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1. .=b STATE OF COLORADO DIVISION OF MINERALS AND GEOLOGY DupartmeN n( Natural Resources 1313 Sherman 5t., Room 215 Denver, Colorado 80203 Phone: (3U3~ R66-3567 Fn%: (3031 872-5 106 interoffice MEMORANDUM to: from: subject: date: Reference: Janet Binns yS r G. Walker Grassy Gap Inspection June 30, 1998 Memo, Binns to Walker, June 11, 1998, same subject llEPARTMENT OF NATURAL RESOURCES Roy Romer Governor James S. Lochhead Executive Director Alichael B. Lonq Division Direcor At your request, I accompanied you on an inspection of a slide at Grassy Gap. The inspection was made on 16 June 1998. Weather was overcast. The purpose of the inspection was to determine if there was a relationship between a sedimentation pond and the slide. I suspect there is a relationship between the two. The pond is located at the crest of a steep slope (approximately 35 °, or 70%). It is located directly above the slide inspected, and a previous but not so extensive a slide. Relative locations are shown in Pictures 3, 4 and 5. An overview of the site is provided in Picture 2. There was some water in the pond. The water surface was below the primary spillway riser. This portion of the mine is on the west flank of anorth-plunging anticline. The formations likely dip approximately 14° to 16° to the west in the vicinity of the slide (Permit, page II.D-5). These beds outcrop under a thin mantle of soil and talus debris where they intersect with the 70% slopes of the valley. There are indications of water seepage at such intersections at the elevation of the crest of the slides. Note the vegetation line on Picture 2 extending to the south around the nose of the of the ridge. In addition, there is an unidentified linear feature that runs to the north from the crest of the slide {see Pictures 1 and 2). This linear feature might have been a ditch (now obliterated by in-filling), capturing spring water from the precise location of the slide crest. Inherent seepage is also inferred by the seepage in the slide area at the time of the inspection (see water seepage in Picture 6). The pond is located directly above the slide area and close to the edge of the steep slopes. The pond extends across the area (north to south) of both slides. The older of the slides, the less extensive one, is located to the north (left in Picture 3} of the one that resulted in the inspection. The tree growth located below the north end of the pond in Picture 3 locates the crest area of this