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. '.,.' r-. -, .: ~\.n '-. .- ,,'>J.... ,-_.,":;.; ROCK UNITS AND THEIR WATER-BEARING PROPERTIES The Precambrian rocks in the reservoir are are at depths of more than 500 feet. They are composed of granite, gneiss, and schist, which are relatively impervious except where they are fractured or jointed. The Morrison Formation is composed of about 450(7) feet of varicolored red, green, and gray-white shale, bentonitic clay, and siltstone. In places it contains lenses of sandstone, which may yield small quantities of water. Test core PR 114 probably pene- trated a sand lens in the upper part of the Morrison (figs. 2 and 3). A flow of half a gallon per minute was obtained. The Dakota Sandstone is composed of about 340 feet of gray-white to brown fine- to coarse-grained moderately to highly cemented sandstone. Carbonaceous shale, siltstone, and some coal are common in the upper half of the formation. Joints and fractures are apparent in exposures and are noted in many test-core logs. Water seeps from the lower third of the Dakota along the canyon walls at the damsite. Test-core logs DH 124, 125, and 3, show water in the lower beds of the Dakota at levels indicated by the piezometric surface. (See fig. 4.) Test-core holes penetrating the lower beds of the Dakota in the canyon bottom, upstream from PR 114, generally flowed from 1/2 to 5 gpm. A strike of N. 20 W. and a dip of 60 E., were calculated for the Dakota by the three-point method from test-core logs showing the contact between the Morrison and the Dakota. From the approximate line of strike along the contact between the Mancos Shale and the Dakota Sandstone north of DH 3 (fig. 4), the top of the Dakota was projected eastward at about the true dip, where it intercepts the King well at 80 feet below the land surface. There is no log of the well to indicate the actual Dakota top. I I