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2 • 58 <br />2.5.3 GEOLOGICAL CONDITIONS <br />• As the development of the mine has increased, so has knowledge of roof and floor <br />conditions. Although some top coal was left in various parts of the mine to stabilize roof <br />conditions, an understanding of strata type that is most likely to cause roof support problems <br />has been gained. In general, based upon knowledge from mining, widely scattered drill holes, <br />and surface evidence, the roof rock appears to consist of either gray to black shales, competent <br />sandstones, or extensive zones of thinly laminated altemating beds of each. This depositional <br />environment appears to be part of the delta system with frequent transgressive and regressive <br />impulses. Because of this very dynamic depositional environment, correlation of specific sands <br />or shales over extensive areas is very difficult or in most cases impossible. The result is rapidly <br />changing roof rock types and competence. Generally, the thinly bedded altemating sands and <br />shales, as well as the shales on the edges of channel sands whose competence is lessened by <br />differential compaction are given the most attention. The floor rock is most often shales, or coal <br />in the thick seam areas, which has few troubles with heaving. Analysis of the roof and floor rock <br />at the McClave Canyon site (n accordance with 30 CFR 783) are presented in Tables 2.1.4 and <br />2.1.5. Based upon these observations, the roof and floor rock appears to be competent with <br />minor areas of spalling, which is a manageable problem. <br />Overburden thickness in the permit area varies from 0-2000 feet (See Fgure 2.1.8). Where <br />excessive overburden, over 1,400 feet, was encountered wrthin the graben, considerable <br />deterioration of mining conditions occurred. Whether this condition is exclusive to the graben <br />remains to be seen. Extensive rib sloughage and increased difficulty holding the thinly laminated <br />beds forming the roof contribute to make coal recovery uneconomical. Coal from drill cores in <br />• Section 15 and further northeastward in the graben also exhibits extensive slickensides and <br />slips suggesting multidirectional and unpredictable forces. <br />M~ Volume 1 4-29.96 <br />