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Gilcrest/LaSalle Pilot Project <br />Hydrogeologic Characterization Report <br />approximately 4,620 ft MSL. The valley floor elevation in Beebe Draw is approximately 4,800 ft MSL <br />where it enters the southeast corner of the Study Area and approximately 4,680 ft MSL at the <br />confluence with the South Platte River alluvial valley, near Lower Latham Reservoir. The South <br />Platte River alluvial valley generally slopes 10 to 20 feet per mile to the northeast across the Study <br />Area. The only relatively steep terrain in the alluvial valley occurs at the slopes (risers) separating <br />adjacent alluvial terraces. Wetlands and depressions related to subtle topographic variations are <br />shown on historic USGS topographic maps along with irrigation canals and drains. <br />Several alluvial terraces have been identified in geologic mapping by Colton (1978) and Smith <br />(1964). These terraces are typically identified by broad, flat, gently sloping surfaces interrupted by <br />risers above South Platte River. Surface topography is important with respect to shallow water table <br />conditions. Changes in surface topography above a shallow, gently sloping water table can bring the <br />land surface in contact with the water table. Of particular interest are locations at the foot of risers <br />between terraces such as west of LaSalle where the Lower Latham Drain follows the base of the <br />riser. Figure 6 illustrates the relationship between topography and shallow groundwater. <br />Primary Study Area Geologic Units <br />The bedrock formations listed below only outcrop in limited extent within the Study Area however <br />these units underlie the eolian and alluvial deposits <br />Laramie Formation (KI) — The upper part, 600-650 feet thick, is mostly gray claystone, shale, sandy <br />shale, and scattered lenticular beds of sandstone and lignite. The lower part, about 75 to 120 feet <br />thick, is light -gray to light yellowish -gray sandstone and sandy shale interbedded with clay, shale, <br />and several beds of coal. <br />Fox Hills Sandstone (Kfh) — The upper part consists of cross -bedded tan sandstone. Grades <br />downward into brown, fine-grained silty sandstone interbedded with gray fissile shale. Locally it may <br />contain thin coal beds. The thickness of this unit is about 300 to 500 feet. <br />Pierre Shale (Kp) —Marine shale mainly deposited in outer and deeper marine environments. The <br />Pierre Shale deposits are intercalated with shallow to coastal marine sediments (Dechesne et al, <br />2011). The contact between the Fox Hills Sandstone and underlying Pierre Shale is gradational <br />consisting of sandstone interbedded with shale and shale becoming more prevalent at greater depth. <br />Though the Pierre Shale does not outcrop in the Study Area, this formation comprises the lower- <br />most confining unit among the hydrogeologically significant strata present. <br />Overlying the bedrock is a series of unconsolidated geologic units that comprise the South Platte <br />alluvial aquifer in the Study Area. These geologic units influence the groundwater flow, aquifer <br />productivity, and groundwater levels. <br />7 <br />