The Vasquez Rocks consist mainly of coarse-grained conglomerate and breccia sediments, which were deposited adjacent to active faults during rapid uplift and consequent erosion of the San Gabriel Mountains. Approximately 25 million years ago (late Oligocene time), the collision of the North American and Pacific tectonic plates uplifted the area along the Elkhorn Fault. Energetic erosion of the highland along with uplift and volcanism caused debris flow sediments to be distributed in alluvial fans into a rapidly subsiding rift known as the Soledad Basin. These sediments were buried and lithified through the Miocene and became exposed more recently via activity along the San Andreas fault system. The strata are now highly tilted and disconnected from their source area. The strata were deformed and offset by the later fault activity. The distinctive hogback ridges of steeply inclined strata serve to graphically demonstrate the significant fault activity in the area. It is possible to see the extreme variance in grain size between layers of sediment where the strata are exposed. These sedimentary rocks, named the Mint Canyon Formation, were laid down about 8-15 million years ago. In the upper layers, many animal fossils have been found, including camels, horses and rodents.