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Gilcrest/LaSalle Pilot Project <br />Hydrogeologic Characterization Report <br />Slope Wash Deposits (Qsw) — This unit is a modification based on mapping by Smith (1964) who <br />describes slope wash as consisting of gravel and sand interbedded with clay and silt feathering out <br />against upland areas, but lapping onto, and interlayering with, stream deposited valley fill deposits. <br />Slope wash deposits are likely mobilized from uplands by precipitation resulting in sheet flow events <br />and deposited below on gentler slopes. <br />Unnamed 3rd Level Terrace (Qt3) — This unit is based on physiographic evaluation of the Study <br />Area and is mapped by Smith (1964) as present in the western and southwestern portion of the <br />Study Area. <br />Eolian Deposits (Qe) — (windblown clay, silt [loess], and sand) Light -brown to reddish -brown to <br />olive -gray deposits of windblown clay, silt, and sand mainly as sand dunes in the east half of the <br />area but also as a blanket of loess between the Front Range and the South Platte River. Loess is as <br />much as 15 feet thick but generally is less than 3 feet thick; sand dunes are as much as 50 feet thick <br />but generally are less than 15 feet thick. <br />Post -Piney Creek Alluvium (Qpp) — Dark -gray humic, sandy to gravelly alluvium. This unit <br />underlies flood plains of major streams and terraces less than 10 feet above stream level. Thickness <br />is from 5 to 15 feet. <br />Piney Creek Alluvium (Qpc) — Dark -gray humic sandy to gravelly alluvium containing organic <br />matter. Underlies terraces whose surfaces are 10 to 20 feet above a nearby flood plain. Areas <br />underlain by this formation along the South Platte River were partly flooded in 1965, again in 1973, <br />and very likely in 2013). <br />Broadway Alluvium (Qtb) — Sand and gravel deposited by the South Platte River and its tributaries. <br />Well -sorted and well -stratified sand and fine gravel. Along the South Platte River, Broadway <br />Alluvium is as much as 125 feet thick but averages approximately 35 feet thick. <br />South Platte Alluvial Vallev Surficial Deaosits <br />The South Platte alluvial aquifer consists of Quaternary -age unconsolidated alluvial deposits filling a <br />paleo-channel incised into Upper Cretaceous -age mudstones and sandstones of the Laramie <br />Formation and Fox Hills Sandstone. The alluvial aquifer deposits consist of gravel, sand, silt and <br />distinct silt and clay layers generally deposited by flowing water. These deposits are comprised <br />primarily of material from the eroding mountains to the west where the main rivers and streams <br />originate. However, local ephemeral streams and slope wash contribute material eroded from the <br />mudstone-dominant Laramie Formation and coarse-grained material from older high terrace <br />deposits. <br />N. <br />