Oklahoma covers 181,035 sq km (69,898 sq mi), including 3,188 sq km (1,231 sq mi) of inland water, and it ranks 20th in size among the 50 states. Along its southern border, Oklahoma measures 507 km (315 mi). The Panhandle, which is 269 km (167 mi) long, brings Oklahoma’s northern border to 747 km (464 mi). From north to south the length ranges from 267 to 357 km (166 to 222 mi), except in the Panhandle, which measures only 55 km (34 mi). The mean elevation is 400 m (1,300 ft). Oklahoma has mountainous lands as well as vast areas of level plains. Soils vary from rich black grassland soils to sterile blow sand, and vegetation ranges from sagebrush to grassland to dense forest. The climate varies from semiarid to humid.
Three of the natural regions, or physiographic provinces of the United States extend within Oklahoma’s territory. These are the Coastal Plain, the Interior Highlands, and the Interior Plains. Of these, the Interior Plains make up the greater part of the state, the Coastal Plain and Interior Highlands flanking these plains on the south and east. Elevations in Oklahoma range from under 90 m (about 300 ft) in the southeast corner to 1,500 m (5,000 ft) in the northwest edge of the Panhandle.
The Gulf Coastal Plain forms a narrow strip along the southeastern Texas-Oklahoma border. The Red River Plains, as it is known to some because it parallels that stream, are low, relatively flat, and sometimes swampy.
The topography changes dramatically in the Interior Highlands north of the Coastal Plains where peaks in the Ouachita Mountains reach as high as 800 m (2,600 ft). The Ouachita Mountains, a series of steeply folded ridges and valleys, resemble parts of the Appalachians farther to the east. Ouachita peaks such as Winding Stair, Kiamichi, Blackfoot, and Rich tower 460 m (1,500 ft) above their valleys.
North of the Ouachitas, set apart from them by the valley of the Arkansas River, is the Ozark Plateau, also part of the Interior Highlands. Broad, flat-topped hills are separated one from another by narrow, V-shaped river valleys. Elevations here range from 180 m (600 ft) to 365 m (1,200 ft). Cookson Hill and Boston Mountains are names sometimes attached to this region. By far the greater part of the state is the Interior Plains province. From east to west the elevation in this region, reaching 600 m (2,000 ft), divides it into the Osage Plains on the east and the High Plains to the west. The Osage Plains are themselves divided into subregions: the central Red Bed Plains and the Prairie Plains (or Arkansas River valley); the hilly sections of the Sandstone and Gypsum hills; the folded limestones, shales, and other strata of the Arbuckle Mountains and the more rugged, chiefly granite Wichitas.
The High Plains, a part of the Great Plains, occupy northwestern Oklahoma and the Panhandle. Elevations on the plains in the western Panhandle exceed those of the mountains farther east. The highest point in the state is located here at Black Mesa (1,516 m/4,973 ft), the remnants of an ancient lava flow. "Oklahoma" © Emmanuel BUCHOT, Encarta, Wikipedia
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