The basement of the region is formed by various metamorphic and sedimentary rocks intruded by plutons ranging in their age from the Paleozoic to the Tertiary; one of these is the 65 million years old Cerro Colorado pluton. However, in the area of San Pedro this basement is entirely buried beneath volcanites of Miocene age, including ignimbrites, remnants of composite volcanoes and volcanic debris. At least three individual ignimbrites have been found; one of the ignimbrites, the San Pedro Ignimbrite, may have originated from a vent now covered by San Pedro volcano although the neighbouring Caracanal volcano is also a candidate source. The older composite volcano centres include Cerro Carcanal and Cerro Huiche south-southeast of San Pedro and Cerro del Diablo due north. This surface slopes down to the Loa River in the west.