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1- 8 <br />~~ swamps deltas, and floodplains. A retreat of the sea produced erosion of • <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 caiciurn carbonate <br />were deposited and ltthified to forth the Graneros Shale, Greenhorn Lime- <br />1~ stone, Carlile Shale, Niobrara formation and Pierre Shale. Appan:ntly <br />J epeirogenic movements that signalled the beginning of the Laramidie Revolution <br />1~ 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 ener!ly 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 />1' and, relatively coarse sediments being shed to the study area as the Raton <br />Basin began to forto. 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 />I elastics 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 />l the;Sangre de Cristo Range and created several open synclinal folds to <br />the east. Sediments of the Cuchara formation were deposited in environ- <br />)~ menu similar to the Poison Canyon. The sediments were folded before being <br />eroded from the study area. The Eocene Huerfano formation was deposited <br />{ on piedmonts and floodplains 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 erasion and deposition <br />i occurred followed by erosion to present levels. <br />(II Geomorpholoar <br />The geomorphic history of the region is complex and replete with <br />numerous controversial aspects. Levings, 1951, provides a comprehensive • <br />