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Spring 2009 Subsidence and Geologic Field Observations <br />Box Canyon, Apache Rocks, and South of Divide Mining Areas <br />V <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 typical subsidence cracks. Some of the largest desiccation cracks in the West <br />Elk Mine area were observed in clays of the Barren Member of the Mesaverde Formation in the <br />Horse Gulch /Minnesota Reservoir area where there had been no mining. The larger, more <br />regular desiccation cracks and construction cracks may be confused with subsidence cracks in <br />areas where mining has occurred. However, transverse and longitudinal tension cracks caused by <br />subsidence have a definite spatial relationship to the geometry 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 SOD mining areas (Figure 4). The cracks look like <br />subsidence cracks but are found in areas where no mining has occurred. The extensive crack on <br />a narrow ridge on the northern part of West Flatiron (as much as 3.5 inches wide and 150 feet <br />• long on August 27, 2002), is a good example of a gravity- induced crack (Dumud 2002). The <br />831 -032.791 Wright Water Engineers, Inc. Page 8 <br />November 2009 <br />