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l • Sandstone, the Mesaverde formation consists of interbedded weakly to moderately cemented fine- <br />grained sandstone and silty clay shale layers. Some economically important coal beds are found <br />in the lower Mesaverde Formation. Most of the slopes above the Rollins Sandstone, where not <br />covered by landslide material, expose formational rock with little colluvium or soil cover. Rock <br />slopes near the project area vary from somewhat steeper than 1:1 to about 2H:1V. <br />The ancient landslide mass (Lsl) is of unknown age, but has mature physiographic <br />features, with no significant hummocky topography, well-defined and oversteepened sides, no <br />discernable head scarp or toe bulge, and no tension crack features that would be expected of an <br />active landslide. The ground surface along the axis of the slide has a slope of about 3 to SH:1 V. <br />The ancient slide was probably triggered by a strong earthquake and flowed as a viscous mass <br />on the relatively weak shale beds above and below the Rollins Sandstone and moved for a <br />distance of about 1/4 mile during a time of a much wetter climate. This is evidenced by the <br />• matrix of fine to coarse fragments of the softer shale and coal components of the Mesaverde <br />formation and large boulders of sandstone found at unpredictable orientations throughout the <br />deposit. Since the last major movement, the slide mass has drained and consolidated into a <br />highly overconsolidated clay matrix surrounding scattered to numerous boulders. The base of <br />[he slide is thought to contain a more permeable bouldery rubble, as evidenced by drilling water <br />circulation losses in the deeper holes, and the almost uncanny lack of historic surface water Flow <br />in the side drainages to the slide. The slide was found to be in excess of 70 feet thick by the <br />three borings that penetrated the mass. <br />A major, recent rock slope failure (Ls2) is located outside of the project area, just to the <br />east of the portal area. This slope failure has anear-vertical head scarp estimated to be about <br />. ~ 6 <br />