The Stones of Perugia: the art of photography and the geological assessment of a city.

Perugia is the capital city of Umbria region (central Italy) and is located on a gentle hill (493 m a.s.l., area 27 km2) with a triangular shape where four narrow and elongated ridges spread out from the historical center (in the highest and northern apex) decreasing in altitude and reaching the Tiber river valley. The drainage network shows a radial pattern flowing from the apex downward to the valley. The morphology and the morphometry of the hill reflect the structural factors and the shaping geomorphological processes. Fluvial lacustrine deposits (Pleistocene – Olocene) deposits are present on the hill, included in a sandstone-marly formation (Marnoso Arenacea, Miocene). The deposits are the result of the infilling of the Tiber Basin, the largest of the intermountain basins in the Umbria region, due to the extensional tectonic phase started in the Pliocene. Pebbles, sands and clays are all present in the Perugia hill with a general coarsening upward sequence from the bottom to the top. Moreover the extensional tectonic phase, still active, is responsible for the tilting and faulting of the relief. The main aim of this study is the updated and complete knowledge of the geological and geomorphological assessment of the Perugia hill. Moreover a geotouristic network is provided linking the subsurface geology to the building stones of the most important historical buildings in the old town. The lithological and mineralogical nature of the stone, the origin of the mining areas and the meaning of the use of a particular kind of lithotypes are provided. Furthermore, in the main stops of the touristic route, the connection between the geological assessment of the hill and the setting of the main historical buildings is specified in order to provide a complete vision of the urban geology of the entire area.