Ouachita Highlands, 8,800 mi2 (22,800 km2)
The fold mountains here were eroded from sedimentary rock formations compressed into great folds; the upturned edges of the resistant formations form the mountain ridges. The linear ridges reach maximum altitudes of about 2,600 ft (790 m), about 1,500 ft (460 m) above the adjoining valleys. The folds and the mountains trend east-west.
The climate is similar to that found in adjoining parts of the Subtropical Division. Winters are warm and summers hot. Rain falls throughout the year, but summers are relatively dry. On the outskirts of this province, in Fort Smith, Arkansas, the average annual temperature is 63F (17C). Average annual precipitation is 41 in (1,050 mm).
The area supports oak-hickory-pine forests. The primary overstory species are southern red oak, black oak, white oak, and hickories. Pine constitutes as much as 40 percent of the cover (shortleaf pine in the uplands, with loblolly pine on lower lying alluvial soils). The dry sandstone ridges of the Ouachita Mountains are covered on their southern slopes by a mixture of shortleaf pine, oak, and hickory, and on their northern slopes by hardwood forests made up mainly of oak and hickory. Hardwoods populate the rich bottom lands of the valleys, and pines predominate on poorer lands.
The major soils are Ultisols. They are stony and nonstony, with medium textures.
Bird and mammal species are similar to those found in the surrounding Southeastern Mixed Forest. One amphibian, the Ouachita dusky salamander, is found exclusively in this province's rocky, gravelly streams.
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