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Formation, is highly variable in thickness. The Lewis shale is comprised primarily of <br />homogeneous marine shale, estimated to range in thickness from 1,900 feet to 1,500 feet or <br />less. In places, a transition zone, consisting of beds of lenticular sandstone, sandy <br />shale, shale, and coal is located above the main unit of marine shale, thus making the <br />boundary betrveen the Lewis shale and Lance Formation arbitrary. <br />The youngest Cretaceous sequence preserved in the region is the Lance Formation. The <br />Lance Formation conformably overlies the Lewis shale and is approximately 1,000 to 1,500 <br />feet thick, it consists of interbedded shale, sandstone, and a limited number of coal <br />beds. Marine invertebrates of Fox Hill age collected from [he base of the Lance Formation <br />indicate that at least the lower 250 feet of the formation is more closely related to [he <br />underlying Lewis shale than to the overlying fresh water deposits (Bass et al., 1955). <br />The Fort Union Formation, of Tertiary age, Lies above the Lance Formation. It consists of <br />interbedded sandstone, shale, and coal deposited during the Paleocene epoch and is <br />estimated to be 1,400 feet thick. The Fort Union Formation is similar to the Lance in <br />general aspect, but differs considerably in details. The sandstone beds are coarser <br />grained and there are more ferruginous layers present in the fort Union. <br />C. <br />The Wasatch Formation of Eocene age unconformably overlies the Fort Union Formation. It <br />is comprised primarily pf fluvial sandstones and shales. The Wasatch Formation is <br />estimated to be somewhat greater than 1,000 feet thick. <br />Deposits of semiconsolidated and unconsolidated material of Late Tertiary and Quaternary <br />age may be found overlying the Wasatch Formation. Colluvium deposits of boulders, <br />cobbles, and coarse sand form a thin veneer at numerous locals, while silt, sand, and <br />boulder deposits form terraces 1n the valleys at several altt LUdes above the larger <br />streams. <br />Coal Groups <br />Coal horizons are present in the McSaverde Group, lance Formation, and Fort Union <br />Formation, the coals of the Me sa verde Croup have been divided into three distinct groups, <br />The lower coal group includes all coal beds heginning about 400 feet above [he base o! the <br />Iles Formation (the Tow Crr ck sandstone) and extruding upward to the trout Creek sandstone <br />• member. The mitldle coal group rompri srs the coal beds he[ween [he trout Creek sandstone <br />3 <br />