Oklahoma, 17,600 mi2 (45,600 km2)
Land-surface form.--Most of this region is characterized by irregular plains with a relief of less than 300 ft (90 m). Elevations range from 1,600 ft (490 m) to 3,000 ft (900 m), increasing gradually from east to west. On these dissected plains, the broad divides range from nearly level to gently sloping, but slopes are short and steep in the valleys. In southwestern Oklahoma, the Wichita Mountains rise as much as 1,000 ft (300 m) above the surrounding plains.
Climate.--The climate is semiarid-subtropical. Although maximum rainfall occurs in summer, high temperatures make summer relatively dry. Average annual temperatures range from 57 to 64F (14 to 18C). Winters are cold and dry, and summers are warm to hot. The frost-free season lasts from 185 to 230 days. Precipitation ranges from 19 to 29 in (490 to 740 mm), with most of it falling as rain.
Vegetation.--This region is dominated by tall grasses extending west from the oak savanna of the cross timbers area on the eastern edge of the province. Bluestem-grama prairie covers the finer textured soils characteristic of most of the province. Oak savanna occurs along the east border of the province and along some of the major river valleys. Sandsage-bluestem prairies are dominant on the coarse textured soils near the province's western edge.
Soils.--The soils are Mollisols with a minor component of Alfisols.
Fauna.--This province shares some species with surrounding provinces. No bird or mammal species is uniquely abundant here.