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3 is of minor importance and open folding is predominant. The strata dipping southward from the main Uinta Mountain axis are folded into sharp southsoutheast plunging anticlinal noses. South of the principal Uinta Mountain axis and separated from it by the Island Park syncline, are two parallel westward-plunging anticlines, the Split Mountain anticline and the Section Ridge anticline. These anticlinal folds are separated by the shallow Daniels Draw syncline. To the west of Dry Fork of Ashley Creek, the northwest-trending faults, comprising the Deep Creek fault zone, intersect the South Flank fault zone that generally bounds the pre-Cambrian core of the mountains. Three periods of deformation are indicated in the area. The first deformation involved the faulting of pre-Cambrian rocks and occurred prior to the deposition of the limestone unit of Mississippian age. The second deformation followed the deposition of the Moenkopi formation and included gentle westward tilting of the area prior to the deposition of the Shinarump conglomerate. The third and principal deformation of the area followed the deposition of the youngest Cretaceous rocks and continued during the deposition of the early Tertiary rocks. Deformation of the area had almost ceased by the time the Bishop conglomerate was deposited in Miocene ( ?) time. Three widepread erosion surfaces, the Gilbert Peak surface, the Lake Mountain surface, and the Jensen surface, were developed during late Tertiary and Quaternary time. The Gilbert Peak surface underlies the Bishop conglomerate and bevels all older rocks. The Lake Mountain surface was cut on the Bishop conglomerate, and the Jensen surface was formed by coalescing pediments late in the erosion period of the Tertiary sedimentary rocks of the Uinta Basin. Two strath terraces, the Vernal and Thornburg surfaces, were formed along major streams during the glaciation of the Uinta Mountains in Pleistocene time. Lenticular coal beds, 14 inches or more in thickness, occur in the upper part of the Frontier sandstone member of the Mancos shale. The coal is high-volatile C bituminous in grade. Coal is reported in the black shale unit of latE> Mississippian age, and lignite is present in the Mesaverde formation of Late Cretaceous age. Approximately 240,000 tons of coal has been mined from the Frontier sandstone member of the Mancos shale. Sixty million tons of coal under less than 1,000 feet of overburden, 54 million tons under 1,000 to 2,000 feet of overburden, and 29 million tons under 2,000 to 3,000 feet of overburden are considered as indicated reserves. A. number of oil and gas wells have been drilled in the area. Commercial gas was discovered on the Ashley Creek structure in 1925 and was produced from 1929 to 1940. The producing horizon was a sand in the lower part of the Morrison formation. Five hundred and thirty-six million cubic feet of gas were produced. Oil was discovered in the Weber sandstone on the Ashley Creek structure at a depth of 4,152 feet in September 1948. Twenty-eight commercially producing wells had been drilled by February 1950. One of the wells is producing from a sand in the Park City formation. Small quantities of copper, lead, and iron have been produced from the Carbonate district (T. 1 S., R. 21 E.), which is located in the limestone unit of Mississippian age. Low-grade phosphate rock occurs in the lower part of the Park City formation. The phosphate rock averages 20 percent of P205 for a thickness of 18 feet in the Ashley Creek-Brush Creek area. Two hundred and fifty million tons of phosphate rock under 43 feet or less of overburden, 650 million tons under 78 feet or less of overburden, 1,800 million tons to stream level, and 2,450 million tons to 1,000 feet below stream level are listed as indicated reserves in .the Ashley Creek-Little Brush Creek area. 4 GEOLOGY OF UINTA RIVER-BRUSH CREEK AREA, UTAH Sand and gravel for concrete and road construction occur in terrace deposits adjoining the major streams. Reserves are adequate for the normal demands of the area. FIGURE 1.-Iondex map of Utah showing location of Uinta River-Brush Creek area.

Geology of the Uinta River-Brush Creek area, Duchesne and Uintah Counties, Utah