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_, <br />R. J. Irish <br />Consulting Engineering <br />Geologist, Inc. <br />soils, mainly silty to clayey, gravelly, cobbley sands that appear to be thin generally (on the <br />order of inches to 10 feet thick}. The ground surfaces underlain by those soils typically are <br />sloped at about 2:1 or shallower. The gorge section bends from a southeastern trend at the damsite <br />to esouth-southwestern trend about 100 feet downstream from the damsite to the mouth of the <br />valley about 600 feet farther downstream. <br />At the damsite (Figs. 2 end 3} the granodiorite is chiefly medium to coarse grained to <br />pegmatitic, end porphyritic, but some of the rock on the southwestern side of the gorge is fins io <br />medium grained and porphyritic. Incidentally, the term "porphyritic" means that larger crystals <br />(phenocrysisl of the rock seemingly float in a matrix of smaller crystals. Dikes of pegmatite <br />{very coarse grained granitic rock) lace the granodiorite, particularly across the northeastern <br />two-thirds of the floor of the gorge at the damsite; thin quartz dikes are common; and 2 thin dikes <br />of dacite, a very fine-grained igneous rock minerelogicellyaquivalent to granodiorite, have been <br />mapped in the reservoir area. The intergranular bond of the minerals that make up the pegmatite <br />is much weaker then the bond of the minerals that make up the finer grained grenodiorites, and <br />some iron oxide staining is apparent on crystal faces of some of the pegmatite. No faults were <br />discovered, although boring DH-M-2 penetrated sheared rock from 42 to 45.5 feet. This may <br />mark the contact between the finer grained and the coarser grained granodiorite. The contact is <br />likely to trend northwestward at an high angle through the damsite. <br />Trout Creek appears to have stripped away any weathered rock during the creation of the gorge <br />section. The granodiorite facing the rock walls there is essentially fresh, although iron oxide <br />typically veneers the surfaces of fractures and joints that constitute the rock discontinuities. <br />Those discontinuities, generellywidely spaced--on the order of 5 to 20 feet apart--form several <br />sets. The joints of the most prominent of these sets at the damsite strike N. 05 W. to N. 05 E., N. <br />30 E., N. 65 W., N. 50 to 55 W., and S. 85 W. These are near-vertical structures that dip 75 <br />degrees or more to one side or the other of the stri kes, end change commonl y from one side to the <br />other along strike. Additionally, near horizontal joints spaced 10 to 20 feet apart disrupt the <br />granodiorite. At the damsite, most of these di p upstream. <br />At the damsite on both sides of the gorge many of the mein near-vertical joints are open as <br />much as 5 to 12 feet wide. Predominantly these ere the joints of the N. OS E. to N. 05 W.- <br />trending set on the left abutment end of the northwest-striking sets on the right abutment. These <br />were scoured out by Trout Creek as it cut down through the granitic rock to create the gorge <br />section, and possibly by early Ice-Age floods that topped the right abutment ridge and deposited the <br />Nebraskan{?}-age alluvial soils there 1 to 2 million years ego. Those soils may have filled the <br />Trout Creek Valley upstream from the damsite, causing Trout Creek to overflow its gorge- <br />bounding ridges. At any rate, the rock along these joints, which are typically 10 to 30 feet apart, <br />is not full y i ncised, thus open, through either ridge, although the stream-cut re-entrants are <br />deep, both horizontally and vertically, on both sides of the ridges, but particularly well developed <br />3 <br />