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<br />, .' <br /> <br />~ (' <br /> <br />. <br /> <br />r~ <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />. <br /> <br />-3- <br /> <br />on Fig, 2, Our reconnaissance inspection verifies that the location <br />of the mapped geologic units is accurately represented on that map, <br />except that. since the map was published. fill has been placed along <br />the north edge of the reservoir for the highway relocation. <br />The geologic materials exposed at the surface in the reservoir <br />area consist mostly of stream deposited alluvial materials. These <br />alluvial materials are comprised of sands. gravels. cobbles and <br />boulders with minor amounts of silt and clay fines. The average <br />thickness is reported by Scott to vary from 5 feet to greater <br />than 25 feet throughout the Quandrangle. The alluvial materials <br />are underlain by bedrock consisting of sandstone. siltstone. and <br />shales (claystones) of the Pierre Formation of Cretaceous Geologic <br />Age (about 9D million years before the present). The bedrock <br />strikes northwest-southeast and dips moderately toward the east. <br />The Pierre Formation is generally recognized as being impervious <br />although in some local areas. it does contain pervious zones (aquifers). <br /> <br /> <br />I <br /> <br />Geology - Dam Axis <br /> <br /> <br />The geology on the Dam Axis is similar to that described above <br /> <br /> <br />(alluvium underlain by bedrock) except that the north abutment will <br /> <br /> <br />be on recently constructed man-made fill which is not displayed on <br /> <br /> <br />the geologic map. The central portion of the dam will rest upon <br /> <br /> <br />recent alluvium (symbols Qp and Qpp on the Geologic Map) while the <br /> <br /> <br />I <br /> <br />; <br /> <br />I <br />