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-21- The geologic units occurring within the Twentymile Park Basin range in age from Late Cretaceous to Quaternary (Figure 4). The oldest exposed sedimentary rocks are that of the Mancos Shale, which was deposited under marine and near-marine conditions. After deposition of the Mancos Shale, sediments of the Iles and Williams Fork Formations of the Mesaverde Group were deposited mostly in terrestrial environments. Following deposition of the Mesaverde Group, sediments of the Lewis Shale were deposited in a predominantly marine environment (BLM, 1976). Younger sedimentary rocks of the Late Cretaceous and Cenazoic periods do not occur in the Twentymile Park Basin due to erosion or non-deposition. The following description of the geologic units occurring within the Twentymile Park Basin is modified from Warner and Dale (1981) and Brogden and Giles (1977). The Mancos Shale is a thick (approximately 5,000 ft.) homogeneous light-gray to dark-gray fossil iferous marine shale with interbedded sandstone and limestone beds and is the oldest unit exposed in the area (Figure 4). The sandstones are generally thin bedded, fine grained, tan, and fossiliferous, and form resistant ledges in the basal and upper parts of the formation. The overall area occuppied by the Mancos Shale is characterized by rolling hummocky topography. The fesaverde Group is approximately 3,000 feet thick and conformably overlies the Mancos Shale (Figure 4). It consists of the Iles Formation and ~dilliams Fork Fornation. The Iles Formation is the lower unit and is approximately 1,500 ft. thick. It consists of interbedded light-brown to white, massive, fine-grained, ledge-forming sandstones, brown to black carbonaceous shale, sandy shale and coal beds. The coal beds are assigned to the lower coal group (Figure 4) of the Mesaverde, and are distributed throughout the middle and upper parts of the formation. Four persistent sandstone beds occur within the Iles Formation. They are: 1) the Tow Creek sandstone member at the base, 2) a double ledge-forming sandstone sequence 400 feet or more above the base, 3) a light-gray sandstone sequence of variable composition associated with the upper (NO. 3) coals of the lower group situated about 900 to 1,000 feet above the base, and 4) the Trout Creek sandstone member which caps the fornation (figure 4). The Trout Creek sandstone is a 50 to 100 foot thick, light-brown to light-gray, fine-grained, massive sandstone. The upper unit of the Mesaverde Group is the Williams Fork Formation which is approximately 1,000 to 2,000 feet thick and includes ail beds between the top of the Trout Creek Sandstone Member and the base of the overlying Lewis Shale (Figure 4). The formation is conformaoie to its base and top, and it is divided into three units, the lower, middle and upper units.