Seems to make sense; when the wood coffin decayed they would sink in. These sandstone slabs helped to prevent that. Used a lot during the 1800's here in Tennessee.
Interesting. I wonder if the graves are as deep as typical graves. Could be they are not, and the stones would keep animals from digging down through the loosened dirt?
Fascinating. Since some of them are family cemeteries, wouldn't the families have the history of why they were like that?
Here's an article on the ones in Tennessee, and another one, this from Vintage News. The latter mentions Champ Ferguson, who is apparently buried in such a grave. I read his autobiography not long ago, written while he was awaiting execution. He fought for the Confederacy but was executed because he wasn't officially in the military. According to his autobiography, he never go around to filling in the paperwork required to set up the Confederate military company that he organized, thus he was tried as a guerilla.