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the Coal Creek site the region is composed of parallel ridges and vall eys ~ the 1 ess resi stant rocks form the vall eys, and the stronger rocks form the ridges. The oldest rocks in the region, Precambrian metamorphic and igneous rocks, crop out west of the Coal Creek site. These are overlain in turn by Mesozoic era formations of the Fountain Formation from the Pennsyl vani an/Permi an age, Lyons Sandstone from the Permi an age, the Lykins Formation of Triassic age, and the Dakota Formation and Pierre Shale of the Cretaceous age. Rocks ranging from Precambrian to Late Cretaceous age crop out along Coal Creek; however, only the Fox Hills Sandstone and Laramie Formation are present at the proposed Coal Creek site, and underlie a large portion of the area. All the coal mined in the area has been obtained from the Laramie Formation. Deposits interpreted as late Pliocene or early Pleistocene include pediment gravel on Rocky Flats. The Rocky Flats gravels contain coarse sand deposits of quartzite, schist, gneiss, granite, pegmatite, sandstone, and siltstone. Terrace gravels occur on benches immediately below the upland gravel. A deposit of cobble gravel lies along the northwest side of South Boulder Creek, and deposits of alluvial gravel are found along Coal and South Boulder Creeks. The Piney Creek alluvium fill is present in the flood plain of South Boulder Creek and part of the flood plain of Coal Creek. The Crescent site is mountainous and is underlain by Precambrian igneous rocks. Upturned Paleozoic and Mesozoic rocks form the Flatirons and hogback ri dges to the east. South Boul der Creek and its tri butari es dissect these rocks and form steep-walled canyons to a depth of 800 feet. The dam would be built in an area of Precambrian-aged Boulder Creek granodiorite. Precambrian-aged quartz monzonite is also present. Geologic Stability There are three major faults in the area, the Valmont Fault northeast of Boulder, the Golden Fault, and the fault near Morrison, that show evi dence of di spl acement more than 1 mi 11 i on years ago to 1 ess than 10,000 years ago. Only one event of Quaternary movement could be confi rmed for each of the faults. Comparable movements today coul d cause earthquakes. Colorado has had high-magnitude earthquakes in the Quaternary period. Earthquakes in Colorado in historic times have caused only minor damage to buil di ngs or mi nor rocksl ides. There is no hi stori c record of an earthquake in Colorado having an estimated intensity higher than VII on the modified Mercalli scale, which indicates very strong movement. 6