This paper describes the geology of a part of the region between the southern Sierra Nevada and Death Valley, one of the most rugged portions of the Great Basin. The topographic features of the mountain ranges in this region strongly suggest that each range owes its present height to faulting. In the western half of the area mapped, displacements of extensive basalt flows of probable early Pleistocene age support the topographic evidences of faulting. On the summits of the ranges are areas of low relief, believed to be remnants of a single old-age erosion surface which extended across the entire region before the beginning of the range-forming fault movements. The undisturbed erosion which produced this surface ended shortly before the deposition of the fossiliferous late Pliocene or early Pleistocene Coso formation; therefore, this surface is tentatively correlated with the Ricardo erosion surface of the Mohave Desert region, which bevels tilted early Pliocene strata and which is also dislocated by range-forming faults.

The ranges are composed dominantly of pre-Tertiary rocks. The pre-Cambrian metasediments, chiefly mica schists and dolomites, have an exposed thickness of 15,000 feet. Limestones, dolomites, shales, and quartzites of Paleozoic age are more than 30,000 feet thick, and fossils collected in them indicate the probable presence of all the Paleozoic systems. During the late Jurassic Nevadian orogeny the pre-Mesozoic rocks were folded, faulted, and intruded by plutonic bodies ranging from granite to gabbro. All the post-Mesozoic rocks are believed to be Miocene or younger; they include a wide variety of volcanic and sedimentary types.

Most of the faulting to which the region owes its present relief occurred in the early or middle part of the Pleistocene, probably after the first (McGee) glacial stage in the Sierra Nevada. The activity on one of the major fault zones, however, has continued into the Recent epoch. All the range-forming faults whose attitudes could be determined are high-angle normal faults.

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