2015/11/13

Tennessee's Cumberland Plateau

Cumberland Plateau, Tennessee
How it got here

About 325 - 260 million years ago, a mountain building event, called the Alleghanian orogeny happened as a result of the collision of the North America and Africa continental plates. The Cumberland Plateau represents the least deformed and westernmost region affected by this event.

The Cumberland Plateau is defined by Pennsylvanian sandstones. The Pennsylvanian sandstones are often referred to as "caprock" by geologists because they lie atop the more easily weathered Mississippian sedimentary rocks and provide a protective roof.

The Plateau is known for many unique physical features, as well as its notably biodiverse rivers and streams. In this blog post I will explore some of my favorite Cumberland Plateau physical features, and discuss a little about their geomorphology.

Geology

Crossbedding, Crab Orchard sandstone, Highway 68, Rhea County, Tennessee Rockcastle Conglomerate crossbedding, Pickett SF & WMA, Picket Co, TN
Crossbedding

You're looking at ancient sand dunes. The sloping lines are the receding limbs of the dunes on what was once a coastal plain. The picture from the upper right (Rockcastle Conglomerate crossbedding) also shows more coarse gravels in the matrix, which suggests a higher energy environment than purely aeolian (wind). It was likely a river, or even a tidal environment.
You can find crossbedding in most of the sandstone outcrops on the Cumberland Plateau.

Lepidodendron fossil, Graves Gap Formation, Morgan County, Tennessee Lepidodendron fossils, Brandon Page, Graves Gap Formation, Morgan County, Tennessee
Fossils

The Cumberland Plateau's sediments were deposited during the Pennsylvanian (Carboniferous for those of you not in the Americas). During that time, vast forests covered the continents. Those forests would later become coal, and on occasion, fossils, which we find in a handful of places on the Cumberland Plateau.
Fossils are more prevalent on the eastern flank of the Cumberland Plateau (Walden Ridge). If you are in that area, find shale beds and look carefully. Lepidodendron fossils are easy to find!
http://www.dcnr.state.pa.us/cs/groups/public/documents/document/dcnr_014599.pdf

Liesegang rings, Claire Murphy, Fentress Formation, Big South Fork NRRA, Scott Co, TN Liesegang rings, Rockcastle conglomerate, Eagle shelter, White County, Tennessee 2
Liesegang Rings

Liesegang rings are often visible on the underside of cliffs and rock shelters of the Cumberland Plateau. They appear as iron-rich extruded ribbons from the surface of the stone. They are slightly more erosion resistant due to their high iron content, and the softer adjacent rock will weather away more quickly, resulting in the extruded surface.
I have noted that liesegang rings are more common the further north one goes along the Cumberland Plateau, with places like the Big South Fork NRRA, or the Daniel Boone National Forest in Kentucky having lots of examples.
https://en.wikipedia.org/wiki/Liesegang_rings_(geology)

Limonite, Graves Gap Formation, Morgan County, Tennessee Cumberland River, Cumberland Falls State Resort Park, McCreary County, Whitley County, Kentucky
Minerals

Hematite, and limonite are both found in places, in varying qualities on the Cumberland Plateau. But the mineral that is most known of and sought after in the region is coal.
Coal is found in seams which follow the bedding planes of the local strata, which are generally horizontal. Low quality hematite tends to be found more frequently on the southern reaches of the Plateau, at places like the aptly named, South Pittsburgh.

Crab Orchard sandstone quarry, Rhea County, Tennessee Art Circle Library, Crossville, Tennessee
Sandstone as a Resource

Sandstone has been used historically as long as records have been kept on the Cumberland Plateau. Early settlers immediately recognized its use in construction. Specific types of sandstone, like the world famous Crab Orchard, are quarried on the plateau for use in construction.
Many places on the Plateau employ sandstone construction, with one of the more spectacular examples of it being the Art Circle Library in Crossville (pictured right).

