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Assuming here might be someone who knows something about this, I wanted to ask what is behind this round structure I have spotted today on Google Earth:

Google Earth map

There seems to be a large (~200 km), nearly perfect half-circle covering the states of Alabama, Mississippi and Tennessee. How could this regular structure possibly originate? I did some research on the web, but could not find anything. It looks a bit like an impact crater, but there is none listed in this location and especially of this large size. So how did this structure emerge?

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This is a sedimentary sequence representing the shoreline of a Cretaceous-Paleogene inland sea, the Western Interior Seaway. You can look at the sequence of sediments laid down in the USGS Geological Map of North America. I recommend downloading the Southern Sheet in high resolution and the Explanation Sheet to explain what's going on.

The land use pattern as seen by other people answering this question is actually putting the effect in front of the cause; due to the nature of these sediments being a positive influence on the fertility of the land, it is more likely to be used for farming.

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    $\begingroup$ Okay, thank you. So this is a prehistoric shoreline. But is there any reason for it to be so perfectly circular? Or did that just happen by chance? $\endgroup$
    – adjan
    Commented Jan 14, 2016 at 13:00
  • $\begingroup$ It's a very hard question to answer. We can look at the sedimentary sequence from road cuttings and drill core, and determine lots of things such as paleocurrent flow direction and the environment of deposition (tidal flat, beach, shallow ocean etc) and piece together a vague description of what was going on at the time, but in the end it's a 60-odd million year old shoreline. Looking at the geology map it seems like there might be a headland of crystalline basement that the curve may have formed around, but there's probably a Masters project in geomorphology to be done to answer this. $\endgroup$
    – Ben MS
    Commented Jan 14, 2016 at 21:36
  • $\begingroup$ Well I thought you had some direct information about the origin of this shape. Thank you for your answers. $\endgroup$
    – adjan
    Commented Jan 15, 2016 at 10:35
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The clue is its location at the southwestern end of the Appalachians. Compressive mountains have to terminate somewhere, somehow. In this case the differential strain energy between the Appalachians and the Alabama-Mississippi sedimentary trough has been taken up by deformation of the softer younger sediments. Browse through mountain chain terminations on Google Earth, and you may find other examples (e.g. Carpathians, north east Iran) although your example is one of the clearest.

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It is indeed a land use pattern, but I would guess it is driven by geology. I'm sure someone will come up with some details, but in the meantime check this wikipedia on the geology of the Appalachians.

See how this map corresponds to what you're seeing on the landscape.

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It's the Black Belt Prairie. NASA's Earth Observatory featured it:
https://earthobservatory.nasa.gov/images/92321/black-belt-prairie

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  • $\begingroup$ While this link may answer the question, it is better to include the essential parts of the answer here and provide the link for reference. Link-only answers can become invalid if the linked page changes. $\endgroup$
    – hichris123 ♦
    Commented Mar 8 at 5:58
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I was looking at this as well. It looks to me like a sedimentary formation created from the southerly flows potentially off the glaciers in the north at the end of the younger dryas. Follow the St.Lawrence seaway and look at the shape of the Adirondacks, its almost like an island formed in a torrent of waters suddenly released. There was a recent find in Greenland of a large impact on the ice sheet called Hiawatha. If that was the case I would postulate that sudden melt-flows would careen down the left side of the Appalachian mountains along with an incredible amount of debris and sediment. At some point it would have found a delta area or created a debris mount that might create a sedimentary lake. It shows in the natural geology to have a sharp incline on one side and a gradual one to the north. This would line up with a depositing location along an older impact crater, mountain rise or sweep in the natural geology of the area. It looks HUGE.

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