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'~'^~ The coal beds are the most continuous strata of the unit. <br />2.7.1.3 Structural Geology <br />As [he Cretaceous period came to a close, a transition from relatively <br />stable crustal conditions to a period of intense tectonic activity occurred. <br />This resulted in the sediments being warped into a broad trough-like <br />structure. The margins of these troughs were folded into a series of anti- <br />clines and synclines. As tectonic activity continued, minor faulting also <br />occurred in the region. <br />A large east-west trending structural depression known as the Sand Wash <br />Basin underlies much of northwestern Colorado. It appears to be an ex- <br />tension of the Washakie Basin to the northwest. Thus the regional dip of <br />~'' strata in the area is to the northwest. The regional trend is interrupted - , <br />by several smaller scale folds. <br />In the Craig area, the major structural features are the Big Bottom syncline, _ <br />and the Williams Fork and Breeze anticlines. Figure 2.7-4 shows structural <br />contours of the Trout Creek sandstone which is 1,000 feet below [he Twenty <br />Mile sandstone in the Iles formation. The Trapper Mine lies on the south <br />limb of the Big Bottom syncline. The axis of the Big Bottom syncline <br />passes directly beneath the Craig Generating Station, running east and west <br />until it passes beneath the Yampa River where it turns northwest. The <br />syncline also plunges to the west and northwest. The contours of the <br />Trout Creek Sandstone indicate a local low point directly beneath the ~ <br />~~~ Yampa River. <br />4 <br />2-355 / <br />1 <br />