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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 between 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 the base of the Lance Formation <br />indicate that at least the lower 250 feet of the formation is more closely related to the <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 />The Wasatch Formation of Eocene age unconf ormably overlies the Fort Union Formation. It <br />is comprised primarily of 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 Ouaterna ry <br />age may be found overlying the Wasatch Formation. Colluvi um deposits of boulders, <br />cobbles, and coarse sand form a thin veneer at numerous locals, while silt, sand, and <br />boulder deposits form terraces in the valleys at several altitudes above the larger <br />5t reams. <br />Coal Groups <br />Coal horizons are present in the Mesaverde Group, Lance Formation, and Fort Union <br />Formation. The coals of the Mesaverde Group have been divided into three distinct groups. <br />The lower coal group includes all coal beds beginning about 400 feet above the base of the <br />Iles Formation (the Tow Creek sandstone) and extending upward to the Trout Creek sandstone <br />member. The middle coal group comprises the coal beds between the Trout Creek sandstone <br />3 <br />