Fluvial erosion of inscriptions and petroglyphs at Siega Verde, Spain

Low-grade metamorphics such as slates and schists tend to be subjected to relatively rapid surface deterioration by both weathering and fluvial wear. If the rate at which such processes are effective on these relatively soft rocks could be calibrated through their effects on surfaces of known ages, this would provide a possibility of estimating the age of petroglyphs on similar facies and in similar environments. An attempt of quantifying these processes is presented, using the petroglyph site of Siega Verde in western Spain as an example. A series of dated inscriptions occurring at a site of zoomorphic petroglyphs provide the data to plot rock surface deterioration against time. The involvement of numerous variables prevents this method from providing much precision, but it is adequate to offer indications of order of magnitude of age. The zoomorphs at Siega Verde have been attributed to the Upper Palaeolithic, but this method suggests that most of the site's anthropic markings are under 200 years old. This finding is supported by several other factors, such as the degree of repatination of the supposedly Pleistocene petroglyphs, and the presence of a recent terrace predating these.