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Physiography of Alabama

Sunday, January 10, 2016 1:04
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(Before It's News)

Physical Divisions of the United States

In my article “Blood and Soil: How Southerners Became a Separate and Distinct People,” I noted how the South is dominated by two major physiographic regions: the Gulf and Atlantic Coastal Plains, which stretch from San Antonio, TX to Wilmington, DE, and the Appalachian Mountains, which stretch from Birmingham, AL to Maryland and beyond into Pennsylvania and the Northeast.

There are a few other important regions: the Piedmont, which forms east of the Appalachian Mountains and stretches from Alabama to Maryland, the Cumberland Plateau, which forms west of the Appalachian Mountains and stretches from Alabama to West Virginia where it becomes the Allegheny Plateau, the Interior Low Plateau, which stretches from northwest Alabama to Kentucky, the Ozarks and Ouachita Mountains in Arkansas and Missouri, and the Central Lowlands in Texas and Oklahoma.

Image 1: Physical Divisions of the United States (Courtesy of Google Images)

Alabama Physiographic Regions

In Alabama, we have 5 of the 9 physiographic regions found in the South:

1.) The Highland Rim – The Highland Rim is found in extreme Northwestern Alabama. It is a sliver of the Interior Low Plateau which dominates Middle Tennessee and most of Kentucky. This region of Alabama was part of the ancient geological core of North America – Laurentia – and became part of the supercontinent Laurasia – which combined Laurentia with Eurasia – around 500 million years ago.

2.) Alabama Valley and Ridge – The Alabama Valley and Ridge province is the southern end of the Appalachian Mountains that runs through Northeast Alabama. Around 300 million years ago, the supercontinent Laurasia collided with the supercontinent Gondwana (Africa, South America, Australia, Antarctica, India) which thrust up the Appalachian Mountains, which at one time was the size of the Alps in Europe. This created the supercontinent Pangaea which united all the major continents on earth into one megacontinent.

3.) Cumberland Plateau – Like the Valley and Ridge, the Cumberland Plateau was uplifted by the collision of Laurasia and Gondwana hundreds of millions of years ago, although the deformation was not as severe. Over millions of years, the erosion of the Appalachian Mountains deposited vast amounts of sandstone, shale and silt in the region, which in turn was severely eroded so that now only small areas of level land remain.

4.) Piedmont – The Piedmont contains the most ancient rocks found anywhere in Alabama. Some are billions of years old. The Piedmont is the remains of a micro-continent that was smashed in between Laurasia and Gondwana when Pangaea was created.

5.) Gulf Coastal Plain – As you can see, the Gulf Coastal Plain contains all or part of 45 of the 67 counties in Alabama, and dominates 70 percent of the state. When Pangaea broke apart 175 million years ago, it ripped away part of Africa and South America which was fused to North America. The fall line that divides the Gulf Coastal Plain from the other regions of Alabama used to be the shoreline of the Gulf of Mexico.

Over the course of millions of years, an unimaginable amount of silt and sediment was carried down from the highlands to the coast where it was deposited in a shallow sea. In a series of stages, the shore of the Gulf of Mexico retreated to its present location leaving behind bands of sediment. In fact, the shoreline has advanced into and out of the Gulf Coastal Plain at least 20 times. The delta of the Mobile River used to be beyond Dauphin Island and what is now Mobile Bay is a flooded river valley.


The Tallahassee-Suwanee Terranae

Like Madagasar, Florida is a detached fragment of the African plate.


Alabama Physiographic Subregions

This map contains a useful breakdown of the Gulf Coastal Plain.

In Southeast Alabama, we have the Fall Line Hills, the Black Prairie, the Chunnenuggue Hills, the Southern Red Hills, and the Dougherty Plain. Apparently, the geology of this part of the state was more influenced by sedimentary deposits from rivers while Southwest Alabama was more influenced by the sea that covered the area.

Although you can’t see it from the map, I recently learned that the course of major rivers in Alabama has changed across history. The Alabama River used to flow south of its present location. The area that is now the Tennessee River in North Alabama used to be the headwaters of the Black Warrior River and the Tallapoosa River used to connect to the Conecuh River in Bullock County.



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