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The alluvial material associated with the Yampa River is the youngest <br /> material (quaternary) associated with this study. The alluvium and <br /> floodplain form a belt on either side of the river which va ries in width <br /> from 1.5 miles in non-resistant material to approximately [he width of <br /> the river when crossing resistent strata. The widest area associated <br />with the river crossing the Lewis Shale is in the area of the Big Bottom <br />syncline. Drill cuttings (see Appendix A) in this area indicate [he <br />thickness of [he alluvium to be 10 to 12 feet and consist of gravel and <br />sands. <br />4.1.2 Structural Geology <br />As [he Cretaceous period came to a close, a transition from <br />relatively stable crustal conditions to a period of intense <br />tectonic activity occurred. This resulted in the sediments being <br />• \ warped into a broad trough-like structure. The margins of these <br />troughs were folded into a series of anticlines and synclines. As <br />tectonic activity continued, minor faulting also occur rd in the <br />region. <br />A large east-west trending structural depression known as the <br />Sand Wash Basin underlies much of northwestern Colorado. It <br />appears to be an extension of the Washakie Basin to the northwest. <br />Thus [he regional dip of strata in the area is to the northwest. <br />The regional [end is interrupted by several smaller scale folds. <br />•j <br />-23- <br />