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-19- a general elevation of 7800 feet on the south boundary of the site to an el- . evation of around 700 feet at the north boundary (cf. Figure 5). The plateau surface within the mine site area is cut by several north-south trending arroyos, the largest of which is Streeter Canyon. The geology of the major part of the study area consists largely of sandstones of the Iles and Williams Fork formations while Mancos shales predominate in the Axial Basin and Lewis shales occur in the northern part of the study area. All of the above named formations were formed during the Upper Cretaceous. Alluvial soils of Quat- ernary origin occur in the Yampa and Williams Fork flood plains and the many remnants of old river channels and oxbows that can be seen in the flood plains indicate that river meandering is a relatively active process in the area. While the meander rate is unknown, it appears unlikely that prehistoric sites of any great antiquity would have remained intact. Any flood plain sites that may occur would most likely be buried under varying depths of alluvium. 4.2.2 Climate and Hydrology Availability of water is an important factor in understanding the pat- terns of man's past and early historic activities in the area. While the climate and hydrology of the study area cannot be assumed to have been the same at all times during man's occupation of the area, the present situation does provide a basis from which to work. The Yampa and Williams Fork are the two predominant drainage channels in the study area and ate presumed to have carried water at all times of the year during human occupation of the area. In the Axial Basin, permanent stream channels today, as indicated on the Axial and Monument Butte 7~' Quad maps, include Morapos Creek in the eastern part of the basin and Milk Creek, Wilson Creek, Good Spring Creek, and Stink- ing Gulch farther west. Jubb Creek and Taylor Creek are both intermittent tributaries of Wilson Creek. According to local informants, Stinking Gulch now carries runoff irrigation water and was also an intermittent drainage before irrigation activities became common in the Basin. According to the same informants, the water flow in Milk Creek is sometimes so low during late summer that it is occasionally difficult to water cattle. On the Colo- wyo Mine site, the only source of readily available water is a spring at the head of Streeter Canyon that now feeds a stock tank. According to informants, the spring runs throughout the summer in some years and dries up in late • summer other years. A patch of snow directly above the spring was the only