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Fall 2007 Subsidence and Geologic Field Observations <br />Box Canyon, Apache Rocks, and South of Divide Mining Areas <br />2.4 Gravity-Induced Cracks <br />I 11 <br />u <br />C~ <br />Gravity-induced cracks have been observed on steep ridges, near clilfs, and in landslides within <br />the 13ax Canyon Apache Racks, and SOD mining areas (Figure -O. The cracks look like <br />subsidence cracks but are t~~und 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 1 ~0 teet <br />long an August 27, 2002), is a good example of a ~,~r-avity-induced crack (Dunrud 2002). It was <br />easy to deterninc that this was not amining-related crack because no mining had occurred in the <br />area. The possihility of gravity-induced cracking in the rugged country above planned mining <br />activities at the West Elk Mine is a gaud reason to perform baseline studies of the area prior to <br />mining so that these features can he documented. <br />Y +~:, f <br />.:f~ 7%:f~'~ <br />y <br />~ L <br />~ ~ _ .. <br />~ ~~` - ~r ~ti <br />o ` J~,. ~ r t ~' ' . <br />.,r (, ` <br />Figure 4. Gravity-induced tension cracks located above unmined coal <br />in the upper, massive sandstone of the Ohio Creek Member of the <br />Mesaverde Formation, (looking northwest). Joints have opened up a <br />foot or so due to gravitational stresses under little or no lateral <br />constraint. <br />831-032.780 Wright Water Engineers. Inc. Page 8 <br />November 2007 <br />