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<br />. <br /> <br />0847 <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />APPENIDx F - GEOLOGIC DATA <br /> <br /> <br />1. tlensral topographY and geology of the area. - The upper Purga_ <br /> <br /> <br />toire River basiq lies in the southwestern part of the Great Plains <br /> <br /> <br />physiographic province, adjoining the Rocky Mountain province, in the <br /> <br /> <br />southeastern part of Colorado, and includes a very small part of north- <br /> <br /> <br />eastern New IVlexico. The surface features range from rugE,ed mountains tower- <br /> <br /> <br />ing over 14,000 feet above sea level in the western portion to gently undul- <br /> <br /> <br />ating plains abollt 6,000 feet elevation in the eastern portion. Along the <br /> <br /> <br />eastern slope of the mojntainous western portion, sedimentary strata are <br /> <br /> <br />s.teeply inclined to the east end form prominent highlands that stand well <br /> <br /> <br />above the basin. The sedimentary strata form narrow zones of out.cropj and <br /> <br /> <br />col1a~tivelymake UP? belt of ~idges and strike valleys, paralleling the <br /> <br /> <br />'mountains, 'that have ,smooth 'dip' slopes to the easta.ildfough pre:clpitous <br /> <br /> <br />'cl.iffstcl'the Jvest: throughwMch- thes.treams have incised deep canyons': <br /> <br /> <br />"The fOdthilharea" from t'hecouspicuous eastern enscarpment' 'to the - eity- : <br /> <br /> <br />of Trinidad,- Colorado, slopes gradually eastward v;ith narrow tributary <br /> <br /> <br />valleys and canyons. <br /> <br /> <br />2. In the western part of the watershed, the mountains have been <br /> <br /> <br />formed by relative uplift and have undergone extensive erosio~ whjch <br /> <br /> <br />has exposed areas of a crystalline complex of granites, shcists, and <br /> <br /> <br />gneisses of Archean or pre-Cambrian age. There appears to be no Cam- <br /> <br /> <br />brian, Ordovician, Silurian, or Devonian rocks in this part of the Rocky <br /> <br /> <br />lVlountains, but such beds may lie beneath some parts of the Great Plains. <br /> <br /> <br />Carboniferous limestones and sandstones, Pennsylvanian and Permian, rest <br /> <br /> <br />on older granites and metamorphics where these are exposed, ,and overlap <br /> <br /> <br />at most other places along the mountain front. The Morrison marls and <br /> <br /> <br />sandstones, of Jurassic age, are a thin but relliarkably persistent for- <br /> <br /> <br />mation and overlap all the older strata. On this formation lie, in appar- <br /> <br /> <br />ent conformity, sedimentary beds of Cretaceous age. The formations of <br /> <br /> <br />this age represented in the watershed are, ascending: Dakota sandstone, <br /> <br />APPENDIX F <br /> <br />