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<br />formation are sandstone, siltstone, shale, and coal. Deposition <br /> <br /> <br />occurred concurrently with the uplifting of the Sangre de Cristo <br /> <br /> <br />Mountains, where Precambrian, Pennsylvanian, and Permian rocks were <br /> <br /> <br />exposed and eroded. The basal layer of the Raton formation is a <br />conglomeratic sandstone which is used to identify the contact with <br />the underlying Vermejo formation. The Vermejo formation is composed <br />of sandstone cemented by clay and calcium carbonate, coaly and silty <br />shale, and coal beds. The formation thickens rapidly westward from <br />the dam site. The Trinidad sandstone, lying below the Vermejo forma- <br />tion, c?nsists of fine-grained sandstone with occasional thin interbeds <br />of gray silty shale throughout. Cementing agents are calcium carbonate, <br />clay, and silica. Underlying the Trinidad sandstone is the Pierre <br />Shale, the oldest exposed rock in the area. The Pierre shale is dark <br />gray to black, fissile, noncalcareous, and gypsiferous. Because of <br /> <br />the intertonguing of sandstone and shale upwards from the Pierre, the <br /> <br />contact between the Pierre and the Trinidad sandstone is somewhat <br /> <br />indistinct. <br /> <br />b. Damsite ~e010RY. The Pierre shale underlies un- <br /> <br /> <br />consolidated deposits of gravel, sand, silt, and clay in the <br /> <br /> <br />Purgatoire valley at the damsite. The total thickness of the <br /> <br /> <br />shale in the Trinidad area probably ranges from 1,800 to 1,900 feet. <br /> <br /> <br />The shale is impervious and provides an excellent floor for the <br /> <br /> <br />reservoir. The north and south abutments of the dam in the main <br /> <br /> <br />valley are formed by strata of the Trinidad formation, Which has <br /> <br /> <br />a total thickness of about 120 feet. Toward the eastern part of <br /> <br />11-1& <br /> <br />. <br /> <br />~ <br />