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• the Iles Formation is epproxime [ely 1,500 feet thick (Figure 2). At the base of the i <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, and coal- ~, <br />Within this interval are three coal zones which represent the lower coal group of the <br />Mesaverde Group. Each of these zones contain several thin coal beds and are numbered 1 <br />through 3 with 1 being the deepest. The Trout Creek sandstone member is the uppermost <br />rock unit of [he Iles Formation. It is massive, fine-grained, end averages about 100 feet <br />in thickness (Bass et al., 1955). <br />The Williams Fork Formation conformably overlies the Iles Formation, and is the upper <br />formation of the Mesaverde Group. It ranges in thickness from about 1,100 feet to about <br />2,000 feet, and includes all of the rocks between the Trout Creek sandstone member of the <br />Iles Formation and the overlying Lewis Shale. The Williams Fork Formation can best be <br />described by dividing it into three units - the (over unit, the Twentymife sandstone <br />member, end the upper unit (Bass et al., 1955). the lower unit consists of interbedded <br />shales, sandy shales, sandstones, end coal. Several important coal beds located within <br />• this lower unit form the middle coal group of the Mesaverde Group. They will be discussed <br />in more detail in the section titled Site Specific Geology. In the Wi Lliams Fork <br />Mountains, some of the shale and thin sandstone outcrops above and within the middle coal <br />group have a reddish-rocky appearance. This characteristic results from the natural <br />burning of the adjacent coal outcrops and is referred to as burn, scori e, or clinker. pn <br />interval of predominantly shale about 500 feet in thickness lies above the Lennox coal bed <br />(Figure 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. !t 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 Williams Fork Formation, which contains the upper coal group of the <br /> Mesaverde gr oup, consists of layers of sandstone, sandy shale, shale, and coal. The <br /> thickness of this unit varies from approximately 200 to 850 feet throughout the region. <br /> Some of the sends canes ere qv i ce massive, resemb2 rrtg the i~,rrtcyimi2e sartdstarne members <br />. (Bass et al . , 1955). The actual number of coal beds with(n the Mesaverde Group varies <br /> from area to area and may range from one to more then several. One of the workable coal <br />3 <br />