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The upper surface of atp terraces is mantled by 2 to 4.5 feet of medium <br />• brown silt containing a fe w pebbles and cobbles. These deposits are interpreted as <br />slopewash and colluvium. The larger clasts are thought to be derived from the <br />alluvium below. The underlying alluvium represents a variety of fluvial environments <br />including point bar deposits, channel sediments and overbank deposits of fine sand <br />and silt. The best exposure of the alluvium -bedrock contact is in the "riser" of the <br />terraces where this contact is regular, and in places, marked by a spring line. Where <br />observed, the bedrock-alluvium contact in the ~t2 terrace was above the atl <br />terraces. Clearly, there is not a hydrologic connection between these bodies of <br />alluvium. <br />The Qt3 terraces range from 35 to 85 feet above Trout Creek. Their <br />topographic expression is markedly less distinct than the lower terraces and the <br />alluvium that comprises them is more weathered and has a higher proportion of fines. <br />At least some of these terraces are composite and all contain a mantle of slopewash- <br />colluvium similar to that described above. <br />The Qtq terraces range from 65 feet above Trout Creek in the upper reaches <br />of the study area to over 100 feet above the creek at the lower end. This mapping <br />unit groups together several different levels of terraces. Their morphology is more <br />• subdued than the younger terraces and some have been breached by erosion, <br />producing o less continuous sequence than the lower terraces. The alluvium in these <br />terraces is more weathered and locally contains a significant proportion of clay. At <br />least one of these terraces, near The center of Section 36, was found to be <br />composite, while others, in Section 24, were clearly alluvial terraces. Evidence of <br />groundwater in Qt3 and Qtp terrace alluvium was not found. <br />Alluvial fans, Qaf, have been mapped based on morphology and geomorphic <br />context. Although no datable materials were found and terraces correlations to the <br />glacial sequences in Upper Trout Creek were not made, a preliminary interpretation <br />of the geomorphic history, consistent with the local deposits and The history of the <br />Colorado River Basin, can be made. During late Tertiary time (approximately seven <br />million years ago) the final broad uplift of the Rocky Mountain region occurred. <br />These events defined the gross configuration of the landscape. Streams in uplifted <br />terrains were rejuvenated and widespread erosion followed. A 9 to 14 million year <br />old basalt that underlies the Flat Tops, in the head waters of Trout Creek, was eroded <br />and boulders of this material deposited throughout the drainage. During The <br />Quaternary, episodic glaciation provided abundant glaciofluvatile sediments and <br />• meltwater. Periods of glaciation resulted in valley filling with outwash. Inier- <br />2.5-63 <br /> <br />