Mineralogical origin of the chromatic contrast enhancing the Mount Bego petroglyphs (Alpes-Maritimes, France)

The conservation of archaeological heritage such as open-air petroglyphs is a major challenge due to the vulnerability of surfaces exposed to local environmental conditions. A precise knowledge of their nature therefore constitutes the cornerstone of their preservation. The surfaces of the valleys of the Mount Bego are characterized by a chromatic contrast between the thin red layer covering the outcrops and the underlying greenish substrate, revealed by thousands of prehistoric petroglyphs. To establish the mineralogical nature of the red layer, a spectroscopic, crystallographic, and petrographic study was carried out. Collectively, the results of this study evidence that, unlike rock coatings, this layer formed through the mobilization of structural iron, in the form of Fe (hydr)oxides, from the clay minerals that constitute the rocks. We thus discuss how the succession of events along the geological history of the site has created the environmental conditions for its formation. This study eventually provides useful data on the actual state of the petroglyphs toward their preservation. In a more global context, the conclusions of this study bring insights into the physicochemical mechanism leading to the reddening of recently exposed glacial valleys.