Arkansas Oklahoma
In a color-enhanced satellite image, the swirling folds of central Arkansas’s Ouachita Mountains tell the story of an ancient tectonic collision.
The middle of the North American continent is notoriously flat, but a beautiful exception is the Ouachita Mountain range. From the air, the complicated east-west trending folds of the range stand out crossing between Arkansas and Oklahoma, to the north of Lake Ouachita. These folds form the Interior Highlands, an uplifted mountainous region that also includes the Ozark Mountains.
Rising to 2753 feet at Mount Magazine, the Ouachita Mountains are the highest mountain range east of the Rockies and west of the Appalachians. The range has very deep roots, which extend all the way to southern Texas and date back 500 million years, when this region existed as a great ocean trough. Sediments collected in this trough until they were piled thousands of feet thick and compressed into layers of shale and sandstone.
During the Pennsylvanian Period, 300 million years ago, the coastline of the Gulf of Mexico was located in central Arkansas. A tectonic plate that had previously been attached to South America and Africa broke off and collided with the North American Plate from the south, buckling the overriding plate. In an episode known as the Ouachita orogeny, the layers of shale and sandstone from the area’s ancient ocean trough were uplifted in this buckling, and the ancestral Ouachita Mountains were created. The plate collision from the south accounts for the unusual orientation of the Ouachita Mountains, which run east and west—most mountain ranges in North America are oriented north and south.
These peaks once rose to Rocky Mountain heights, but 300 million years of erosion have worn them down to their roots—even lower than the Appalachians, parts of which formed around the same time. This is likely due, at least in part, to the dominance of erosion-prone sedimentary rock in the Ouachitas. The Appalachians are made up of a sturdier mixture that includes harder sedimentary, volcanic, and metamorphic rocks.
A waterfall in the Ouachita Mountains flows over an outcrop of sedimentary rock, the type of rock that dominates the area.
Look for the folds of the Ouachita Mountains en route to Little Rock or Hot Springs, Arkansas.
