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2004 Geologic Hazard Field Observations <br />South of Divide Mining Area <br />• 8.2.1 Aerial Photo Observations at Location 2 <br />This is a good location to evaluate the potential mining effects, if any, on the alluvium and <br />adjacent colluvium in Dry Fork and also its tributaries (Figure 6). <br />The alluvium and most of the colluvium contains a lazge amount of clay and silt that was <br />eroded from the Wasatch Formation and deposited in the Dry Fork valley and its tributaries. <br />This alluvium and colluvium in Dry Fork, which ranges from an estimated 25 to 150 feet <br />thick, contains more Wasatch clays and silts and smaller amounts of sand- to boulder-size <br />intrusive rocks (porphydtic granodiorite and quartz monzanite derived mostly from Mount <br />Gunnison) than does the alluvium in Deep Creek. The alluvium and colluvium in the <br />tributaries of Dry Fork are approximately similaz in grain size and texture to the alluvial and <br />colluvial deposits in Dry Fork. The overburden depth to the B Seam above mined longwall <br />panels 14 through 17 in Deep Creek ranges from about 950 to 1,050 feet. The overburden <br />depth to the B Seam above longwall panels 14, 15, and 16 beneath the upper part of Sylvester <br />• Gulch ranges from about 800 to 1600 feet. <br />• No cracks were observed in the alluvium and coluuvium of Deep Creek and the upper reaches <br />of Sylvester Gulch when longwall panels 14 through 17 were mined beneath Deep Creek and <br />longwall panels 14 through 16 were mined beneath the upper part of Sylvester Gulch. Based <br />on these observations, and the fact that the Dry Fork alluvium contains a higher percentage of <br />clay and silt than does the alluvium in Deep Creek, it is unlikely that mine subsidence- <br />induced cracks will occur in the alluvium and colluvium in the South of Divide mining azea, <br />particularly where the overburden depth to the E Seam is equal to, or more than about 800 <br />feet. <br />• The occurrence of subsidence cracks also is considered unlikely where the E Seam <br />overburden depth range is between 350 and 800 feet (350 to 400 feet is the minimum <br />overburden depth to the E Seam for the northwestern part of the mine). This statement is <br />• <br />931-032.621 Wright Water Engineers, Inc. Page 28 <br />November 2004 <br />