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I I I I I I I I I I I I I I I I I I I A.4. GEOLOGY 1. PhvsiolZraohv. The existing diversion structure 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 Culorado 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 foothills and outlying areas. The Colorado Piedmont and Great Plains are separated by the Great Plains Escarpment or Tertiary Escarpment near the Colorado-Wyoming border. Elevations in the Colorado Piedmont are on the order of 5,000 feet to 7,000 feet in this area. A variety of geologic processes has 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. StratilZraphv. The GEOLOGIC MAP OF COLORADO by Ogden Tweto indicates stratigraphy of the site consists of the Upper Cretaceous Pierre Shale overlain by young gravels and alluvium of the Piney Creek Alluvium and younger. Adjacent to the site are gravels and alluvium of the Pinedale and Bull Lake Age including Broadway and Louviers Alluvium. The Pierre Shale consists of shales, claystones, siltstones, and sandstones with a maximum thickness in this area of approximately 8,000 feet. Five (5) sandstone Units have been described in the Pierre Shale; in 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 in some areas of Larimer County. Fox Hills Sandstone is found near the site but is not found in the Poudre River floodplain in the site area. 4. Mineral and Enerl!V Resources. Northeastern Colorado contains various economical mineral resources including petroleum, natural gas, uranium, gravel and crushed-rock ag- gregate, and limestone. This project will not impact any of these resources as no expansion of the project site is planned. A.5. SEISMOLOGICAL CONSIDERATION 1. General. The seismic risk zones published by the Uniform Building Code indicates Colorado is in the Zone 1 category which indicates minor seismic risk corresponding to Modified Mercalli Intensities V to VI. Seismicity maps produced for the U.S. Geological Survey show the site to be located in a zone which has a maximum potential Intensity VII. PAGE5.CbapcerV