...an oblong horizontal lid (ledger stone) made of local soapstone (local greenish-grey soft chloritic schist) with two slots cut into it, through which the vertical head and foot stones are fitted, called a 'through mortise-and-tenon joint' in woodworking. The ledger stone is thus suspended over an internal empty cavity... to create an above-ground stone box monument that resembles a tomb. The body is presumably underground since no remains have been seen inside exposed internal cavities.The website catalogues 166 such tombs found in rural northeast Georgia, most for English or Scotch-Irish families, and dating from the late 19th century.
It looks like something a furniture-maker would have crafted, rather than a stonemason. More details at the link, which is quite nicely doe.