During the late Miocene, in the central Italian Apennines, sedimentation occurred in a few distinct basins resulting from synsedimentary tectonics. Alluvial fan sediments of the Messinian Colombacci Formation accumulated in the most western of these basins, the Pietrarubbia basin. The Colombacci Formation consists of coarse-grained, predominantly clast-supported conglomerates, pebbly sandstones, sandstones, and marls, which were deposited by high-viscosity debris flows and braided streams. The alluvial-fan deposits owe their development to an area of high relief that was caused by synsedimentary folding. The mode of deposition and the occurrence of caliche nodules point to a semiarid climate.