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• shale beds with an upper or capping cliff-forming rock unit named <br />the Trout Creek Sandstone. <br />3.1.2 Williams Fork Formation. This unit includes three rock <br />sequences, all of which crop out in the area mapped. The lower <br />sequence in this formation is mainly "-- soft sandstone, thin <br />bedded sandstone, sandy shale, coal beds, and thin beds of dark- <br />gray to black shale. Although the upper half of the unit contains <br />a few beds of sandstone, it consists chiefly of shale." (Bass, <br />et. al. 1955, p. 158). Most of the area mapped on Exhibit 13-12-2 <br />(Drawing 9316-2) is underlain by this sequence which crops out on <br />both sides of the Fish Creek anticline between the Iles Formation <br />below (on the south) and the Twentymile Sandstone on the north, <br />east and west. The Yoast mine will extract coal from beds in the <br />lower part of this sequence. <br />The Twentymile Sandstone is resistant to erosion and forms a <br />prominent bluff along the west side of the main drainage (unnamed) <br />flowing northward from the mine area. This massive sandstone unit <br />is between the lower and upper parts of the Williams Fork <br />Formation. The top of the Twentymile Sandstone was mapped on the <br />eastern and western flanks of the Fish Creek anticline and is shown <br />on the map as a key bed. <br />The upper sequence of the Williams Fork Formation is about 200 <br />feet thick in the Fish Creek area according to Bass, et. al. (1955, <br />p. 158). He notes (p. 159) that "-- the unit consists of beds of <br />sandstone, sandy shale, dark gray shale and one coal bed about 3 <br />• feet in thickness." In the area mapped the upper Williams Fork <br />Formation is probably 700 to 800 feet thick. <br />11 <br />