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in thickness over a large area. The Trout Creek Sandstone Member, <br />a common ridge-forming unit in the Danforth Hills, has been called <br />the "White Rock" because of its characteristic exposures. <br />The deposition of the Trout Creek Sandstone marked a major <br />regression of the Late Cretaceous seaway over a large region. <br />The resulting clean, well sorted blanket sand formed by this <br />migrating beach and barrier island complex is an aquifer of <br />regional extent in Northwest Colorado. Under the area to be mined <br />within the Colowyo Permit area, the Trout Creek sandstone underlies <br />the lowest surface recoverable seam to be mined by 800 feet. <br />Considering the nature of the intervening strata which is comprised <br />of primarily claystone, shales, and siltstones; it is very doubtful <br />the Colowyo operation will impact the Trout Creek sandstone. <br />The Iles Formation in the permit area is estimated to be 1,375 feet <br />thick. The Iles Formation forms most of the cliffs that surround <br />the Axial Basin. The Iles Formation lies approximately 1,700 feet <br />beneath the actual mining area; however, minor folds or faults in <br />the regional trend bring the formation nearer the surface. <br />Williams Fork Formation - The Williams Fork Formation consists of <br />alternating beds of sandstone, sandy shale and coal. It is <br />distinguished from the Iles Formation by its thick zones of brick <br />red sandstone and shale, which have been colored from adjacent <br />naturally-burning coal beds, a common occurrence in the Axial area. <br />The coal beds in the formation are uniformly distributed in the <br />vicinity of the mine but generally vary greatly within the regional <br />extent of the unit. The stratigraphic thickness of the Williams <br />Fork Formation in the permit area has been estimated to be about <br />1,600 feet; its degree of areal extent in the Axial Basin is <br />similar to that of the Iles Formation. <br />The Williams Fork Formation is the predominant coal-bearing <br />formation in the Mesa Verde Group. Mesa Verde coals associated <br />with the Fairfield Group, the coal-bearing zone above the Trout <br />Creek Sandstone, rank in quality about midway between bituminous <br /> <br />2.04.6-4 <br />