Appalachian Plateau Geology and Landmarks

Obed Wild & Scenic River
The Obed River drains part of the Appalachian Plateau in Tennessee. Posnov / Getty Images

Stretching from Alabama to New York, the Appalachian Plateau physiographic region makes up the northwestern portion of the Appalachian Mountains. It is divided into several sections, including the Allegheny Plateau, Cumberland Plateau, Catskill Mountains and Pocono Mountains. The Allegheny Mountains and Cumberland Mountains serve as a boundary between the Appalachian Plateau and Valley and Ridge physiographic region.

Although the region is characterized by areas of high topographic relief (it reaches elevations upwards of 4,000 feet), it is technically not a mountain chain. Instead, it is a deeply dissected sedimentary plateau, carved into its present-day topography by millions of years of erosion. 

Geologic Background

The sedimentary rocks of the Appalachian Plateau share a close geologic story to those of the neighboring Valley and Ridge to the east. Rocks in both regions were deposited in a shallow, marine environment hundreds of million of years ago. Sandstones, limestones and shales formed in horizontal layers, often with distinct boundaries between them.

As these sedimentary rocks formed, the African and North American cratons were moving towards each other on a collision course. Volcanic islands and terranes between them sutured onto what is now eastern North America. Africa eventually collided with North America, forming the supercontinent Pangea around 300 million years ago.

 

This massive continent-on-continent collision formed Himalayan-scale mountains while uplifting and pushing the existing sedimentary rock far inland. While the collision uplifted both the Valley and Ridge and Appalachian Plateau, the former took the brunt of the force and therefore experienced the most deformation.

The folding and faulting that affected the Valley and Ridge died out underneath the Appalachian Plateau. 

The Appalachian Plateau has not experienced a major orogenic event in the past 200 million years, so one might assume that the sedimentary rock of the region should have long since eroded down into a flat plain. In actuality, the Appalachian Plateau is home to steep mountains (or rather, dissected plateaus) with relatively high elevations, mass wasting events and deep river gorges, which are all characteristics of an active tectonic area.

This is due to a more recent uplift, or rather a "rejuvenation," from epeirogenic forces during the Miocene. This means that the Appalachians didn't rise again from a mountain building event, or orogeny, but rather through activity in the mantle or isostatic rebound. 

As the land rose, streams increased in gradient and velocity and quickly cut through the horizontally-layered sedimentary bedrock, shaping the cliffs, canyons, and gorges that are seen today. Because the rock layers were still horizontally layered on top of each other, and not folded and deformed like in the Valley and Ridge, the streams followed a somewhat random course, resulting in a dendritic stream pattern.

 

This image from the USGS offers a good look at the topography and underlying bedrock for the entire Appalachian Highland region, and this image from Radford University focuses on the bedrock of just the Appalachian Plateau (left) and Valley and Ridge (right).

Limestones in the Appalachian Plateau often contain different marine fossils, remnants of a time when seas covered the area. Fern fossils may be found in the sandstones and shales. 

Coal Production

During the Carboniferous period, the environment was swampy and hot. The remains of trees and other plants, like ferns and cycads, were preserved as they died and fell into the standing water of the swamp, which lacked the oxygen needed for decomposition. This plant debris accumulated slowly - fifty feet of accumulated plant debris can take thousands of years to form and produce only 5 feet of actual coal - but consistently for millions of years.

As with any coal-producing setting, the rates of accumulation were greater than the rates of decomposition.  

The plant debris continued to stack on top of each other until the bottom layers turned to peat. River deltas carried sediment eroded from the Appalachian Mountains, which had recently uplifted to great heights. This deltaic sediment covered the shallow seas and buried, compacted and heated the peat until it turned into coal. 

Mountaintop removal, where coal miners literally blow away the top of a mountain to get to the coal underneath, has been practiced in the Appalachian Plateau since the 1970s. First, miles of land are cleared of all vegetation and topsoil. Then, holes are drilled into the mountain and packed with powerful explosives, which when detonated can remove up to 800 feet of the mountain's elevation. Heavy machinery digs away the coal and dump the overburden (extra rock and soil) into valleys.  

Mountaintop removal is catastrophic to the native land and harmful to nearby human populations. A few of its negative consequences include:

  • Complete destruction of wildlife habitats and ecosystems
  • Toxic dust from explosions causing health problems in nearby human populations
  • Acid mine drainage polluting streams and groundwater, destroying aquatic habitats and ruining drinking water
  • Failure of tailings dams, flooding large areas of land

While federal law requires coal companies to reclaim all land destroyed by mountaintop removal, it is impossible to restore a landscape formed by hundreds of millions of years of unique natural processes.

 

Places to See

Cloudland Canyon, Georgia - Located in the extreme northwest corner of Georgia, Cloudland Canyon is an approximately 1,000 foot deep gorge carved out by Sitton Gulch Creek. 

Hocking Hills, Ohio - This area of high topographic relief, featuring caves, gorges and waterfalls, can be found about an hour southeast of Columbus. The melting of glaciers, which stopped just north of the park, carved away the Blackhand sandstone into the landscape seen today. 

Kaaterskill Falls, New York - Ignoring a ledge that separates the falls into an upper and lower section, Kaaterskill Falls is the highest waterfall in New York (at 260 feet high). The falls were formed from streams that developed as Pleistocene glaciers retreated from the area. 

Walls of Jericho, Alabama and Tennessee - This karst formation sits at the Alabama-Tennessee border, one hour northeast of Huntsville and an hour and a half southwest of Chattanooga. The "Walls" form a large, bowl-shaped amphitheater of limestone rock.