systematic excavation of the sites MF1 and MF3. Some concomitant morphological factors have created very favorable conditions for prehistoric settlements in the Plan di Modzon area. For example, the exceptionally wide valley floor directly perched on the main Dora Baltea Valley, in consequence of the Verrogne Glacier diversion promoted by the P. Leysser DSGSD. The easy and direct accessibility to this area, because of the gently-dipping slope of the Dora Baltea Valley affected by the DSGSD. The ridged and grooved morphology as the result of the glacial and gravitational interaction, offering wide surfaces free from geological hazards (debris flow and avalanche processes). The DSGSD is, therefore, one of the primary causes of the archaeological settlement of this area, having contributing to create a morphology adapted to prehistoric settlements.