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ween Section 36, Township 6 South, Range 91 West and Section 1, Township 7 South, Range 91 West. No geologic or geotechnical investi- gations have been made of the dam sHes. The following discussion is based on a reconnaissance made on July 25, 1985. I II I I The geology of the East Divide Creek damsites is that of the area southeast of the Piceance Creek Basin and southwest of the Grand Hogback. The bedrock is the lower and middle parts of the Wasatch Formation of Tertiary Age. The Wasatch Formation consists of sandstone, and conglomeratic sandstone near the base, interbedded with siltsone and claystone. The West Divide Creek anticline is about two miles southwest of the damsite. III the vicinity of the damsite the beds dip 1. to 10. southwest. No major faults have been identified in the areas of the damsites, but joints are conspicuous in the more com- petent beds. The most abundant joInts are parallel to the bedding. Other joint sets are at right angles to the bedding approximately parallel to the strike of the beds, and at right angles to the strike of the beds. I I I I I I II I I The bedrock is concealed for the mos t part by surficial deposits. The surficial deposits include colluvium, including landslide deposits, and alluvium including terrace deposits. The colluvium is derived by weathering from the underlying bedrock. It consists of angular blocks and gravel of sands tone in a sandy and silty clay matrix. Colluvium accumulates wi th time on the slopes. During periods of high precipitation or rapid snowmelt the colluvium becomes saturated and fails, forming landslide deposits. A large landslide has been mapped (Ref. 36) on the left abutment and in the reservoir area of the proposed damsite. In the bottom of the East Divide Creek is alluvium, primarily derived by erosion of the collu- vial deposits. It consists of gravel, sand and silt. Locally are cobbles and gravel of basalt derived form the older terrace deposits. Between East Divide and West Divide Creeks on and north of East Divide Creek are terrace deposits. The terrace deposits consist of cobbles and gravel, chiefly of basalt, but containing pieces of other hard igneous and metamorphic rocks, in 8, sandy and silty matrix. On the surface of the terrace deposit is a fine silt, probably Aeolian in origin. (See Figure IV-5) I I I IV-l9 I I I I I Material suitable for an earthfill or rollercrete dam pro- bably can be developed from the colluvium, including the landslide deposits. Aggregate and sand for concrete and for drain material could be processed from the terrace deposits. The terrace deposits could also possibly furnish enough coarse material for riprap. There is no local source of sui table materIal for a rock fill dam.