Carta Geologica della Riserva Naturale del Furlo - Geological Map of Furlo Natural Reserve

The history of the Furlo gorge, which can be read in the rocks that emerge here, began around 200 million years ago, when — in a sea similar to that of the present Bahamas — limestone sediments were deposited both in shallow and deeper sea areas. No sediments from land emerged in this area and therefore only materials derived from the shells of calcareous single-cell microfossils (nannoplankton and foraminifers) could be deposited, to a lesser extent silicatic (radiolarians) and their rearrangement. Massive limestone and other formations, such as Bugarone, are deposited in this environment, linked to the redeposition of materials that from the highest parts of the seabed collapsed towards the depressed parts. The formation of Rosso Ammonitico, rich in the fossils from which it takes its name, is an example, since it occurs discontinuously throughout this area. Later, the sea will deepen due to complex phenomena of tectonics in the Mediterranean area in which not only the two African and European macro-clods face each other, but other microplates, separated by sea and ocean basins, come into play. These will greatly complicate the subsequent tectonic evolution and the work of current geologists. The deepening of the seabed allows the sedimentation which becomes increasingly stratified with little disturbance from the carbonate sediments of the Cretaceous and Paleogene formations (eg Maiolica and Scaglie). Occasionally however, some events that have also had an impact on a global scale, have differentiated the sedimentation of the rather monotonous carbonate stratification. Among these, the Bonarelli Level (after Guido Bonarelli, a famous geologist who lived between the 19th and 20th century who studied these rocks and much more) consisting of bituminous rocks and volcano-derived materials, is an indicator of an anoxic event, i.e. scarcity of oxygen in the seawater column. Or the anomalous abundance of iridium and other elements in a thin layer of clay between the limestone layers of the Red Scale, which is found in outcrops in other areas of the planet, marks an upheaval in the living conditions on earth similar to those noted during the last great mass extinction between the Cretaceous and the Paleogene. After this geological period, the sedimentation conditions begin to change with increasing contributions of clayey materials coming from dry land. In fact there was a transition from a relaxing tectonic regime, which opened deeper and deeper basins, to a compressive shortening regime, with the nearing of Africa to Europe. The formations that will come (from the upper part of the Scaglie Group to the Schlier) are increasingly marly and clayey. Among these units stands out the formation of the Bisciaro very rich in cineritic material deriving from volcanoes that mark the passage from the relaxing to the compressive tectonics. Thus, during the collision / compression phase (in the upper Miocene) all the more ductile levels, such as the clayey ones, constitute areas of detachment of the sedimentary succession from the underlying rock units, allowing the formation of folds and faults at different scales. The most important detachment surface is the basal one, which is located below the large concretion of the Limestone Massif. It consists of anhydrites (from the Burano Anhydrites Formation), or anhydrous gypsum of evaporitic derivation. This plastic formation allows the shortening with the development of folds with a wide radius of curvature linked to the mechanical response and the thickness of the overlying formation, the Calcare Massiccio. In particular, at Furlo the stratified rocks with their orientation identify a defined anti-formal fold (convex upwards) and anticline (at the core of which more ancient rocks emerge) constituting the relief of the Paganuccio and Pietralata mountains. This large fold, with an elongated axis from North-West to South-East, is cut transversely in the central part by the gorge of the Candigliano river, and we can see this in reality, and — even more clearly — on the plan of the geological map. This situation is in itself rather strange if you think of the behavior of watercourses which in general tend to go around raised formations. This phenomenon was explained by the presence of the river prior to the folding — a folding which had no effect on the stream’s course. This way the river continued to erode the rocks during the formation causing a lifting of the fold.