The Ouachita Mountains are a mountain range located in west central Arkansas and southeastern Oklahoma. The range's subterranean roots may extend as far as central Texas, or beyond it to the current location of the Marathon Uplift. Along with the Ozark Mountains, the Ouachita (pronounced [wɑˈʃiɾə], pseudophonetically: "wah-shi-tah") Mountains form the U.S. Interior Highlands, the only major mountainous region between the Rocky Mountains and the Appalachian Mountains.
The Ouachita Mountains are fold mountains like the Appalachian Mountains to the east, and were originally part of that range. During the Pennsylvanian part of the Carboniferous period, about 300 million years ago, the coastline of the Gulf of Mexico ran through the central parts of Arkansas. As the South American plate drifted northward, a subduction zone was created in this region. The South American oceanic crust was forced underneath the less-dense North American continental crust. Geologists call this collision the Ouachita orogeny. The collision buckled the continental crust, producing the fold mountains we call the Ouachitas. At one time the Ouachita Mountains were very similar in height to the current elevations of the Rocky Mountains. Due to the Ouachitas' age, the craggy tops have eroded away leaving the low formations that used to be the heart of the mountains.
Unlike most other mountain ranges in the United States, the Ouachitas run east and west rather than north and south. Also, Ouachitas are distinctive in that volcanism, metamorphism, and intrusions are notably absent throughout most of the system.
The mountains are known to contain at least 15 endemic plant species (i.e. plants that occur nowhere else). These include Sabatia arkansana, Valerianella nuttallii, Liatris compacta and Quercus acerifolia.
The Ouachita Mountains contain the Ouachita National Forest and Hot Springs National Park as well as numerous state parks and scenic byways. They also contain the Ouachita National Recreation Trail, a 223 mile long hiking trail through the heart of the mountains. The trail runs from Talimena State Park in Oklahoma to Pinnacle Mountain State Park, near Little Rock, Arkansas. It is a well maintained, premier trail for hikers, backpackers, and mountain bikers (for only selected parts of the trail).
The Battle of Devil's Backbone was fought here at the ridge of the same name in 1863.