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<br />0:183J3 <br /> <br />Geologv <br /> <br />Three phases of geologic landforming have occurred in the unit. The first phase involved the <br />depositing of sedimentary sandstone and shales. This process occurred about 70 to 135 <br />million years ago when the region was covered by sea. <br /> <br />The second phase occurred about three to 70 million years ago when the entire San Juan <br />chain was uplifed and repeated outpourings of lava and ash occurred from a cluster of <br />volcanoes. Now, these igneous materials cover the major portion of the unit. <br /> <br />The last phase of the landforming process occurred during recent geologic time (3 million <br />years ago to the present). The landscapes during this period have been shaped by the forces <br />of erosion, glaciation, and the accompanying deposits of glacial till, flood plain alluvial <br />material, and landslides and other mass movements. <br /> <br />The areas where materials from these phases of geologic development occur are shown on <br />the following map: <br /> <br />I <br />