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~J <br />Section 779.15 (a) Continued. <br />Revised 11/20/80 <br />at the aquifer outcrop toward the middle of the syncline, and then <br />to the south along the plunging axis of the syncline. Ground water <br />flow direction is schematically shown on Map 15a, Kerr Mine Area <br />Ground Water Hydrology. <br />AcLuifer Lithology and Thickness <br />The lithology of the upland terrace deposits is sand, gravel and <br />conglomerate (Kinney, 1970). The thickness of these deposits is <br />highly variable, ranging from 70 feet in drill hole 028-79-16 (see <br />Exhibit 22a, Drill Hole Logs), to zero where the deposits have been <br />removed by erosion. <br /> <br />Information on the lithology of the stream-laid deposits is taken <br />from logs of backhoe pits dug in the intermittent stream valleys. <br />The alluvium is mostly tight, sticky clay with isolated lenses of <br />sand and gravel. The depth of these deposits in the intermittent <br />stream valleys in the mine plan area is variable, and ranges from 8 <br />feet to over 15 feet, as shown on Map 40a, Kerr Mine Area Stream <br />Channel Cross Sections. <br />The lithology of coal aquifer and associated strata is taken from <br />logs of the 28 bedrock monitor wells in the mine plan area, and <br />reconnaissance investigations by the U. S. Geologic Survey <br />(Voegeli, 1965). As described in Section 774.14, the Sudduth coal <br />seam is part of the Coalmont Formation of Tertiary age. The imper- <br />meable shales of the Pierre Formation underlie the Coalmont Forma- <br />tion, and the interbedded siltstones <br />I • <br />779-31 bb <br />