. Fig. 4. Index map of Colorado, locating principal physiographic features discussed in text. tiary mantle has not been completely removed, but has been sorted and deposited as dune sand. Further character is given to much of the western part of the piedmont by Quater- nary alluvium. The Platte-Arkansas Divide may never have been covered by the Tertiary mande characteristic of the plains. It was, however, covered by an earlier mantle, derived from the Ancestral Rocky Mountains, and seen to- day as a resistant conglomerate. These rocks give the western end of the divide strong re- lief and affe
. Fig. 4. Index map of Colorado, locating principal physiographic features discussed in text. tiary mantle has not been completely removed, but has been sorted and deposited as dune sand. Further character is given to much of the western part of the piedmont by Quater- nary alluvium. The Platte-Arkansas Divide may never have been covered by the Tertiary mande characteristic of the plains. It was, however, covered by an earlier mantle, derived from the Ancestral Rocky Mountains, and seen to- day as a resistant conglomerate. These rocks give the western end of the divide strong re- lief and affect the groundwater regime. Eco- logically the area is related closely to the foothills and lower mountains, but is nonethe- less a part of the Great Plains Province. Raton Section.—At the foot of the moun- tains, all sedimentary units dip steeply east- ward, forming a great trough, the so-called "Denver ; The western wall of the basin is marked by hogbacks in the Dakota, Lyons, and Fountain sandstones and other sedimen- tary formations. South of Pueblo an upward fold extends southeastward, bringing the sand- stones back to the surface. The exposed crest of that fold defines the Raton Section. The Raton Section merges gradually with the Colo- rado Piedmont in some localities, but gener- ally is higher and more complex, hence dis- tinctive. Characteristic topographic features include mesas, cuestas, dissected plateaus, deep canyons, and volcanic features of various ages. Basaltic rocks cap Raton Mesa (the west- ern terminus of which is Fisher's Peak, 9586 feet) and Mesa de Maya. Locally the cap reaches a thickness of some 500 feet. The elevation of the groups of mesas declines east- ward with the volcanic cap, reaching the gen- eral level of the plains at about the 103rd
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Keywords: ., bookcentury1900, bookcollectionbiodiversity, booksubjectmammals
