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Spring 2009 Subsidence and Geologic Field Observations <br />Box r­­ Apache Rocks and South of Divide Mining Areas <br />• <br />Figure 18. Southwesterly view of the drill pad for MDWS 21 -32. 21 -36, and 21 -38 showing a <br />sink hole in the re- graded and reclaimed site. This sink hole appears to be along the obscured <br />easternmost crack trend which leads to the rockfall on the east side of West Flatiron (Location 5 <br />• of this report). <br />5.5.2 Location 5 <br />This location is north of Location 4 approximately 400 feet and at the eastern cliff face of West <br />Flatiron. As shown in Figure 19, this site is where some of the numerous cracks, previously <br />observed in the MDW drill pad (Location 4 of this report), project into the sandstone cliff face on <br />the east side 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. As such, 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 />531- 032.791 Wright Water Engineers Page 32 <br />November 2009 <br />