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<br />1 <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br /> <br />1.4 HYDOLOGEOLOGIC SETTING <br /> <br />The setting for this study is a high mountain valley called the San Luis Valley. The valley is <br /> <br /> <br />located along the Grande Rift which, from a geologic perspective, formed by tectonic movement <br /> <br /> <br />whereby the earth's crust pulled apart creating a north-south trench. The basement rock in this <br /> <br /> <br />trench is shallower on the west side of the valley where it is hinged and tipped downward toward <br /> <br /> <br />the east. On the east side of the valley near the Sangre de Cristo Mountains this basement rock <br /> <br /> <br />has dropped to depths of as much as 15,000 feet below present land surface. The downward <br /> <br /> <br />tilting of this basement rock easterly is not uniform. In the pulling apart process, a ridge <br /> <br /> <br />remained in the basement rock along a north-south direction near the center portion of the valley. <br /> <br />This ridge is referred to by geologists as the Alamosa horst. The deeper down faulted sections of <br /> <br /> <br />basement rock are called the Monte Vista graben and Baca graben. An east-west cross section of <br /> <br />the valley from a paper by Brister and Gries (1994) is included in Figure 1-2. Figure 1-3 shows <br /> <br /> <br />the location of the cross section in the valley. <br /> <br />As this trench expanded over tens of millions of years, it was filled by material eroded from the <br /> <br /> <br />nearby mountains encompassing the valley. Significant portions of the filled material that <br /> <br />accumulated in this trench entered from the San Juan Mountains while they were being built <br /> <br /> <br />through volcanic activity. Several fill beds extending across this valley trench from west to east <br /> <br /> <br />have been identified as volcanic flows. One of these flows, called ash-flow-tuffs, extend from the <br /> <br /> <br />surface on the western side of the valley and dips downward as it extends across the entire valley. <br /> <br />These ash-flow tuffs interbedded with sediments are found at depths of over 9,000 feet below <br /> <br /> <br />ground surface easterly of the Town of Hooper. <br /> <br />The top few thousand feet ofthe valley fill are rocks, gravels, sands and clays deposited by <br /> <br /> <br />erosion of the San Juan and Sangre de Cristo Mountains. Extensive erosion of these mountains <br /> <br /> <br />occurred as the ice age ended. As recently as 20,000 years ago, the high mountain valleys were <br /> <br /> <br />still partly clothed in glacier ice. Large stream and river flows out of the mountains carried the <br /> <br /> <br />sands and gravels found in the alluvial fans located around the edge of the valley and carried <br /> <br />1-4 <br />