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• thickness of the group ranges from 2,750 feet to 3,450 feet. The Mesaverde Croup in the region has been dived into two formations. The Iles Formation, the lowermost formation, consists of marine and continental sequences of sandstone, shale, sandy shale, and coal beds, with a total thickness around 1,500 feet. The Tow Creek sandstone member, the basal unit of the Iles Formation, is a massive, fine-grained, well-sorted sandstone ranging in thickness from 35 feet to 725 feet, and is believed to have been deposited in near shore or beach environments. Above the Tow Creek sandstone is a 1,200-foot interval of sandstone, sandy shale, shale, and coal. The Trout Creek sandstone member is the uppermost unit of the Iles Formation, and consists of fine-grained, massive, cliff-forming sandstone about 100 feet thick. The coal zones of the Iles Formation are distributed throughout the middle and upper portions of the formation, and have been assigned to the lower coal group of the Mesaverde Group. The Williams Fork Formation, conformably overlies the Iles Formation and is the upper formation of the Mesaverde Group. The Williams Fork ranges in thickness from 1,100 feet to about 2,000 feet and includes all of the strata between the top of the Trout Creek • sandstone member of t he Iles Formation and the overlying Lewis shale. The formation is comprised of a lower unit, approximately 1,000 feet thick, consisting of shale, thin sandstone beds, sandy shale, and coal zones assigned to the middle coal group of the Mesaverde Group. The middle unit, a massive, cliff-forming sandstone varying in thickness from 100 feet to 200 feet, has been designated the Twenty-mile sandstone member, and is similar in composition to the Trout Creek sandstone member of the Iles Formation. The upper unit of the Williams Fork Formation lies above the Twenty-mile sandstone member and consists of sequences of sandstone, sandy shale, shale, and coal. This upper unit exhibits the greatest variation in thickness of any part of the formation. This unit ranges from 200 feet to 850 feet in thickness, with a westward increase due primarily to a change of facies of the strata in the lower part of the Lewis shale, which overlies the Mesaverde Group. Marine shale strata in the eastern portion of the region are placed in the Lewis shale, while a westward facies change to sandstone, sandy shale, shale, and coal are placed in the Williams Fork Formation. The coal zones deposited stra tigraphically between the top of the Twenty-mile sandstone member and the Lewis shale have been assigned to the upper coal group of the Mesaverde Group. L_J The Lewis shale conformably overlies the Mesaverde Group and, like the Williams Fork 2