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December 198G <br />• <br />4.0 SITE CONDITIONS <br />4.1 Topography <br />863-2060 <br />As shown on Figures 2 and 12, the ground surface in Area 9/10 <br />slopes northward at an average gradient of 12 percent (7 de- <br />grees). The southern boundary of Area 9/10 coincides with a <br />sharp ridge formed from the northward-dipping Mesaverde strata. <br />Elevations in the study area range from 7,705 feet at the high <br />point of the southern rim to a minimum of 7,240 feet at the <br />northern boundary of Area 9/10. <br />4.2 Geology <br />Geologic units investigated at the site are part of the Williams <br />Fork Formation of the Mesaverde Group. These sedimentary rocks <br />• of Upper Cretaceous age primarily consist of interbedded sand- <br />stone, shale, and coal. The Wadge coal seam, which is approxi- <br />mately 10.5 feet thick, dips to the northwest at approximately 7 <br />degrees. A top-of-coal structure map, which also defines major <br />faulting within the area, is presented as Figure 11. Mesaverde <br />strata encountered in Area 9/10 are quite similar to the units <br />present in Area 51. <br />Typically underlying the Wadge seam is a black shale unit that is <br />approximately 1 foot thick. The shale unit is generally slightly <br />weathered, of low to medium strength, and is often highly carbon- <br />aceous. Examination of this shale unit in Area 51 indicates that <br />it is relatively competent, and does not appear to be a weak unit <br />of low shear strength. Such weak seams, frequently termed <br />"underclays", are widely recognized as a principal source of <br />highwall and spoil pile instability in surface coal mines. <br />• <br />Golder Associates <br />