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<br /> <br />e <br /> <br />e <br /> <br />e <br /> <br />'. <br /> <br />0651 <br /> <br />river is a rolling plains area. The elevation at Hoehne, located in the <br /> <br /> <br />plains section, is 5,742 feet. From the high plateaus and mesas on the <br /> <br /> <br />southern divide, the basin slopes steeply to the river. <br /> <br /> <br />16. From the confluence of North and Middle Forks, the Purgatoire <br /> <br /> <br />River flows through a valley in a narrow mountainous section to Trinidad, <br /> <br /> <br />river miles 186 to 157. This valley varies in width from a few hundred <br /> <br /> <br />feet to about one-fourth mile, Between Trinidad and the mouth of San <br /> <br /> <br />Francisco Creek, river miles 157 to 122, the v~lley reaches a maximum <br /> <br /> <br />width of about three-fourths mile. Topographic details of the Purga- <br /> <br /> <br />toire River valley from river mile 163 to river mile 122 are shown on <br /> <br /> <br />plates 2 to 7, appendix C. <br /> <br /> <br />17, q~plogy. ~ The principal ~ocks of the upper Purgatoire River <br /> <br /> <br />basin range in geologic age from pre-Cambrian through TeftiarY with <br /> <br /> <br />minor amounts of Quaternary gravels and alluvium. Except for the small <br /> <br />representation of pre-Cambrian plutonics and metamorphics in the extreme <br /> <br />headwaters of the basin, the only igneous or metamorphic material present <br /> <br />occurs as dikes and flows of Tertiary age. Upstream from Sopris, Colo- <br /> <br />rado, due to an uplift of older rocks in the headwater reaches, the <br /> <br />streams flow over successively younger strata which dip steeply to the <br /> <br />east and crop out in north-south ridges. Downstream from Sopris, Colo- <br /> <br />rado, the strata dip west, and exposures become successively older. In <br /> <br />the reach between Trinidad and the mouth of San Francisco Creek, the <br /> <br />river flows over soft shale with few prominent topographic features. The <br /> <br />Pierre formation of Cretaceous age occurs along the area of transition <br /> <br />from foothills to plains. This material has a slow. rate of infiltration <br /> <br /> <br />and readily erodes to form gentle slopes. Above the Pierre are the coal- <br /> <br /> <br />bearing Trinidad and Vermejo formations of Cretaceous age, which form <br /> <br />the abrupt break between the foothills and the plains. The other Cre- <br /> <br /> <br />taceous formations form relatively flat mesas with a shallow soil mantle. <br /> <br />6 <br /> <br />'e <br />