The very high alluvial formation of early Pleistocene age in the Creuse River Valley at Eguzon (Indre) : cryoturbation patterns, prehistoric occupation sites and absolute dating

Around twenty prehistoric sites of the very early Palaeolithic were discovered on the northern edge of the Massif Central, in the Creuse Valley between Crozant and the Paris Basin, and on the Aigurande crystalline plateau that borders it. Concentrated more particularly around Eguzon, the sites are all closely linked to fluviatile formations, whose study, despite difficult exploration conditions, revealed many remnants of alluvial deposits. Those of the very high formation, some up to 15 m thick, were the best preserved. The most significant site, at “Pont-de-Lavaud”, is in a thin bed of gravel made up mainly of quartz pebbles and debris with small, interbedded sand lenses. Tilted 6 to 7° in a small depression in the weathered foliated crystalline basement, the bed is at mid-slope of an interfluve between the Creuse River and one of its small tributaries. Excavation done between 1983 and 1995 uncovered many prehistoric artefacts carved out of quartz in the upper part of the sediment and, below this, various cryoturbation patterns (sorted circles, polygons, inclusions in the alluvium of alterite from the bedrock) and scattered, less abundant artefacts of the same type. Due to the outstanding state of conservation of these findings, they could be studied in a particularly detailed manner and, when coupled with the study of the alluvial formations, enabled us to : – place them at the base of the very high terrace, located 90–110 m above the river’s present-day low water level – attribute the unusual position of the outcrop to the tilting of a block of bedrock by new displacement of an old fault, thus confirming the role of recent tectonics in the regional morphology ; Absolute age dating by Electron Spin Resonance (ESR) of the best conserved outcrops of the very high alluvial formation gave an age of 1 to 1.2 million years, thus placing them at the end of the early Pleistocene. However, dating of alluvial remains near the site and of the site itself, gave ages between 0,9 and 1 million years. Differences are attributed to the role that greater erosion and pedogenic weathering might have played. Therefore, the “Pont-de-Lavaud” occupation sites are among the oldest presently known in Europe.