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OUGHt B< Geology The rocks underlying the study area are in three area groups: the sedimentary deposits of the Little Snake River Drainage, the pre-Cambrian rocks of the Sierra Madre Mountains, aed the sedimentary deposits of the Jack Creek Drainage. The south and west portion of the study area in the Little Snake River Drainage is underlain by a sequence of sedimentary deposits consisting of marine and continental sandstone, s~ales, mudstones, and marlstones. These sedimentary deposits become as thick as 16,000 feet near Baggs. The sedimentary rocks vary in age from Cambrian to Miocene. In addition to and within these sedimentary deposits near the Wyoming-Colorado border, occur a series of basaltic dikes and sills, which intruded into the area in the late Miocene age. The most dominant bedrock in this portion of the study area is the Browns Park Formation. The Browns P~rk Formation is an eolian deposit of sand, silt, and volcanic ash, early to Middle Miocene in age, and is as thick as 1,800 feet. Numerous domestic and stock wells are developed in the formation. The Haystack Formation of the Mesa Verde Group and the underlying Lewis Shale are two significant formations for this study in the Little Snake River drainage area. The Haystack Formation, consisting of sandstones and shales, and at times the overlying Browns Park Formation, will slide on the contact of the Lewis Shale under adverse ccnditions. Several stream valleys, such as Savery Creek, have large, massive landslides developed alon2 the walls and floodplains. The potential effects of these slides were considered in the evaluation of several dam sites. The uplift which formed the Sierra Madre uplift ~long with the erosion of sedimentary rocks has exposed pre-Cambri~n rocks. The pre-Cambrian rocks in the study area are 3 variety of igneous. metamorphic, and metasedimentary rocks. The igneous rocks in the study area include granites. volcanic rocks, gabbros, 1II-2