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The area in southern California that is bounded by the San Andreas fault on the southwest and by the Garlock fault on the north and northeast, known as the Mojave Block, shows many unique geologic features, especially in the kinds and extent of the deformations it has undergone. Between 1900 and 1945, several parts of the area were studied and mapped, but under an intensive program of work by members of the U. S. Geological Survey in 1952 much is now known about most of an area of about 30,000 square miles, of which the Mojave block includes about 15,000 square miles.

From the distribution, thickness, and lithologic characteristics of Paleozoic and Mesozoic sedimentary sections exposed within the Mojave Block, compared with those shown north of the Garlock fault, as well as those southwest of the San Andreas fault, it seems clear that following the late Mesozoic orogeny, probably beginning in early Tertiary time, the block rose at least 15,000 feet and was eroded as fast as it rose, before the earliest Tertiary sedimentary rocks (middle Miocene) were deposited.

Tertiary sedimentary rocks and included tuffs, breccias, and flows are confined to several basins that probably covered about half the area of the block. The earliest fossil-bearing sedimentary rocks are middle Miocene in age, but some underlying volcanic rocks may be Oligocene. Sections of middle and upper Miocene beds range from 5000 to 11,000 feet. Other basins contain as much as 8000 feet of lower Pliocene beds and 5000 feet of lower middle Pliocene beds. About middle Pliocene time, all the earlier Tertiary sedimentary rocks were greatly deformed and now show open and close folds and faults.

Before deposition of the next sedimentary rocks of known age (Manix, late Pleistocene), there were several periods of erosion separated by extensive flows of andesite and basalt. It is concluded that the Mojave Block has stood above the near-by areas, both to the north and to the south, during all of the Tertiary and much of Quaternary time. Within the block, there was an integrated system of drainage outward, largely to the southeast. In places, this system of drainage has recently been broken by movement along a group of northwest-trending faults that took form in early Tertiary time.

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