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_z- <br />Regional Geology <br />The San Luis Valley is a north trending structural depression bounded by <br />mountain ranges. The valley has a maximum fill of 30,000 to 36,000 feet of <br />material consisting of alluvial fan gravel, volcanic debris and interbedded <br />basaltic lava flows. The valley is approximately 150 miles long and 50 miles <br />wide flanked by the San Juan Mountains volcanic) on the west and the Sangre <br />de Cristo (Paleozoic - Precambrian) on the east. <br />The San Luis Valley is a part of the much larger Rio Grande Rift zone which <br />can be traced from Mexico through the upper Arkansas Graben and possibly past <br />Leadville to the Wyoming border through a series of en-echelon north trending <br />faults. The geology along the Rio Grande Rift zone is very similar to the <br />Basin and Range province. The zone is probably caused by differential <br />movement of crustal plates resulting in normal faulting and extensive <br />volcanism. <br />Rifting in this area began approximately 18 million years ago with concurrent <br />volcanism and hot spring activity all continuing to the present. Structural <br />relief is greatest along the eastern side of the San Luis VAlley with the <br />highest mountain peaks and deepest part of the structural depression. This <br />can be explained by the thicker portion of the crust riding upon the upward <br />bulge of the mantle which protrudes into the thinned section of the crust <br />within the San Luis Valley. The crustal plates are apparently moving westward <br />at differing rates on either side of the Rio Grande Rift zone. The area is <br />still technically active as evidenced by truncated spurs, aligned mountain <br />Eront, and fault scarps in recent alluvium. <br />The San Luis Valley is divided into 5 distinct physiographic subdivisions. <br />The Alamosa Basin comprises the main northern part of the San Luis Valley. It <br />is roughly triangular in shape bounded by the San Juans on the northwest, the <br />Sangre de Cristo on the northeast, and a line from the sand dunes Co Antonito <br />on the southwest. The basin floor is generally flat gently sloping inward in <br />all directions to the low point near the Sangre de Cristo mountains. The area <br />