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8 <br /> swam s deltas and flood plains. A retreat of the sea produced erosion of <br /> P , P <br /> the Purgatoire before the next incursion, still in early Cretaceous time. <br /> A broad, shallow sea invaded the area depositing the massive quartz sand- <br /> stones and thin interbedded shales of the Dakota formation. As the sea ad- <br /> vanced and water depth increased, mud, silt, fine sand and calcium carbonate <br /> were deposited and lithified to form the Graneros Shale, Greenhorn Lime- <br /> stone, Carlile Shale, Niobrara formation and Pierre Shale. Apparently <br /> J epeirogenic movements that signalled the beginning of the Laramide Revolution <br /> are recorded by the fine sands in the upper Pierre. The sea withdrew to the <br /> east with the Trinidad formation being deposited in the high energy coastal <br /> environment. As the sea continued to retreat, mud, silt, sand, and carbona- <br /> ceous material of the Vermejo formation were deposited in floodplain, delta <br /> and swamp environments. <br /> In latest Cretaceous time source areas to the west were being uplifted <br /> ' and relatively coarse sediments being shed to the study area as the Raton <br /> Basin began to form. These sediments are preserved as the upper Vermejo and <br /> Raton formations and include the Cretaceous-Tertiary boundary. A second <br /> pulse of tectonism, in the Paleocene resulted in the shedding of coarse <br /> clastics from the west to form the lower beds of the Poison Canyon formation. <br /> 1� <br /> Following deposition of the Poison Canyon, tectonic forces uplifted <br /> theiSangre de Cristo Range and created several open synclinal folds to <br /> the east. Sediments of the Cuchara formation were deposited in environ- <br /> ments similar to the Poison Canyon. The sediments were folded before being <br /> 3 eroded from the study area. The Eocene Huerfano formation was deposited <br /> on piedmonts and floodolains during a time of thrust faulting that estab- <br /> lished the present structure of the Sangre de Cristos. The Laramide <br /> Revolution culminated with deepening of the Raton Basin, uplifting of the <br /> Sierra Grande uplift, normal faulting and the intrusion of acidic to basic <br /> igneous rocks. Later in Tertiary time periods of erosion and deposition <br /> occurred followed by erosion to present levels. <br /> Geomorphology <br /> The geomorphic history of the region is complex and replete with <br /> �� numerous controversial aspects. Levings , 1951 , provides a comprehensive <br />