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<br />00 <br />c:;l <br />C\.! <br /> <br />pP ;!/' <br />1188ja!l,!;e; )ll lling at the dam site shows a deep cover of <br /> <br />alluvial material such that same ground water movement in overburden <br /> <br />rn(llc res 8l . -.:_ <br /> <br />may occur. <br /> <br />At the dam site Rifle Creek cuts direct~ across the Grand Hogback, <br /> <br />a steep limb of a monocline of Mesa Verde sandstone, shale and coal <br /> <br />beds which have been tilted nearly vertical dipping 80 to 85% downstream. <br /> <br />The medium grained sandstones are relative~ hard. The intervening <br /> <br />shale beds are firm and impervious. The harder sandstone fOrBllpre- <br /> <br />dominant ridges between slides where the shale has been more deeply <br /> <br />eroded. The sandstone ridges run at right angles to or directly across <br />C)l..kt.-~ <br />the canyon, val'3'iag its: thickness from a few inches up to 20 feet. <br /> <br />;;, <br /> <br />The bedrock series is not unusually fractured or broken in spite <br /> <br />of the extreme dip. The brittle sandstone beds are jointed rendering <br /> <br />them permeable but the alternating tight shale beds will form impermeable <br /> <br />cut-off members to water attempting to by-pass either rock abutment or <br /> <br />in the bedrock foundatiqn. <br /> <br />The overburden condition at the dam site is of some concern. This <br /> <br />cover J:'anges in tlrl.ckness up to a "",,'lrl..,m of 125 feet over undisturbed <br /> <br />: I <br /> <br />Mesa Verde formation. The side ,gullies have spread fan deposits into <br />~ '. <br />the1 bottom crowding the stream first to one side and then to the other. <br /> <br />^ <br />.~~, <br /> <br />Under such condition the deposits became thicker and the stream <br /> <br />elevated. In such a process tight wash and talus deposits became <br /> <br />interbedded wi th more permeable stream bed gravels and boulders. <br /> <br /> <br />Diamond drillings in the canyon show porous gravel, sand and boulder <br /> <br />:~~.T)ro~ 15 to 25 feet thick beneath approximate~ 100 feet of <br />ovef'lilQt t 9111 containing clay, sand, gravel and boulders. <br /> <br />,i <br />