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