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OC2189 - 4 - Geology.- The bedrock formations which crop out in the watershed consist of sedimentary rocks of Upper Cretaceous age or younger. The structure of the formations is simple with the strata having a very gentle dip in a southwesterly direction. The principal formations are the Pierre shale, Trinidad sandstone, and Vermejo forma- tion of Upper Cretaceous age and the Raton formation of Upper Cretaceous and Paleocene age. The oldest strata exposed are the upper beds of the Pierre shale. They crop out in the lower part of the watershed and consist mainly of gray to nearly black thin-bedded, silty and sandy shale beds. Overlying the Pierre is the Trinidad sandstone, which consists mostly of light-gray, gray, and buff slightly arkosic sandstone beds. The sandstone beds of the Trinidad crop out as resistant ledges across the middle portion of the watershed. The Vermejo and Raton formations crop out in slopes and ledges in the upper part of the watershed. The rocks of these two formations include buff, gray, gray-green slightly arkosic sandstone; buff, gray, and dark-gray, and nearly black carbonaceous, coaly, and silty shale; and coal beds. Unconsolidated deposits of Pleistocene age consisting of silt and clay interbedded with sand, gravel, and boulders occur on remnants of terrace and pediment surfaces in parts of the middle and upper areas. Colluvial and alluvial deposits mantle much of the upper area. However, in the lower and middle portions, alluvial deposits along the major drainages are thin or absent; and in most places the stream channels are cut into Pierre shale. 80ils._ The soils in the watershed are generally shallow where they overlie sandstone beds on the steeper slopes. The soils developed on the Vermejo and Raton formations resemble those of the Traviscilla series but in many cases a heavier surface soil exists. Same deeper soils exist in the valleys and drainageways below rooky outcrops of the Raton and Vermejo formations. They are similar to soils of the Carnero series except that they have weaker structure and show less development. Lithosols of the Lismas series have developed on the Pierre shales. These are very thinly developed on the steeper slopes and in many places the parent materials are exposed. The deeper alluvium, derived from materials of Pierre shale origin, probably can be placed in the Orman series or Winifred series. These soils also thow some influence from the geologic materials lying above the Pierre formation.