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e rises 130 ft above river level. Sandstone and shale form the lower portion of the north canyon wall. South of the river, the bank rises gradually to about 70 ft above the channel in a distance of 850 ft. At this point, a steep slope, the remnant of the old south wall of the canyon, rises for 90 ft to a level bench. Mapping did not reveal any bedrock at the surface on the south river bank. e Within the old river channel, the lower river gravels are covered by alternating layers of debris-flow deposits and a thin layer of river alluvial-terrace gravel. The debris-flow deposits consist of basalt gravel and boulders, mixed fines, and cobbles and boulders of sedimentary rocks. The interbedding of debris-flow deposits and layers of rounded river gravel indicates that periodic debris flows were depositing material faster than the river could move it downstream; therefore the river channel was being filled. Additional debris-flow deposits, consisting mostly of subangular basalt cobbles and boulders, lie over the above-mentioned mixed layer of debris-flow deposits and river gravel. They cover much of the ridge north of the river and the uplands south of the river. Debris flows most likely blocked the river at this location several times through geologic history. No faults were observed in the Wasatch Formation during the mapping, and none are shown on the published geologic maps of the vicinity of the site. The nearest faults along the perimeter of the Piceance Basin recorded in published geologic literature are along the eastern edge, 14 to 16 mi northeast of the site. The nearest recorded fault within the basin is approximately 20 mi north. No documented recent earthquake activity has been associated with these faults. e C. FIELD INVESTIGATION A drilling program was conducted along the proposed dam axis, and six of ten test holes were successfully completed. The locations of the test borings are shown on Drawing No. 2 in ATEC's report (Appendix A). 3686-a/8 IV-2