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Gilcrest/LaSalle Pilot Project <br />Hydrogeologic Characterization Report <br />The topography and unconsolidated sedimentary deposits of the South Platte River Basin record a <br />gradual progression of incision overprinted by cycles in alluvial sediment supply. These cycles are <br />associated with periods of glacial advance and retreat (Lindsey and others, 2005) and have resulted <br />in a series of terraces flanking the modern stream course. The lowest terraces lie closest to the river <br />and are youngest; older terraces step upward in elevation above the youngest. Many of these <br />Quaternary terraces are identified in the Study Area (Figure 3). Upper Holocene Post -Piney Creek <br />alluvium fills the lowest floodplain of the South Platte River with the next step up to Upper Holocene <br />Piney Creek alluvium (Colton, 1978). Above the Piney Creek alluvium and covering the largest part <br />of the alluvial aquifer in the Study Area, is a terrace identified by Colton (1978) and Smith (1964) as <br />Pleistocene -age Broadway alluvium. Smith (1964) also identified an intermediate terrace between <br />the Piney Creek and Broadway terraces. The alluvial valley topography indicates another possible <br />terrace, above the Broadway terrace, just south and west of Gilcrest. Smith (1964) identified this as <br />the "3rd Unnamed Terrace" as shown in Figure 3. All of these terraces are believed to overlie and be <br />hydraulically connected with the alluvial aquifer. <br />Colton (1978) also mapped several isolated occurrences of older Slocum, Verdos and Rocky Flats <br />alluvium on uplands above the South Platte valley and Beebe Draw. These higher gravelly deposits <br />contribute coarser grained material to side tributary wash and slope wash deposits flanking the <br />southeastern edge of the main South Platte alluvial valley and are likely to have only limited <br />hydraulic connection, if any, with the main alluvial aquifer. <br />Locally, eolian sand and loess blanket both alluvium and bedrock. In other places, slope wash <br />deposits consisting of fine-grained sediments spill from hillsides across lower terraces. Eolian and <br />slope wash deposit thicknesses are generally 20 feet or less; yet, these deposits can conceal the <br />contacts between adjacent terraces as well as contacts between alluvium and bedrock. On uplands <br />covered with permeable deposits, a thin layer of alluvium or eolian sediment above less permeable <br />bedrock of the Laramie Formation exists and can locally be saturated, however this is not <br />considered to be a significant aquifer in the Study Area. <br />Bedrock Topography and Geolo <br />Figure 7 shows the bedrock surface elevation contours with bedrock subcrop geology concealed <br />beneath Quaternary valley fill deposits. Contouring of the bedrock surface accommodates 309 data <br />points in the Study Area with a refined conceptual model and greater detail than previous <br />investigations. The refined conceptual model identifies individual incised tributary paleochannels <br />within a broad paleovalley now filled with alluvial deposits. Bedrock formations in the Study Area <br />have only limited exposure due to a blanket of eolian deposits in the uplands and valley fill deposits <br />in the alluvial valley. As seen in Figure 7, the bedrock surface topography underlying the alluvial <br />9 <br />