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I I I I I I I I I I I I I I I I I I I 6. Groundwater. The groundwater depths are not known for sure at this time as no piezometers exist at the site. The reservoir had some water In It when the Investigations were conducted by Chen In 1979 and had been filled that year to gage height 20 feet. The water levels thus recorded In the borings would be expected to be higher than at the present time. The reservoir has been empty, except for the dead storage pool, for approximately five years and the water levels would not be expected to be very high at the time of construction. A.4. GEOLOGY J. Physiography. North Poudre ~eservolr No. 6 Is located at the northern portion of the Colorado Piedmont Section of the Great Plains Physiographic Province, east of the Southern Rockies Physiographic Province, and southwest of the Great Plains Escarpment. The Colorado Piedmont Is an elongated trough In the Great Plains, adjacent to the Front Range of the Southern Rockies which has been eroded due to uplift of the area In Miocene-Eocene times. Erosion has exposed bedrock stratum next to the foot hills and outlying areas. The Colorado Piedmont and Great Plains are separilted by the Great Plains Escarpment or Tertiary Escarpment near the Colorado-Wyoming border. Elevations In the Colorado Piedmont are In the order of 6,000 to 7,000 feet In this area. A variety of geologic processes have formed the present natural surface Including erosion and deposition due to wind, water, and mass wasting. Land forms created from these processes In this area Include dip slopes, colluvial slopes, pediment gravels, deflation basins, and alluvial and aeolian deposits. The folds of the bedrock strata In this area have had major influence on the topography of the site. 2. Stratigraphy. The stratigraphy of the site consists of the Upper Cretace- ous Pierre Shale overlain by colluvial, alluvial, and aeolian deposits. The Pierre Shale was deposited In a shallow-water marine environment from sediment derived from the land area to the west during late Cretaceous times. The Pierre Shale consists of shales, claystones, siltstones, and sandstones with a maximum thickness In this area of approximately 8,000 feet. Five sandstone units have been described in the Pierre Shale; III ascending order they are the Hygiene, Terry, Rocky Ridge, Larimer, and Richard Sandstone members. The Richard sandstone member Is the youngest of the sandstone members and outcrops near the surface west and east of Reservoir No. 2 but does not outcrop near Reservoir No.6. Quaternary deposits overlie the Pierre Shale with colluvial, alluvial, and aeolian deposits. The solis and subsoils encountered at the site are clays containing varying quantities of silt and sand and derived from the nearby bedrock stratum. The upper zones of the solis are primarily colluvial deposits with aeolian and alluvial processes Involved. Residual clays are located over the bedrock stratum. 3. Structural Geology. The reservoir lies on the western edge of the Denver Basin, a large structural basin (or asymmetrical syncline) which Is located from north of the Arkansas River, adjacent to the mountain front, to Just west of the Kansas and Wyoming borders. The Denver Basin was formed as a down warping of sediments during late Cretaceous and elilrly Cenozoic as a result of the Laramide PAGE ( Chapler V - No.6 l'euibUlty