Detection of occupational surface remnants at a heavily eroded site; case study of archaeological soils from La Terrasse, Bibracte oppidum

Abstract

The area of La Terrasse is located at one of the higher parts of the Celtic oppidum Bibracte. No traces of building activities, except for the fortification system which surrounds the plateau from three sides, were archaeologically detected and the area can be therefore labeled as “empty space” with an enigmatic history. Multiproxy investigations of sediments in trenches cutting across various parts of the enclosed area and excavated during the 2019 season revealed a complicated history of the formation, being influenced by erosion and by anthropogenic stabilization.Although the recent relief of the La Terrasse area appears quite stable, there is evidence that the site (and Bibracte oppidum in general) were subject to intense erosion in the past and that the former surface with the archaeological soil dated to the Late Iron Age is preserved only as a relict expressed geochemically by the increase of CEC. The reason for the recent surface stability is the presence of the Iron Age ramparts, which enclose the area and protect it against erosion. An OSL sample collected from the surface of the buried archaeological soil dates the overburden not later than to the early Medieval period (AD 561). The archaeological soil represented by the overburden did not reveal any significant geochemical signal indicative of intensive use despite its location in the most suitable and stable area of the site.It is clear that the detection of former surfaces in eroded and exposed archaeological sites and the properties of the archaeological soils is always a complex matter and can only be addressed through a combination of field observations, geochemical and micromorphological proxies.