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The Iles Formation is approximately 1,500 feet thick (Figure 6-21. At the base of the formation <br />• is the Tow Creek sandstone member, a massive, fine-grained, well-sorted sandstone ranging <br />from 35 to 125 feet in thickness (Bass et al., 19551• Above the Tow Creek sandstone is a <br />1,200-foot interval of interbedded shales, sandstones, and coal. Within this interval are three <br />coal zones which represent the lower coal group of the Mesaverde Group. Each of these zones <br />contain several thin coal beds and ere numbered 1 through 3 with 1 being the deepest. The <br />Trout Creek sandstone member is the uppermost rock unit of the Iles Formation. It is massive, <br />fine-grained, and averages about 700 feet in thickness (Bass et al., 1955). <br />The Williams Fork Formation conformably overlies the Ites 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 lower unit, the Twentymile sandstone member, <br />and the upper unit (Bass et al., 195b1. The lower unit consists of interbedded shales, sandy <br />shales, sandstones, and coal. Several important coal beds located within this lower unit ~. <br />(Lennox, Wedge, Wolf Creek) form the middle coal group of the Mesaverde Group. They will <br />be discussed in more detail in the section titled Site Specific Geology. In the Williams Fork <br />Mountains, some of the shale and thin sandstone outcrops above and within the middle coal <br />group have areddish-rocky appearance. This characteristic results from the natural burning of <br />• the adjacent coal outcrops and is referred to as burn, scoria, or clinker. An interval of <br />predominantly shale about 500 feet in thickness lies above the Lennox coal bed (Figure 6-21. <br />Above this shale interval lies the Twentymile sandstone member. It ranges in thickness from <br />100 to about 200 feet and in some areas is divided into several separate beds. It is very <br />similar in nature to the Trout Creek sandstone member of the Iles Formation (Bass et al., <br />19551. <br />The upper unit of the Williams Fork Formation, which contains the upper coal group of the <br />Mesaverde group, consists of layers of sandstone, sandy shale, shale, and coal. The thickness <br />of this unit varies from approximately 200 to 850 feet in thickness throughout the region. <br />Some of the sandstones are quite massive, resembling the Twentymile sandstone members <br />(Bass et al., 19551. The actual number of coal beds within the Mesaverde Group varies from <br />area to area and may range from ohe to more than several. One <br />CI <br />3 Flevised 9/96 <br />