OK, not trying to crash the party, but I think I have some direction for the conversation. Virtually every single one of these is south and east of the ancient coastline that ran from macon to augusta to columbia to cheraw to raleigh - the present day Sandhills. Literally 99.99% are southeast of the "sand" in the Sandhills that was that ancient beach.

That leads me to believe it is some sort of natural phenomenon that had to do with the ancient sea or seabed. But, how interesting this all is! And they look really cool from above...