Tafoni, Pennington formation, Putnam Co, TN Stone Cove Arches, Putnam County, Tennessee
Tafoni

These small cave-like features often occupy the underside of a cliff or a rock shelter. These shapes form on rocks in all manner of environments. While it's not exactly understood how they form on the sandstones of the Cumberland Plateau, it's certain that water moving through the porous medium of the rock plays some role in it.
Tafoni can often be found adjacent to liesegang rings, and is very common in the Big South Fork NRRA.
https://en.wikipedia.org/wiki/Tafoni

Geography

North Twin Arch, Big South Fork NRRA, Scott Co, TN Princess Arch 3, Daniel Boone National Forest, Wolfe Co, KY Campground Natural Bridges, Putnam Co, TN Rock Bridge, Swift Camp Creek, Daniel Boone National Forest, Wolfe Co, KY
Natural Arches

One of the most spectacular features of the Cumberland Plateau, natural arches occur on ridges and cliffsides as the result of differential weathering along a cliff face.
While each arch has a slightly different story, the general rule is that the sandstone cliffs here erode from the base, not uniformly along their vertical face (see Rock Shelters). Many cliff faces have joints which run parallel to the cliff face, and when the erosion process finds that joint it can create an arch.
Tom Dunigan's Tennessee Landforms webpage is a great resource for finding out more about where you can find these amazing features.
https://en.wikipedia.org/wiki/Natural_arch

Welch Point, Bridgestone Firestone WMA, White County, Tennessee 10 Sunrise, Rocking Rock, White County, Tennessee 2 Buzzard's Roost, Fall Creek Falls State Park, Van Buren County, Tennessee 1
Overlooks

The rule of a good overlook is that the cliff its on must be higher than the trees below it, lest your view be obscured. There are dozens of great overlooks, many of them on private property on the Cumberland Plateau. If you're unsure of where to find a great overlook, try Buzzards Roost in Fall Creek Falls State Park, or Welch Point in the Bridgestone Firestone Centennial Wilderness WMA.

Glade Creek, Bledsoe SF, Bledsoe County, Tennessee 3 Glade Creek, Bledsoe SF, Bledsoe County, Tennessee 5 Emory River, Morgan County, Tennessee 1 North Fork Honey Creek 2, sandstone, Big South Fork NRRA, Scott Co, TN
Rivers

The rivers of the Cumberland Plateau are beautiful, rugged, and full of life. They are internationally recognized for their biodiversity, especially with regard to their fish and mollusk populations.
I also think it's pretty neat that the Emory River flows through an ancient fault line here.


Indian Rockhouse Nr1 twilight, Big South Fork NRRA, Scott Co, TN Rock shelter 1, Pogue Creek SNA, Pickett Co, TN Rockcastle rockhouse 3, Middle Creek Nature Loop Trail, BSF NRRA, Fentress Co, TN
Rock Shelters

As mentioned with natural arches, sandstone cliffs of the Cumberland plateau tend to erode from the base as opposed to unformly along the face of a cliff. This results in overhung features that are called rock shelters, or as the locals refer to them, rock houses.
https://en.wikipedia.org/wiki/Rock_shelter

Stone Door 2, Savage Gulf SNA, Grundy Co, TN Alfred Crabtree, Abby Harmon, Athawominee, Bledsoe County, Tennessee Devils Cave, Big South Fork NRRA, Scott Co, TN
Slot Canyons / Caves

What? Caves in sandstone? Not really. The difference between a cave, an arch, or a slot canyon on the Cumberland Plateau is both a matter of semantics, as well as chance that organic debris and sediment have managed to make a "roof" above a slot canyon, as is the case through most of Devils Cave in the Big South Fork NRRA.
In other places along the rim of the Plateau, large joints have opened as a result of erosion, or perhaps they have been undermined by a deep cave system in the Mississippian limestones below.

Devils Creek Falls, Putnam Co, TN Cane Creek Falls frozen, Fall Creek Falls State Park, Van Buren County, Tennessee Cane Creek Cascades, Fall Creek Falls State Park, Van Buren Co, TN Greeter Falls, South Cumberland State Park, Grundy County, Tennessee 1 Wildcat Falls, White County, Tennessee 4 Honey Creek Falls 3, Big South Fork NRRA, Scott Co, TN
Waterfalls

The Cumberland Plateau has plenty of them. Tom Dunigan's Tennessee Landforms is a good resource for finding (and sharing!) waterfalls. Another good resource is Greg Plumb's Waterfalls of Tennessee