The 2,390 meters-thick San Hipolito Formation crops out in a 20 square kilometer area on Punta San Hipolito on the Vizcaino Peninsula. The formation is thrust over the Cretaceous Valle Formation and forms a southwest-dipping homocline. The San Hipolito Formation rests on pillow lavas of Middle to Late Triassic age. The formation was deposited in a basin on the seaward side of an island arc. The island arc served as a barrier that prevented the influx of nonvolcanic terrigenous sediments into the basin. Sediments which accumulated in the basin were derived from the island arc in the form of pyroclastic and epiclastic debris, from a carbonate complex that built up in the shallow marine water on the flanks of the island arc, and from tests of pelagic organisms. Ash from the volcanic island settled onto pillow lava and was converted into green, celadonitic chert. Volcanism increased the silica content of the seawater which caused radiolarian blooms which contributed to the formation red, radiolarian chert. Triassic radiolarians from the formation form one of the most diverse assemblages in the world. A sequence of predominantly limestone with lesser amounts of volcanic material accumulated above the chert as carbonate mud and volcanic debris were washed into the basin. Thin-shelled pelecypods of Late Triassic age, including Monotis cf. M. subcircularis and Halobia? sp. contributed to the limestone buildup. Submarine landslide masses containing volcanic debris and blocks of shallow water limestone slid into the basin from the island arc and carved a submarine channel in the limestone sequence below. Landslide accumulations were covered by a thick sequence of sandstones and tuffs. Burial of the formation brought about diagenetic changes in the sandstones that include the formation of authigenic phyllosilicate, zeolite, and albite and the etching and replacement of terrigenous grains by calcite. The formation was thrust over rocks of Cretaceous age during the late Tertiary and uplift continues in the Quaternary.