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having steeply dipping hogback ridges. Elevations in these areas average between 6,000 and 7,000 feet. Among the more prominent plateaus in the basin are Mesa Verde near the southwestern corner of Colorado, and the Defiance Plateau lying between the Chinle Valley and the Chuska Mountains. Elevations in the higher portion of both of these plateau areas average between 7,000 and 8,000 feet. Mesa Verde is a high, deeply dissected plateau that slopes gently southward. The streams that drain.the area have cut deep canyons, many of which reach nearly to the north rim of the plateau and divide it into numerous narrow mesa fingers that trend southward. Large alcoves in the sandstone cliffs at the heads and along the sides of the canyons are the sites of many ruins of ancient cliff dwell ings. The Defiance Plateau is also a deeply dissected tableland dominated by the narrow mesas and deep canyons of the Canyon de Chelly area. The trend of these canyons is mainly to the west, and they range to over 1,000 feet in depth. . r Geology Rocks ranging in age from Precambrian through Quaternary are exposed in the San Juan River Basin (Generalized Bedrock Geology Map follows page 111-6). They consist of crystalline rocks of Precambrian Age, volcanic rocks of Tertiary Age, and a thich sequence of sedimentary rocks of Paleozoic, Mesozoic, and Cenozoic Ages. Distribution of these rocks is controlled by four major structural features: the San Juan structural basin, the Needle Mountains upwarp, the Monu- ment upwarp, and the Defiance upwarp. The older rocks are generally exposed in the upwarped areas where the younger rocks have been removed by erosion, while the younger rocks occur mainly in the central portions of the San Juan structural basin. The San Juan structural basin occupies a large part of the eastern half of the San Juan River Basin. It is a broad structural depres- sion which was also the original basin of deposition .for early Cenozoic sediments. On the north, it is flanked by the Needle Mountains upwarp; on the northwest, it is flanked by the Monument upwarp; and on the southwest, by the Defiance upwarp. III-6