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The Iles Formation is approximately 1,500 feet thick. At the base of <br />• the formation is the Tew Creek sandstone member, a massive, fine <br />grained, well sorted sandstone ranging from 35 to 125 feet in thickness. <br />Above the Tcw Creek sandstone is a 1,200 foot interval of interbedded <br />shales, sandstones, and coal. Within this interval are three coal zones <br />which represent the lower coal group of the Mesaverde Group. Each of <br />these zones contain several thin coal beds and are numbered 1 through 3, <br />with 1 being the deepest. The Trout Creek sandstone member is the <br />uppermost rock unit of the Iles Formation. It is massive, fine grained, <br />and averages about 100 feet in thickness. <br />The Williams Fork Formation conformably overlies the Iles Formation, and <br />is the upper formatior. of the Mesaverde Group. It ranges in thickness <br />from about 1,100 feet to about 2,000 feet, and includes all of the rocks <br />between the Trout. Creek sandstone member of the Iles Formation and the <br />overlying Lewis Shale. The Williams Fork Formation can best be <br />described by dividing it into three units -- the lower unit, the <br />• Twentymile sandstone member, and the upper unit. The lower unit <br />consists of interbedded shales, sandy shales, sandstones, and coal. <br />Several important coal beds located within this lower unit form the <br />middle coal group of the Mesaverde Group. In the Williams Fork <br />Mountains some of the shale and thin sandstone outcrops above and within <br />the middle coal group have a reddish-rocky appearance. This <br />characteristic results from the natural burning of the adjacent coal <br />outcrops and is referred to as burn, scoria, or clinker. An interval of <br />predominantly shale about 500 feet in thickness lies above ±he Lennox <br />coal bed. <br />Above this shale interval lies the Twentymile sandstone member. It <br />ranges in thickness from 100 to about 200 feet and in some area is <br />divided into several separate beds. It is very similar in nature to the <br />Trout Creek sandstone member of the Iles Formation. <br /> The upper unit of the Williams Fork Formation, which contains the upper <br />• coal group of the Mesaverde group, consists of layers of sandstone, <br /> sandy shale, shale, and coal. The thickness of this unit varies from <br />2.04.6-4 <br />