From https://earthobservatory.nasa.gov/images/46485/ouachita-mountains-oklahoma https://earthobservatory.nasa.gov/images/46485/ouachita-moun...: > Starting aro

From https://earthobservatory.nasa.gov/images/46485/ouachita-mountains-oklahoma https://earthobservatory.nasa.gov/images/46485/ouachita-moun...: > Starting around 340 million years ago, a tectonic plate that had previously been attached to Africa and South America collided with the North American plate. The process helped form the supercontinent of Pangaea, and it thrust the ancient sea floor skyward. The similarities between rock layers of the Ouachita Mountains and the Appalachian Mountains indicate that they were part of a massive mountain chain formed by this collision. Once elevated, the mountains soon commenced eroding. > About 200 million years ago, Pangaea began to break apart. The breakup wrenched apart the supersized mountain range, leaving a wide swath of low-lying land between the Ouachita Mountains in the west and the Appalachian Mountains in the east. Low-profile land persists today across much of the southeastern United States.