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<br />III-2 <br /> <br />II <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 />I <br /> <br />Of these tributaries, only the Cache la Poudre heads in the Front Range, <br /> <br /> <br />with the three creeks rising within the Piedmont. Pleistocene glacially- <br /> <br /> <br />influenced deposition by the ancestral South Platte and the Cache la Poudre <br /> <br /> <br />formed several sand and gravel terraces along the modern streams. Due to <br /> <br /> <br />their high spring sediment loads, flash flood potential and sedimentary <br /> <br /> <br />drainage basins, the three creeks have built alluvial fans into the South <br /> <br /> <br />Platte Valley. The terraces and alluvial fans form the rich, nearly flat- <br /> <br /> <br />lying farmland along the South Platte, including Pleasant Valley, Kersey <br /> <br /> <br />Terrace and Green City Flats. <br /> <br />2. Geologic History <br /> <br /> <br />The geologic history of the rocks exposed on the surface in the vicinity <br /> <br /> <br />of the Hardin Dam site began in the late Cretaceous Period, approximately 80 <br /> <br /> <br />million years ago (Table III-I). At this time, a huge, shallow inland sea <br /> <br /> <br />covered most of the central United States, depositing a thick blanket of <br /> <br /> <br />calcareous mud. As this sea retreated toward the present Gulf of Mexico, the <br /> <br /> <br />near-shore sandbar and beach deposits moved southeastward over the earlier <br /> <br /> <br />muds. Behind the beaches were the brackish and fresh water lagoons, deltas <br /> <br /> <br />and swamps. The lagoonal mud, delta sand and silt and swamp peat were also <br /> <br /> <br />deposited further to the east, over the bar and beach sands as the beaches <br /> <br /> <br />retreated eastward. Later, terrestrial deposits--stream channel deposits, <br /> <br /> <br />alluvial fan deposits and mudflow deposits--were deposited over all of the <br /> <br /> <br />earlier units. <br /> <br />Near the end of the Cretaceous Period, the present Rocky Mountains <br /> <br /> <br />began to rise during a period of intense faulting, folding and volcanic <br /> <br /> <br />activity called the Laramide Orogeny. Erosion of the earlier Cretaceous <br /> <br /> <br />rocks began in the early Tertiary Period (Table III-I). The eroded material <br /> <br /> <br />was redeposited as gravel, sand, silt and clay in wide-spread lakes, in <br /> <br /> <br />stream channels and on alluvial fans during the middle to late Tertiary. <br /> <br />At the end of the Tertiary, the Ice Age began. Large alpine glaciers <br /> <br /> <br />covered the Rocky Mountains, growing and retreating as the climate cooled and <br /> <br /> <br />warmed several times. During the cold, dry cycles, runoff from glacier-fed <br />