<Li> every little rill, at times of rain, carves its own little valley . </Li> <P> The central section of the Great Plains, between latitudes 42 ° and 36 °, occupying eastern Colorado and western Kansas, is, briefly stated, for the most part a dissected fluviatile plain . That is, this section was once smoothly covered with a gently sloping plain of gravel and sand that had been spread far forward on a broad denuded area as a piedmont deposit by the rivers which issued from the mountains . Since then, it has been more or less dissected by the erosion of valleys . The central section of the plains thus presents a marked contrast to the northern section . While the northern section owes its smoothness to the removal of local gravels and sands from a formerly uneven surface by the action of degrading rivers and their inflowing tributaries, the southern section owes its smoothness to the deposition of imported gravels and sands upon a previously uneven surface by the action of aggrading rivers and their outgoing distributaries . The two sections are also like in that residual eminences still here and there surmount the peneplain of the northern section, while the fluviatile plain of the central section completely buried the pre-existent relief . Exception to this statement must be made in the southwest, close to the mountains in southern Colorado, where some lava - capped mesas (Mesa de Maya, Raton Mesa) stand several thousand feet above the general plain level, and thus testify to the widespread erosion of this region before it was aggraded . </P> <P> The southern section of the Great Plains, between latitudes 35.5 ° and 25.5 ° lies in western Texas and eastern New Mexico . Like the central section, it is for the most part a dissected fluviatile plain . However, the lower lands which surround it on all sides place it in so strong relief that it stands up as a table - land, known from the time of Mexican occupation as the Llano Estacado . It measures roughly 150 miles (240 km) east - west and 400 miles (640 km) north - south . It is of very irregular outline, narrowing to the south . Its altitude is 5,500 feet (1,700 m) at the highest western point, nearest the mountains whence its gravels were supplied . From there, it slopes southeastward at a decreasing rate, first about 12 ft., then about 7 ft per mile (1.3 m / km), to its eastern and southern borders, where it is 2,000 feet (610 m) in altitude . Like the High Plains farther north, it is extraordinarily smooth . </P> <P> It is very dry, except for occasional shallow and temporary water sheets after rains . The Llano is separated from the plains on the north by the mature consequent valley of the Canadian River, and from the mountains on the west by the broad and probably mature valley of the Pecos River . On the east, it is strongly undercut by the retrogressive erosion of the headwaters of the Red, Brazos and Colorado rivers of Texas and presents a ragged escarpment approximately 500 to 800 ft (240 m) high, overlooking the central denuded area of that state . There, between the Brazos and Colorado rivers, occurs a series of isolated outliers capped by a limestone which underlies both the Llano Uplift on the west and the Grand Prairies escarpment on the east . The southern and narrow part of the table - land, called the Edwards Plateau, is more dissected than the rest, and falls off to the south in a frayed - out fault scarp . This scarp overlooks the coastal plain of the Rio Grande embayment . The central denuded area, east of the Llano, resembles the east - central section of the plains in exposing older rocks . Between these two similar areas, in the space limited by the Canadian and Red Rivers, rise the subdued forms of the Wichita Mountains in Oklahoma, the westernmost member of the Ouachita system . </P>

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