It’s not fair to ask a geologist their favorite thing. The world is vast and wonderful and interesting, and so many amazing features exist at a variety of scales that tell us the history of our home. However, if I were forced to pick my favorite geologic feature in Chattanooga, which I have been asked to do, it would be the Cumberland Plateau.
This extensive, flat-topped plateau stretches through Tennessee, from Alabama to Kentucky, and it’s hard to miss as it towers over and creates the skyline west of Chattanooga. I love it because it’s the result of hundreds of millions of years of vast changes that have taken place in this one area of the planet – the area that affects us, our part of the world.
At the base of the plateau, near the valley and at the bottom of the Tennessee River gorge, the rock that builds the visible foundation of the plateau reveals fossils of shelled organisms called brachiopods and parts of the bodies of crinoids that were once the stalks of organisms that anchored to the sea floor and filtered their food out of the sea.
In these rocks, called limestones, these fossils show us that our area was a warm, shallow ocean in a tropical climate. At the top of the plateau, along the spectacular bluffs such as those at the tips of Walden’s Ridge near Signal Mountain and Lookout Mountain, you can find ripple marks in sandstone rocks. These ripple marks are the same features that you’d see in the sand of the bed or along the banks of a river or stream; the ones in the rocks differ only in that they were set and preserved for millions of years.
There are also beautiful features called cross-beds that look like they were made by a paintbrush. Between many rock layers on the plateau, you’ll also find coal, a rock that has been critical to providing energy to the Chattanooga area and to Tennessee throughout its modern history. The coal we find today represents ancient swamps, thick with vegetation that sat in stagnant water, building up organic matter over time.
The ripples and cross beds, along with rocks containing round, smooth gravels, tell us that large river systems and fanlike deltas moved across the land, laying down sediment and working around the swamps. In the sandstone and coal layers, you can find the fossils of plants called Lepidodendron and, in some areas, the fossils of other fern-like plants that confirm that this area was no longer a shallow sea.
It’s amazing to think that this simple sequence of rocks along the Cumberland Plateau – limestone on the bottom and sandstone at the top – tells a really cool story of a shallow sea that evolved to land over millions of years. Even more amazing to me is that we can interact with these rocks and see the evidence for ourselves.
This shallowing of the ocean to land resulted from the slow collision of two major continental masses to our east that triggered the land to rise up and cause large rivers to flow off a huge mountain range to our east – the ancient Appalachian Mountains, which were once the size of the Himalayas. Since then (over a couple of hundred million years), the landscape has significantly changed, and much erosion has happened to reveal to us the record of the exciting past.
The Tennessee River has cut down and through the landscape, exposing the plateau and the river gorge. Along with the erosion from the river, the limestone at the base of the plateau has been subject to a type of erosion that forms caves and caverns like Racoon Mountain Caverns and Ruby Falls, adding even more exciting geologic features to our area.
So, why do I love the Cumberland Plateau? Because this vast geologic feature holds so much information about our past. It’s a place where we recreate, explore and connect with nature, and even get some of the resources we need to live and work. This broad feature and all the smaller features that contribute to its overall story are like tiny pieces of a puzzle that connect us to our past. I think that’s cool.
“My Favorite Thing” is a regular feature in which Chattanoogans from all walks of life write about the one thing they enjoy the most in the Scenic City. Installments unearth hidden gems, offer fresh perspectives of local mainstays and reveal the rich diversity of Chattanooga.
Amy Brock-Hon is a professor of geology at UTC. She likes rocks. Really, really likes rocks. She also enjoys telling anyone who will listen about how cool rocks are and how awesome the geologic history of Chattanooga is. She also encourages you to get out and explore and learn about the beautiful and interesting place in which we live. Say hi to the rocks for her.