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The Los Angeles lowlands and the offshore continental borderland, south of the northern Channel Islands, taken together, have the shape of an inverted and distorted recumbent L, with an east-west leg 225 mi (362 km) long and a north-south spread of about 120 mi (190 km). The crystalline and semicrystalline basement of this area seems to have been eroded to low relief in middle Cretaceous time, to form what we suggest calling the Los Angeles erosion surface. Onshore this surface, cut on Catalina blue and green schists, quartz plutonites, and other rocks, forms the floor of the Los Angeles basin and, farther inland, the surface of the basement rocks of the somewhat higher Perris block, the latter modified by Tertiary erosion to principal levels of about 1,500 and 1,600 ft ( 00 m) elevation. The basement rocks are middle Cretaceous or older, from both radiometric and fossil evidence. Offshore, the basement everywhere seems to be Catalina Schist, overlain by Cenomanian and younger sedimentary rocks. Probably five-sixths of the area was cut on Catalina Schist, and the remaining, northeasternmost sixth on more siliceous basement. The first notable deformation of the surface was at the end of Cretaceous time, when a low southwest-northeast Los Angeles arch rose in the central and northeastern parts of the area and soon was denuded almost entirely of its thick Cretaceous cover, which was similar to that preserved in the Santa Ana and Santa Monica Mountains on the southeast and northwest respectively. In Cenozoic time, Paleocene(?), Eocene, and later marine and no marine deposits successively overlapped from the west and south onto the denuded basement, in part filling newly developed deep and shallow basins. The overlaps were interrupted in early middle Miocene time by local uplifts. During Pliocene and Pleistocene times, the surface was deformed rather profoundly, as deep basins and high mountains formed.
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