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• several synclinal folds. Ground water occurrence in the basin is <br />controlled by structure, topography and lithology. There are two <br />types of aquifers which occur within the basin in the proposed <br />persist and adjacent areas: confined bedrock aquifers and unconfined <br />alluvial aquifers. <br />In the Twentymile Park area, ground water occurs in confined <br />bedrock aquifers within several formations. The rocks which <br />comprosiise major aquifers are the Tventymile and Trout Creek <br />Sandstones of the Williams Fork Formation and the Iles Formation, <br />respectively. Wells finished in these thick, massive, fine-to- <br />medium-grained sandstones can yield as much as 100 gallons per <br />minute (Brogden sad Giles, 1977). However, these aquifers are <br />• etratigraphically separated from the minor aquifers affected by <br />mining. <br />Minor aquifers in the Williams Fork Formation include thin <br />layers of fine-grained sandstone, siltatone, and, where locally <br />fractured, coal beds. Minor aquifers in the general area yield an <br />average of 10 gpm. The combination of saturated permeable layers <br />confined by impermeable layers results in artesian aquifer <br />conditions. Artesian pressures within the confined aquifers in the <br />heart of the ground water basin have been found to ezceed 700 pounds <br />per square inch at a depth of 1630 feet. Almost all of the yells <br />drilled into confined aquifers in the Twentymile Park ground water <br />. basin display artesian water levels, and same flow water at the <br />surface, as show in Table 26, Ground Water Monitoring Data. <br />2.04-37 <br />