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COLORADO WESTMORELAND INC. DocDate:12/11/2001 <br />ORCHARD VALLEY MINE <br />WATER REPLACEMENT PLAN <br />General Description IIIIII'II'IIIIIIIII <br />999 <br />REGIONAL GEOMORPHOLOGY <br />The Mancos shale and overlying Mesaverde formation of Cretaceous age are exposed <br />along the slopes of the valley of the Nortti Fork of the Gunnison River, Mesa- <br />verde outcrops extend inward from the river along all the major drainages in the <br />)ease area, ranging from isolated, vertical sandstone ledges to extensive expo- <br />sures of interbedded sands and shales, These beds dip 2" to 5` to the NNE. <br />South of the river, beyond the bordering Cretaceous strata, are eroded peaks of <br />an intrusive monzonite stock which reach elevations up to 11,000 feet, compared <br />to ridge elevations of 8,000 feet to 8,600 feet over most of the lease area. <br />North of the lease area, the topography rises to elevations exceeding 9,000 feet, <br />with remnants of the Wasatch formation of Ecocene age overlying the Mesaverde <br />formation. <br />Basaltic rubble consisting of subangular to subrounded boulders up to four feet <br />in dimension in a matrix of clayey soil covered the original post Ecocene topog- <br />raphy of the Wasatch remnants and all the associated erosional surface on the <br />Mesaverde formation at the level of the top of the ridges below the rubble in the <br />lease area. Volcanism of late Ecocene to Pliocene time which produced the basalt <br />_ flows of the Colorado plateau was the source of this rubble. There are volcanic <br />necks in the Wasatch area to the north and in the monzonite peaks to the south of <br />the lease area. <br />Homogeneity of the basaltic rubble, and the size and shapes of the large bouiders <br />indicates gravitational force, aided by water saturation of the sediments, moved <br />the blanket deposits from hearby highlands. This phase of development may have <br />been initiated in Pliocene time and accelerated during Pleistocene time, with <br />subsequent incision of steep walled canyons throuoh the rubble blanket into the <br />Mesaverde formation. Much of the original blanket cover migrated down the steep <br />slopes of the canyons as hummocky landslide deposits and partially filled the <br />deep channels. <br />As the bounteous water of the Pleistocene decreased, the landslide topography <br />became stable, and finer sediments were deposited in the lesser drainages, <br />including Steven's Gulch. The main streams of East Roatcap, West Roatcap, and <br />terror Lreek transport the finer sediments into the North Fork of the Gunnison <br />River Valley. Even to the present, however, the erosional history is one of <br />degradation and the sediments being deposited are mostly from sheet erosion, soil <br />creep and minor movement of soil doom the stream channels during spring run-off <br />and storm events. In recent time, minor amounts of fine sediments have been <br />deposited along gentle stretches of valley floors, where besaverde strata out- <br />crops have formed gentler slopes along upland drainages, and in isolated flat <br />areas, small catchment basins, and low gradient drainages in the landslide topog- <br />raphy.l <br />• further detailed descriptions may be found in Section 2.04.6, Underground Mining <br />and Reclamation Permit, Orchard Valley Kline. <br />1. Mitcheli, Michele D., Alluvial Valley Floor Study, 1984. <br />