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Fall 2008 Subsidence and Geologic Field Observations <br />Box Canyon, Apache Rocks, and South of Divide Mining Areas <br /> <br />r? <br />16 <br />it - . ?!+Yf,? ?. w•. r _._ <br />Figure 3. Combination desiccation crack and gravity-induced crack, located in weathered <br />claystone of the Wasatch Formation above unmined coal. This irregular crack, which is as much <br />as 1.5 inches wide and is roughly parallel to the west cliff of West Flatiron, is most likely above a <br />gravity-induced crack in the massive underlying Ohio Creek sandstone. The crack also appears to <br />be enhanced by the shrinking of the weathered claystone. <br />Desiccation cracks can often be recognized by their irregular. branching. and diverging pattern, <br />less regular than t\ pical subsidence cracks. Some of the largest desiccation cracks in the West <br />lAk Mine area were observed in clays of the Barren Member of the Mesaverde F ornuttion in the <br />f lorse Gulch'NAinnesota Reservoir area where there had been no mining. The larger, more <br />regular desiccation cracks and construction cracks nuty be confused "ith subsidence cracks in <br />areas where mining has occurred. I lowever, transverse and longitudinal tension cracks caused b\ <br />subsidence have it definite spatial relationship to the ,eonictr\ of the longwall mining panel <br />causing the cracks. <br />2.4 Gravity-Induced Cracks <br />Gravity-induced cracks have been observed on steep ridges. near cliffs, and in landslides within <br />the Box Canyon Apache Rocks, and SOI) mining areas (Figure -f). The cracks look like <br />suhsidence cracks but are found in areas «here no mining has occurred. The extensive crack on <br />a narrow ridge on the northern part of* Wcst Flatiron (as much as 3.? inches wide and 150 feet <br />• I0111-1 (M :\uI-rust '7. IN'). is a Lood e\amhle (,f ci ray its -induced crack (Dunrud I he <br />831-032.790 Wright Water Engineers, Inc. Page 7 <br />December 2008