<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 . As already mentioned, 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> <P> This article incorporates text from a publication now in the public domain: Chisholm, Hugh, ed. (1911). "article name needed". Encyclopædia Britannica (11th ed .). Cambridge University Press . </P> <P> During the Cretaceous Period (145--66 million years ago), the Great Plains were covered by a shallow inland sea called the Western Interior Seaway . However, during the Late Cretaceous to the Paleocene (65--55 million years ago), the seaway had begun to recede, leaving behind thick marine deposits and a relatively flat terrain which the seaway had once occupied . </P>

​18 000 years ago huge ice sheets called ____ covered vast areas of north america and europe