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this section (Figure 6-21. The total thickness of these overlying beds is apprazimately <br />500 feet. The sandstones are mostly fine to very fine grained, light gray, and well sorted; the . <br />shales are commonly silty with some carbonaceous material at varying depths. At the top of <br />this sequence lies the Twentymilr: sandstone member of the Williams Fork Formation. <br />The Twentymile sandstone outcrops within the western portion of the Seneca II-W lease area, <br />and like the Trout Creek sandstone, is a marine sandstone. The thickness of the Twsntymile <br />sandstone is 130 feet. Above this Twentymile sandstone is the upper unit of the Williams Fork <br />Formation. This unit, exposed to the west of the permit area, consists of sandstones, shales, <br />and thin coals. <br />The Quaternary age deposits witYiin the permit area consist of thin alluvial (Exhibit 6-1 and 6- <br />iBl and colluvial deposits. There are not any outcrops of Miocene extrusive igneous lock nor <br />eny evidence of intrusive rocks at the mine site. <br />Overburden Assessment <br />Overburden samples obtained from the Seneca 11-W Mine study area were analyzed to identify <br />any acid-forming, toxic-forming, alkalinity producing, or other inhibitory materials present <br />within the mine boundary. Physical properties of the overburden were determined to define <br />compaction, stability, permeability, erodibility, and water holding characteristics. Chemical and • <br />physical analyses also reveal the most desirable overburden strata to be placed at the spoil <br />surface and in the pit base. This baseline information is required prior to mining to eff~sctively <br />prepare a mine plan that restores landscape productivity and which also minimizes <br />environmental degradation. <br />Contained within this report are evaluations of the chemical and physical data, locations and <br />extents of suitable and unsuitable overburden, and proposed methods of mitigation. Spoil <br />mixing phenomenon is discussed and core variability is documented. A sampling program <br />immediately after mining is proposed to investigate and evaluate mining methods, overburden <br />mixing, and reclamation success (see Tab 21, Minesoil Reconstructionl. These investigations <br />were completed to assure adequate postmine vegetation growth and ground water aquifer <br />characteristics. <br />Conclusions and recommendations are based on the interpretation of laboratory data and <br />lithological descriptions developed during the sample collection and analysis. <br /> <br />8 Revised 9/98 <br />