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in the upper part of the section and numerous oil and gas fields located mainly across the central part of the basin in New Mexico, Colorado, and Utah. The outcrop area of shales of Mesozoic con- tains some of the most erodible soils and highest sediment produc- ing areas in the basin. Sedimentary rocks of early Cenozoic age consisting mainly of shales and sandstones occur through the center of the San Juan structural basin covering a large area in New Mexico and a smaller area in Colorado. Socialized badlands areas in the high erosion and sedi- ment yield occur occasionally in the rocks of this age. Volcanic rocks of middle and late Cenozoic age occur along the northeastern edges of the San Juan River Basin in Colorado and form most of the high peaks in the San Juan mountains. They include rocks of flow origin as well as a variety of rocks of pyroclastic origin, such as tuff, welded tuff, tuff agglomerate, and tuff breccia. Masses of intrusive igneous rocks form the cores of such mountain areas as the Ute Mountains, La Plata Mountains, and Rico Mountains in Colorado; the Carrizo Mountains in Arizona, and some of the more prominent peaks in the Chuska Mountains in New Mexico and Arizona. Quaternary deposits ranging in age from Pleistocene to Recent (not shown on the Geology map) are widespread in the San Juan River Basin. Glacial moraines representing several intervals of glaciation are present along most of the major valleys in the San Juan Mountains and the La Plata Mountains in Colorado. Landslide deposits, rock glaciers, and talus deposits are also common in these mountains. Several levels of mesa and terrace surfaces underlain by sandy and gravelly alluvial deposits occur along the larger stream valleys and bordering isolated mountain areas, such as the Ute Mountains in Colorado, the Carrizo Mountains in Arizona, and the Chuska Mountains in New Mexico. Deposits of windblown silt and sand'occur on some of the more extensive mesa surfaces in Colorado, such as the Sage Plain in the Dove Creek-Cortez area and the Red Mesa and Florida Mesa areas south of Durango. Large areas of wind-deposited sand occur in the southeast corner of Utah, the northeast corner of Arizona, and along the west side of Chinle Wash in Arizona. Recent alluvium occurs in the flood plains of most smaller tributaries as well as along the larger streams. MINERAL RESOURCES Natural gas, crude oil, uranium, vanadium, zinc, lead, sand and gravel, and coal are the most important minerals being produced currently in the basin. Petroleum products including helium as well as natural gas and crude oil account for the largest percen- tage of this production. III -8 ,"