How the Ozarks Formed

Recently my wife and daughter and I hiked to Big Bluff, a majestic roost 500 feet above the Buffalo River, which flows through the Ozarks of Northwest Arkansas. Accessible from the “Goat Trail” spur off the Center Point Trail, Big Bluff is situated between Steel Creek campground and Horseshoe Bend (or Hemmed-In Hollow, depending on which side of the river you’re on). It is, in my opinion, the jewel of the Ponca Wilderness Area and one of the most impressive overlooks in Arkansas.

Along the trail there, just before you reach the best view of the river valley and surrounding hills, there is a somewhat famous spot, a hole allegedly chiseled by local residents many generations ago so their children could walk safely to a one-room schoolhouse near the river at the bottom of the hill and not too far from the equally famous cabin inhabited by Granny Henderson.

At this same spot, less than 10 feet from the man-made hole, is a small cavern and pyramid-ish-shaped limestone formation clearly carved out by water erosion. In fact, the whole landing there is a big semi-circular room created, at some point, by swirling water.

Did I mention we were 500 feet above the river?

I started to explain this to my 13-year-old daughter. She lasted through the first sentence of my unsolicited lecture – “You see, Nora, this whole area here was under the sea.” – before she walked away and joined my wife to marvel at the river valley.

My whole life – starting with my dad when I was just a little boy and ending with Ken Smith (author of Buffalo River Handbook) sitting at the front of my canoe during a river trip five years ago – people would always tell me the same thing I tried to tell my daughter: “All this (the Ozarks) used to be under the ocean.”

Okay, I thought. Based on these fossils I keep finding, and a thousand eroded rock walls like the one at the top Big Bluff, I’ll trust you on that. But just exactly how did the Ozarks crawl out of the ocean? No one ever explained that.

Sometimes the answers to questions come without the question being asked. A while back, a reader commented on a story in Research Frontiers. The gentleman had more than an amateur’s knowledge of geology and the Ozarks, but he was not a trained geologist. We put him in touch with Steve Boss, geosciences professor. Below is an edited excerpt of Boss’s reply to our reader.

The Boston Mountains and the Ouachita Mountains were uplifted at the same time, during the assembly of Pangea, when northern South America (present-day Venezuela and Colombia) collided with the southern margin of North America. As the uplift proceeded, the Appalachians to our east and then culminating with the uplift of the Ouachitas to our south, the seas drained away from the Ozarks region and the rocks remained continually exposed and subject to erosion. Thus, the landscape of northern Arkansas and the Ozarks is the result of this continuous exposure over an interval in excess of 200 million years.

The Boston Mountains are one of three “plateaus” of the Ozarks. The other two are the Springfield Plateau (on which we live in northwest Arkansas) and the Salem Plateau (to our north in Missouri and extreme northern Arkansas). There are no volcanic rocks in the Ozarks. All exposed rocks of the three plateaus are from the Paleozoic Era, specifically the Ordovician through Pennsylvanian periods (roughly 299 million to 485 million years ago). All of these rocks are sedimentary, composed of limestone, dolostone, sandstone and shale.

The Boston Mountains are the highest of the Ozark Plateaus, and they also have the deepest gorges. However, the highest elevations in Arkansas are actually along the northern edge of the Ouachitas (Mt. Magazine, Mt. Nebo, etc.). The rocks of the Boston Mountains sit on top of the rocks of the Springfield Plateau, and the rocks of the Springfield Plateau sit on top of the rocks of the Salem Plateau.  Put another way, if you drilled through the Boston Mountains, you would find the rock layers of the Springfield Plateau beneath them and beneath that, the rocks of the Salem Plateau.

For more information about the geology and plateaus of the Ozarks, visit the Arkansas Geological Survey.