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<br />3 <br /> <br />intermixed with the several strands of alluvium in the South Platte <br /> <br />River drainage basin in a complex fashion. More recent blankets of <br /> <br />loess cover much of the surface surrounding Ft. Morgan and the <br /> <br />Narrows Dam site. <br /> <br />THE FOUNDATION <br />Pierre Shale is the. bedrock beneath the dam and reservoir. <br /> <br />The rock is a dense, pre-consolidated siltstone, claystone, and shale <br /> <br />with smaller amounts of fine sand. In places it consists of lime- <br /> <br />cemented slabs and lenses; in other places gypsum may serve as the <br /> <br />cement. Rock samples exposed at the surface and taken from drill cores <br /> <br />show considerable variation in particle size, degree of compaction, <br /> <br />color, an? permeability. The samples obviously do not come from the <br /> <br />same horizon since there is considerable topographic relief on this <br /> <br />bedrock surface. Horizontal bedding is obscure or lacking in most <br /> <br />samples. <br /> <br />The Pierre Shale has a widespread reputation for swelling under <br /> <br />low loading conditions where excess water is available. It also <br /> <br />weathers to a soft clay which is easily removed by running water. <br /> <br />Hence outlet works on exposed Pierre Shale are subject to rapid erosion <br /> <br />by even low velocity discharges. However, when excavated to unweathered <br /> <br />shale, the rock is remarkably strong and capable of supporting <br /> <br />structures and resisting erosion by running water. <br /> <br />It is important that in all critical foundation excavations <br /> <br />the cut is carried into unweathered shale. At some places it is very <br /> <br />~ <br />-, <br />