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2.1.2 STRUCTURE <br /> <br /> During the early part of the Tertiary Period, after the seas had retreated from <br /> Colorado, the entire Rocky Mountain chain was uplifted. This geological event, <br /> commonly referred to as the Laramide Orogeny, involved predominately vertical <br /> movement of fault blocks in the Southern Rocky Mountains. The relatively <br /> incompetent sedimentary rocks became draped over the irregular surface of the <br /> shattered crystalline basement rock. The large scale structural features of the <br /> region have resulted from tectonic uplift, sinking or tiling of the underlying blocks <br /> of basement rock. <br /> Large features such as the Washakie-Sand Wash Structural Basin have been <br /> superimposed upon local structural features such as the Tow Creek and Fish Creek <br /> Anticlines and the Twentymile Park Syncline. The Edna Mine is located on the <br /> eastern limb of The latter (Exhibit 2.1-I ). <br /> At least two systems of faults exist in the region. The major faults trend <br /> approximately northwest and parallel the axes of folds such as The Twentymile Park <br /> Syncline. These faults are represented on Exhibit 2.1-I in the northeast and <br /> southwest parts of the study area. These faults have a steep dip, a normal <br />• displacement, and may likely extend downward into the basement. <br /> The second group of faults are relatively minor features that are oriented <br /> obliquely to the fold axes. One of these faults is shown on Exhibit 2.1-I in Sections <br /> 25, R86W, TSN, and 30, R85W, TSN. <br />w <br />2.1-4 <br />