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3.0 G~FNERAL GEOLOGY <br />The general geology of the area has been reported by the U.S. • <br />Geological Survey (Bass, et. al., 1955). This report adt:uately <br />covers general aspects of the geology at the Yoast mine a in its <br />vicinity. The present report will be confined large i to a <br />discussion of specific aspects of geology along the aul-road <br />alignments and in the vicinity of Sedimentation Pond Glo as they <br />relate to engineering characteristics of the rock units mapped. In <br />addition to the published report noted above regional proprietary <br />photogeologic mapping from AI files was used. <br />The southern limit of geologic mapping is the south line of <br />secs. 15, 16, 17, and 18, T.SN., R.87W. <br />3.1 Rock Units • <br />The entire area mapped (see Drawings 9316-1 and -2) is <br />underlain by bedrock of the Iles, and Williams Fork Formations, and <br />by the Lewis Shale, all of Cretaceous age. The Iles and Williams <br />Fork are predominantly sandstone units with subordinate amounts of <br />shale. The Lewis Shale is composed primarily of shale with <br />subordinate amounts of sandstone. The mine is located on the <br />flanks of the Fish Creek anticline in the southern part of the area <br />mapped. The anticline and an unnamed syncline to the east <br />determine structural conditions of the bedrock along the haul-road <br />alignment. <br />3.1.1 Iles Formation. The upper part of the Iles Formation crops <br />out just east of the axis of the Fish Creek anticline in the • <br />southern part of the area mapped. Here the Iles is a series of <br />to <br />