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Spring 2008 Subsidence and Geologic Field Observations <br />Box Canyon, Apache Rocks, and South of Divide Mining Areas <br />• <br />• Ocsiccation cracks can often be recognized by their irregular, branching, and cliwi-111L, p.lttern, <br />ICY, regulau- than typical subsidence cracks. Sonic o1' the largest desiccation cracks in the `Vc,t <br />f=,lk Mine aura were ohstrvtd in claw of the Barren Member ol' the Mesaverde Formation in the <br />Fiume Gulch/Minnesota Reservoir area where there had been no minino. The ktr,_,cr. more <br />rCLILIku- desiccation cracks and construction cracks nlay he confused %vith suhsidenct cracks in <br />areas where mining has occurred. However, transverse and longitudinal tension cracks caused by <br />I'll hsidtnce have a definite spatial relationship to the geonletry of the luno\vall minim, panel <br />lu?lll? tilt cracks. <br />2.4 Gravity-Induced Cracks <br />Gravity-induced cracks have been observed on steep ridges, near cliffs, and in landslicles within <br />the Box Canyon Apache Rocks. and SOD mining areas (Figure 4). The cracks look like <br />suhsiclenct cracks Nut are found in auras %vhcre no nlinin T hats occurred. The extensive crack on <br />a narrow ridge on the northern part of West Flatiron (as much ati 3.5 inches wide and 150 feet <br />Ions- on AMILIst 27. 2002). is a _100d example of a gravity-induced crack (Dunrud 2002). The <br />is 831-032.790 Wright Water Engineers, Inc. Page 7 <br />August 2008 <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 underlying Ohio <br />Creek sandstone. The crack also appears to be enhanced by the <br />shrinking of the weathered claystone.