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_, s R. J. Irish Consulting Engineering Geologist, Inc. the right abutment topographically trends approximately parallel to the trend of the adjacent. section of the Arkansas River Valley to the west, end is about 800 to 1000 feet wide. The reservoir, given a maximum water surface elevation of about 7930 feet, would extend northeastward and eastward upstream about 2400 feet. For most of that distance it would be about 250 to 300 feet wide. Palley slopes on both sides of the reservoir area are steepl y sloped where bedrock is exposed, but shallowly sloped where the bedrock is blanketed by slopewash and residual soils. SITE GEOLOGY Trout Creek drai ns a sector of the eastern slope of the Arkansas River Valley. That valley topographically evidences the structural extension ofthe Rio Grande Rift,a large graben structure that extends northward from Mexico through west Texas, New Mexico and central Colorado almost to the Wyoming border. In the vicinity of Buena Vista, the down-faulted block that constitutes the graben cuts northward across, and displaces downward, a section of the eastern flank of the Sawatch Mountains uplift. It is bounded on the west by the north-trending faults of the Sawatch fault zone, and on the east by a swarm of north-trending faults that generally are unnsmed. The graben block itself probably is broken by several major, north-trending faults. Itis about 5 to 6 miles wide, end its floor is thought to be about 4000 feet below the present topographic floor of the Arkansas Valley. Faulting to form the rift zone end the graben was initiated in the early Miocene Period about 20 to 25 million years ago, and fault movement is judged by some geologists to continue sporadically into the present time. the graben has been partly refilled bystream-deposited sediments, mainly the interbedded clays, silts, sands and gravels of the Miocene/Pliocene-age { 25 million to 2 million years ago) Dry Union Formation. These sediments locally are blanketed by alluvial fan soils deposited mainly by streams draining eastward from the Sawatch Range, but also by morainal end outwesh sediments derived from glaciers that repeatedl y extended out iota the Arkansas River Valley during the Ice Age from those valleys. The last of those glaciers melted away about 8000 to 10,000 years ego. In fact, the granitic bedrock ridge that forms the right abutment of the planned Trout Creek Dam is covered by glacial outwesh and glacier-derived alluvial soils of Nebraskan(?)-age {about 1 to 2 million years ago} to within about 700 feet of the planned damsite. At the damsite and within the reservoir area, which lie structurally on the eastern flank of the Sawatch Uplift east of the graben breach of that flank, granodiorite constitutes bedrock. A strong, hard member of the granite family of intrusive rocks, it faces the gorge cliffs of the Trout Creek Valley at the damsite and most of the valley slopes that would be inundated by the reservoir wafer. Locally, however, it is veneered across the reservoir-bounding slopes by slopewash end residual