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4.0 Groundwater HvdroloRy <br />The geology, occurrence of groundwater on the mine site, and groundwater <br />movement are discussed in this section co determine possible interactions <br />between mining in [he sipper Williams Fork Formation and groundwater in allu- <br />vial aquifers in the Yampa and Williams Fork Rivers. <br />4.1 Hydrogeology <br />The Trapper Mine is located in a complex geological setting. Pas[ deposi- <br />tional processes formed discon[inous lenticular beds with variable lithology <br />which have been subsequently affected by tectonic activity. The geology <br />along with topography of [he site determines the groundwater regime of [he <br />area. Stratigraphy and geologic structure of the area will be discussed so <br />• ~ a picture of the relationships between geologic units and groundwater flows <br />in the area can be visualized. <br />4.1.1 Stratigraphy <br />Bedrock in the Craig area is comprised of Cretacepis and Tertiary <br />sedimentary Formations underlain by an ancient igneous and metamorphic <br />basement complex. Table 4-1 summarizes the Stratigraphy on the mine <br />plan and adjacent areas. Important to this study are the Lewis Shale, <br />[he Williams Fork Formation and [he alluvial materials associated with <br />[he Yampa and Williams Fork rivers. <br />.1 <br />-17- <br /> <br />