Valleys are narrow, with steep sides and gradients.
The Ouachita mountains eroded from sedimentary rock formations compressed into great folds, with the upturned edges forming the mountain ridges. The folds and mountains trend east-west. The Arkansas River flows across the southern part of the highlands and the Missouri River across the northern. The only major river which passes through the Ozark Plateau and Ouachita Mountains is the middle Arkansas. The Ozark Plateau and Ouachita Mountains are known for their many scenic, small spring-fed streams.