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Alluvial deposits are locally present in stream valleys, particularly along the larger <br />streams. Colluvial slides and slumps are also present in canyons and gullies throughout <br />the area. Slide bodies which could potentially affect the stability of mining related <br />structures in the northern portion of the permit area have been identified, and mitigation <br />measures are addressed in Section VII., "Backfilling and Grading" of this document. <br />The unit of particular interest to the Seneca II-W Mine is the Upper Cretaceous <br />Mesaverde Group. The Seneca II-W Mine will extract two coal seams from this unit. <br />The Mesaverde, in this part of northwestern Colorado, is divided into two formations: 1) <br />the 1550 foot thick Iles Formation, and 2) the overlying 1100 to 2000 foot thick Williams <br />Fork Formation. <br />The Iles Formation is divided into three members. The basal Tow Creek Sandstone is a <br />massive, fine grained, well sorted sand. The Tow Creek varies from 35 to 125 feet thick. <br />Overlying the Tow Creek are 1200 feet of interbedded shales, sandstones, and coals. <br />The coals, while mined in the vicinity, are too deep to be considered economically <br />recoverable at the Seneca II-W Mine. The massive, fine grained, 100 feet thick Trout <br />Creek Sandstone is the uppermost unit of the Iles Formation. <br />Conformably overlying the Iles is the Williams Fork Formation. The lower unit of the <br />Williams Fork Formation is a series of interbedded shales, sandstones, sandy shales and <br />coal. The middle coal group contains the seams of economic importance. Lowermost is <br />the Wolf Creek coal, which is split into an upper and lower unit. The lower seam is <br />about 16 feet thick, while the upper is about 7.5 feet thick. This coal lies too deep <br />within the permit area to be mined by surface methods. 100 feet above the Wolf Creek <br />coal is the Wadge coal. The two seams are separated by interbedded sandstones, shales, <br />siltstones and thin coals. The Wadge coal seam is the most continuous in the area and is <br />the seam which will be extracted. The outcrop strikes about north-south, and dips 8 to <br />14 degrees. The Wadge is also split into upper and lower seams. The lower is two to <br />three feet thick, and is separated from the 10 foot thick upper seam by two to five feet <br />of shales and sandstones. Overlying the Wadge is about 560 feet of sandstones, shales, <br />and coal, including the Lennox Coal Seam, which has been eroded away in places. <br />Overlying this lower unit of the Williams Fork Formation is the 130 feet thick Twenty <br />Mile Sandstone. The upper most unit of the Williams Fork Formation consists of <br />interbedded sandstones, shales, and thin coals. <br />Three coal seams of economic importance are found in the Williams Fork Formation. <br />These seams are, in ascending order, the Wolf Creek, Wadge and Lennox coals. The <br />Wolf Creek coal lies too deep to be mined by surface methods and the Lennox is <br />relatively thin and discontinuous within the permit area. The Wadge coal is the only <br />seam to be mined as part of this operation. <br />Geologic baseline information including local and regional structure and stratigraphy, <br />and information on interburden, overburden and coal geochemistry is provided in <br />Volume 2, Tab 6, of the permit application. <br />5 <br />