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Fall 2009 Subsidence and Geologic Field Observations <br />Box Canyon, Apache Rocks, and South of Divide Mining Areas <br />• <br />• <br />5.5.2 Location 5 <br />This location is approximately 400 feet north of Location 4 and on the east side of West Flatiron. <br />As shown in Figure 20, this site is where several of the cracks, previously observed in the MDW <br />drill pad (Location 4 of this report), project into the eastern sandstone cliff of West Flatiron. <br />West Flatiron is a flat topped mesa nearly completely capped at its uppermost surface with <br />massive sandstone (about 50-feet thick) of the Ohio Creek Member of the Mesaverde Formation. <br />While relatively resistant to erosion (i.e., the principle reason that West Flatiron exists in its <br />present form), the massive sandstone is more brittle than the shales and siltstones which underlie <br />it making it capable of forming near-vertical cliffs. Weathering of the more susceptible <br />underlying formations can cause undercutting of the sandstone making it much more susceptible <br />to cracking and slabbing. In a similar manner, any weakening of the underlying stratigraphic <br />units due to longwall mining can cause reduced formation stability and thus lead to cracking and <br />slabbing of the brittle sandstone unit, particularly at the outcrop where there is no lateral support. <br />As a result, Location 5 is an expected corollary to the rockfall/landslide area on the west side of <br />West Flatiron near Location 4. While noticed for the first time during this fall 2008 field visit, it <br />is likely that the cracking and slabbing of the sandstone outcrop on the east side of West Flatiron <br />has been occurring for at least a year. <br />831-032.791 Wright Water Engineers, Inc. <br />April 2010 <br />Page 32