The Eastern Highland Rim - Part 1 - Geology
Mississippian period, approximately 380 to 320 million years ago, the Eastern Highland Rim was formed through the deposition of sediments. At that time, a large, shallow sea covered the interior of the United States, providing a habitat for corals and other creatures that primarily created their shells from calcium carbonate. As these organisms died, their remains accumulated on the ocean floor, forming deep layers of limestone that have full of aquatic fossils. In other places, the shells dissolved and deposited as lime mud.
As the continents shifted and ocean levels dropped over time, the Cumberland Plateau and the Eastern Highland Rim moved westward and up in elevation. The sediments of the Cumberland Plateau, which largely consist of stream deposits from the former Blue Ridge Mountains, formed a protective layer preventing the complete erosion of the limestones that make up the flank of the plateau.
The rock units that comprise the Cumberland Plateau are shown in warm colors, where the Eastern Highland Rim is shown in cool colors.
The rock units are ideally separated by identification of fossils within each unit. However, in this article I will discuss more broadly the geomorphology of the rocks and the landforms associated with each unit.
In the below geologic map of White County, Tennessee, the stratigraphic column begins with the Pennsylvanian aged Rockcastle Conglomerate and as one loses elevation, one encounters new rocks. Warm colors were used to indicate Pennsylvanian aged rock, which comprises the Cumberland Plateau while cool colors were used to denote primarily Mississippian rocks comprising the Eastern Highland Rim.
A special configuration of karst window occurs between the Bangor, Hartselle, and Monteagle units where an upper cave (occurring in the Bangor) has an outflowing stream. The stream travels a short distance and drops off the edge of the Hartselle into the Monteagle. Example of this include Virgin Falls, Sheep Falls, Rainbow Falls, and many more. When this happens underground, it is often associated with especially deep and significant cave systems.
crossbeds. I most often think of it as providing a "bench" halfway up to the Cumberland Plateau where one finds flattened areas along the escarpment (like here, and here). It provides a basement unit for Bangor Limestone cave and a ceiling for Monteagle Limestone cave. It's also the stone from which comb graves are hewn.
The Hartselle Sandstone / Monteagle Limestone contact is the most prolific cave producing region accounting for approximately 70% of the caves in Tennessee. It tends to produce short caves with jagged limestone that are muddy. These are often miserable caves, but if you can get into them deep enough they can get interesting.
As a result of this units solubility, it is the most prolific cave producing unit. Tennessee's longest cave, Blue Spring, is formed in this strata. Most of the other long caves in the state are also formed in this rock unit - Cumberland Caverns, Xanadu Cave, Rumbling Falls Cave, Mountain Eye System, Nunley Mountain Cave, Big Bone Cave - to name a few. In other states, the Monteagle goes by Ste. Genevieve and Gasper Limestone. Mammoth Cave, the world's longest cave, found just north of Tennessee, in Kentucky, is formed in the Ste. Genevieve and St. Louis limestone (mentioned below).
The Monteagle doesn't tend to form as many sinkholes as the St. Louis or Warsaw units, but the sinkholes which are in it are spectacularly huge, and often the deepest and largest volumetrically.
Because rivers are very dynamic systems, it's not uncommon for the systems to change on human time scales. In February of 2014, a sinkhole swallowed part of the National Corvette Museum as a result of a collapse within the St. Louis limestone.
The St. Louis limestone tends to form many sinkholes on the surface. While they are not as large volumetrically or by area as sinkholes in the Monteagle, they are larger in number.
Much like the St. Louis, many wet caves exist within this unit, and as a result this rock unit tends to produce a large number of sinkholes.
One notable sinkhole in the Warsaw limestone happened in 1994, in Cookeville, Tennessee, when a large collapse exposing a previously unknown cave system (Tires-To-Spare Cave - which is a whole other fascinating story).
The Fort Payne is associated with waterfalls because it is underlain by the Chattanooga shale, a rock that is much more susceptible to mechanical weathering. Where streams cross the intersection of these two units, there will always be a waterfall. Notable examples of this include Burgess Falls, Cummins Falls, Waterloo Falls, and Window Cliffs Falls.
On the below picture of Burgess Falls, on the left part of the image there is an obvious change in the rocks around the rim of the waterfall. That is the contact between the Fort Payne and Chattanooga shale. Below the shale, but harder to differentiate in the photo, is the Leipers-Catheys formation, an Ordovician unit.
The Fort Payne in places tends to produce quartz geodes, some of which can be quite large! But I won't give away my favorite geode hunting locations to the internet, so good luck finding them!
19% of all families and 50% of all genera. The blackness of the shale results from its deposition in an anoxic environment providing us with the evidence of a causal factor of this extinction.
The Chattanooga shale is known to be one of the most radioactive black shales in the United States due to it containing uranium. This unit was explored by the Manhattan Engineer District and later, the Atomic Energy Commission, for suitable fuel for nuclear weapons and energy.
The uranium content of the Chattanooga shale presents potential health problems though to those who live in the area. Uranium's radioactive decay produces radium, which decays into radon. Radon travels through joints and fissures in the bedrock and settles in low lying places (caves and basements) where when inhaled for long periods of time can cause cancer and respiratory illnesses.
trace fossil, as the worm burrows from the image below from taken in the Root Cellar in Blue Spring Cave. Many well preserved fossils in this strata have been silicified, where the organic carbon is replaced by silica.
Rugosa, would form colonies of interlocking hexagonal polyps. When alone, their form looks like a large, slightly curved claw.
trace fossil is the remains of blue-green algae that grew in concentric layers above silt and sediment, creating a algae-sediment-algae sandwich. Stromatolites have largely disappeared from the Earth since many things can eat blue-green algae, but there are still a few places which prevent predation on the colonies.
Following this link takes you directly to the Google Sheet.