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Subsidence and Geologic Field Observations <br />Apache Rocks and Box Canyon Mining Areas <br />July 12-15, 2004 <br />2.3 Desiccation Cracks <br />Desiccation cracks tend to occur in claystones and siltstones of the Mesaverde and Wasatch <br />Formations in the West Elk mining azea where the rocks aze weathered to clays and silts. The <br />process of desiccation involves the shrinking of the clays and silts after a dry period that follows <br />a wet period, when the material swells (i.e. the shrink/swell process). <br />Desiccation cracks can often be recognized by their irregulaz, branching and diverging pattern- <br />less regular than typical subsidence cracks. Some of the lazgest desiccation cracks in the West <br />Elk mining azea were observed in clays of the Barren Member of the Mesaverde Formation in the <br />Horse Gulch-Minnesota Reservoir azea where there had been no mining. The larger, more <br />regulaz 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 longwall mining panel causing the cracks. <br />2.4 Pseudo Subsidence Cracks (Gravity-Induced Tension Cracks) <br />Cracks have been observed on high, steep ridges, near cliffs, and in landslides, in the Box <br />Canyon Lease Tract and Apache Rocks Lease Tract. The cracks look very much like subsidence <br />cracks, but they cannot be, because no mining had been done in the azea. The extensive crack on <br />a narrow ridge on West Flatiron, which was as much as 3.5 in wide and 150 feet long on August <br />27, 2002, is a good example of agravity-induced crack (see pages 11-12 in the 2002 report). It <br />was easy to determine that this was not amining-related crack because no mining had occurred in <br />the area. The possibility of gravity-induced cracking in the rugged county above planned mining <br />activities at West Elk Mine is a good reason to perform baseline studies of the azea prior to <br />mining so that these features can be documented. <br />831-032.640 Wright Water Engineers, Inc. Page 6 <br />September 2004 <br />