The region southeast of Mono Lake is more or less representative of much of the western Great Basin. Pliocene volcanic rocks lie on an old erosion surface cut in pre-Tertiary granitic and metamorphic rocks. The oldest are rhyolite tuff and flows overlain in turn by andesite flows, tuffs, and breccias and by olivine basalt flows. Small erosional breaks separate these three series. In the central part of the area, a mountainous mass of rhyolite overlies the basalt, and eastward streams have spread a mantle of rhyolite tuff and gravel.

All formations are displaced by normal faults, along the largest of which the present ranges have been relatively elevated. Although this faulting has continued until the present, it was nearly complete by middle (?) Pleistocene time. The result is a mosaic of fault-bounded blocks, and statistical correlation of the strikes of these faults and older joints supports the view, suggested by others, that the individual faults are controlled by earlier planes of weakness, notably by joints in the granitic rocks. The factors controlling the location and trend of the major displacements which produced the ranges are unknown.

Volcanic activity, following the initiation of the Basin-Range faulting, continued through the Pleistocene until recent times.

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