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• The Iles Formation is approximately 1,500 feet thick (Figure b-2). At the base of the <br />formation is the Tow Creek sandstone member, a massive, fine-grained, well-sorted <br />sandstone ranging from 35 to 125 feet in thickness (Bass et aL., 1955). Above the Tow <br />Creek sandstone is a 1,200-foot interval of interbedded shales, sandstones, end coal. <br />Within this interval ere three Coal zones which represent the lower coal group of the <br />Meseverde Group. Each of these zones contain several thin coal beds and ere numbered 1 <br />through 3 with 1 being the deepest. The Trout Creek sandstone member is the uppermost <br />rock unit of the Iles Formation. It is massive, fine-grained, and averages about 100 feet <br />in thickness (Bass et, el., 1955). <br />The Yilliams Fork Formation conformably overlies the Iles Formation, and is the upper <br />formation of the Meseverde Group. It ranges in thickness from about 1,100 feet to about <br />2,000 feet, and includes all of the rocks between [he Trout Creek sandstone member of the <br />Iles Formation and the overlying Lewis Shale. The Yilliams Fork Formation can best be <br />described by dividing it into three units - the lower unit, the Twentymile sandstone <br />member, end the upper unit (Bess et el ., 1955). The lower unit consists of interbedded <br />shales, sandy shales, sandstones, and coal. Several important coal beds located within <br />this lower unit form the middle coal group of the Meseverde Group. in the Yilliams Fork <br />Mountains, some of the shale end thin sandstone outcrops above end within the middle coal <br />group have a reddish-rocky appearance. This characteristic results from the natural <br />burning of the adjacent coal outcrops end is referred to as burn, scoria, ar clinker. An <br />interval or predominantly shale about 500 feet in thickness lies above the Lennox coal bed <br />(Figure 6-2). <br />Above this shale interval lies the Twentymile sandstone member. It ranges in thickness <br />from 100 to about 200 feet end in some areas is divided into several separate beds. It is <br />very similar in nature to the Trout Creek sandstone member of the Iles Formation (Bass et <br />al., 1955). <br /> The upper unit of the Yilliams Fork Formation, which contains the upper coal group of the <br /> Meseverde group, consists of layers of sandstone, sandy shale, shale, end coal. The <br /> thickness of this unit varies from approximately 200 to 850 feet in thickness throughout <br /> the region. Some of the sandstones are quite massive, resembling the Twentymile sandstone <br /> members (Bess et al., 1955). The actual number of coal beds within the Meseverde Group <br /> varies from area to area and may range from one to more than sever al. One of the workable <br />• l <br />i <br />hi rou <br />is called the Dr <br />thi <br />C <br />k <br />l b <br />d <br />d i <br />t 400 <br />it <br />t <br />d <br />b <br /> coa <br />seams w <br />t <br />n p <br />y <br />s g <br />ree <br />coa <br />e <br />en s s <br />a <br />ou <br />ua <br />e <br /> 3 <br />