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2.0 Site Geology Although the details of site geology are rather complex from the engineering point of view, they can be simplified as follows: 2.1 The entire area is underlain by the Pierre formation which at the site appears to be nontypical of the Pierre shale. The Pierre is a mudstone or siltstone with muted stratification and foliation. Tests show that it is not saturated and it may not be as clayey (with the associated tendency to swell) as in the more typical locations. 2.2 Adjacent to the river in low terraces above the braid- ing belt are recent alluvial and colluvial deposits consisting of alternate layers of plastic clay, silty sand, and clean sand. Generally the uppermost few feet are more clayey. 2.3 The present braided belt of the South Platte, about 1500 feet wide at the dam site consists of water-washed sands and fine gravels. They are in inter-fingered sinuous bodies that are not easily segregated during construction. '-.::, 2.4 To the north, the Pierre shale rises in low bluffs to about elevation 4400 within about 3000 feet of the river. The rock then rises more gently to elevation 4454 to the crest of the dam nearly a mile from the river (but about 6700 feet along the dam crest). The rising ground on the right is covered with a thin blanket of wind-blown sand and silt. Also there are probably local thin areas of residual silty clay derived from advanced weathering of the Pierre. 2.5 On the right bank the Pierre shale surface dips down- ward gently to the south reaching an elevation 4148 at Station 70+00 22300 feet from the Platte River. The ground surface elevation at this point is approximately 4450, 4 feet below the dam crest but below the maximum water surface. Further south, the Pierre surface rises rapidly, approaching the ground sur- face 1000 feet south of Bijou Creek. The Pierre surface re- flects an earlier river channel. It is possible that there are other undulations in the Pierre surface between Bijou Creek and the river that have not been found in the widely spaced pre- liminary borings. . 2.6 The Pierre shale south of the river is overlain by com- plex alluvial formations consisting of irregular strata of per- vious sand and gravel. The formations appear to be somewhat thinner-bedded, more irregular, and finer grained in the upper- most 100 feet. However, the borings in this area are too widely spaced to develop the complete picture of the texture and strati- fication of the deposits. . 3