2009-10-16_PERMIT FILE - M2009082 (11)

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30" "- 40° 37'30" I f 8 N I r 7 N ENVIRONMENTAL GEOLOGY No. 6-1 FXPI.:4NAT ION _".- -4.?s0 ecdrock 0 )r (dashed where approximate) Shows altitude of bedrock surface above mean sea level. Contour Interval 10 ft. Boundary of study area Ancient channels in bedrock surface DISCUSSION 30 f 6 N The Pleistocene and Recent geologic history of the Windsor study area has resulted in extensive, thick deposits of gravel, sand, and silt in the major stream valleys. This alluvial material conceals the config- uration of the underlying bedrock surface on which the streams once flowed. This map shows the configuration of the bedrock surface as it would look if the alluvial material were removed. The elevation contours show the old stream valleys that were cut into the bedrock during a period when erosion was dominant over deposition. The valley fills shown on the Bedrock Geology Map (Place 6-A) were deposited after the erosional period during times when the stream regime was primarily depositional. In conjunction with the rrap showing Thickness of Unconsolidated Mate- rial (Plate 6-J), this map can be used to identify the trends of maximum thickness of alluvial deposits. This permits the identification of optimum sites for ground-W*dter resources and for sand and yravel re- sources. Movement of water through the alluvium is, in part, related to the shape of the relatively impermeable, older bedrock surface over which the water flows. Maximum yialds from water wells will he obtain- ed from that portion of a given aquifer with the greatest saturated thickness. Similarly, the quantity and quality of sand and gravel re- sources are closely related to old channels in the bedrock. In general then, the value of alluvial ground-water resources and sand and gravel resources will increase near the deeper channels in the bedrock surface. REFERENCES Chinq, P. W., 1972, Ecorximic gravel deposits of the lower Cache la Poudre River: Colorado State Univ., Master .Sci. Thesis, 93 p. Grey, Lydia, 1974, Economic and land use evaluation of piedmont alluvial deposits. Windsor area, Colorado: Univ. Colorado, Master Sci. Thesis 43 p. Shelton, D. C., 1972, Thickness of alluvium and evaluation of aggregate resources in the lower Cache la Poudre River basin, Colorado, by electrical resistivity techniques; Univ. Colorado, Master Sci. Thesis, 67 p. Woodward, Dennis, 1972, A hydrogeologic investigation of the Cache la Poudre River alluvium in the Windsor triangle area, Colorado: Colorado Geol. Survey open-file rept., 63 p.