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7 <br /> and Raton formations and are prominently displayed in the Picketwire <br /> Valley. They range in thickness from a few inches to more than 20 feet <br /> and are, for the most part, basaltic in composition. Quarternary deposits <br /> will be described in the section dealing with the geomorphology of the <br /> area. <br /> Structure and History <br /> The principal structural feature of the region is the Raton Basin, a <br /> broad, asymmetric syncline whose axis trends northward. The eastern limb, <br /> which includes the study area, is gently dipping relative to the steep <br /> dips of the western limb. The Raton Basin was formed during the latter <br /> phases of the Laramide Revolution when the Sangre de Cristo Range was <br /> thrust eastward. Several steep, normal faults, trending east to northeast <br /> and with displacements rarely exceeding 25 feet have been mapped between <br /> Weston and Starkville (Wood, et al . , 1957). <br /> The geologic history of the region can be summarized in four phases <br /> (Precambrian to late Paleozic, Mesozoic, Tertiary and Quarternary). During <br /> the Precambrian many thousands of feet of sediments accumulated, were de- <br /> formed, intruded by large plutons and metamorphosed to primarily gneiss, <br /> schist and quartzite. A prolonged period of dominant erosion persisted to <br /> early Pennsylvanian time. If early Paleozoic sediments were deposited, <br /> they were subsequently removed by erosion. In early Pennsylvanian time a <br /> transgression of the sea concurrent with uplift of the Ancestral Rocky <br /> Mountains resulted in the deposition of detritus. By the middle of the <br /> Pennsylvanian, regression of the sea commenced. Middle Pennsylvanian through <br /> early Permian is represented by shallow marine and terrestrial sediments. <br /> The Permian and Triassic are not represented by rocks in or near the study <br /> area. <br /> 1 , <br /> iEarly Mesozoic events included a transgression of the sea. The Ocate, <br /> Wanakah(?) , and Morrison formations in the study area appear to represent <br /> coastal (beach) , freshwater lake and floodplain environments. In early <br /> Cretaceous time a major transgression of the sea from the south occurred. <br /> Sandstones of the Purgatoire formation represent beach and offshore bar <br /> i deposits and shales resulting from low energy environments such as coastal <br /> 1 <br />