Round structure in southern United States 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:



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?
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.