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Fall 2007 Subsidence and Geologic Field Observations <br />Box Canyon, Apache Rocks, and South of Divide Mining Areas <br /> <br />:. ~ w y. ~ c <br />Y •~~ r .1 yam. .,.yj ~ • ~.~~ <br />i 111 ` 1. Wit'' aj~~, t ; {-{4. l+i.. ~~ ~` r~ . <br />iJ 4• + t L`4~Ir~ .. ~ J .~. <br />' .}, <br />~4\~-~t ~ y C! t~ - f~ ~, <br />'~ .. <br />4 r- ,n <br />Figure 3. Combination desiccation crack and gravity-induced crack, <br />located in weathered claystone of the Wasatch Formation above <br />unmined coal. This irregular crack, which is as much as 1.5 inches <br />wide and is roughly parallel to the west cliff of West Flatiron, is most <br />likely above a gravity-induced crack in the massive Ohio Creek <br />underlying sandstone. The crack also appears to be enhanced by the <br />shrinking of the weathered claystone. <br />Drsiccution cracks can atten he recognized by their in•egular, branclung, and diverging pattern, <br />less regular than typical subsidence cracks. Some of the largest desiccation cracks in the West <br />Elk Mine area were obset-~~ed in clays of the Barren Member of the Mesaverde f=orn~ation in the <br />Horse Gulchr'Minnesota Reservoir area where there had been no mining. The larger, more <br />regular desiccation cracks and construction cracks may be contused with subsidence cracks in <br />areas where mining has occun•ed. However, transverse and longitudinal tension cracks caused by <br />subsidence have a dctinite spatial relationship to the geometry of the lungwall mining panel <br />causing the cracks. <br />• <br />831-032.780 Wright Water Engineers, Inc. Page 7 <br />November 2007 <br />