The first structures built at Fort Frederica were of the "lean-to" variety, but here's a closer look at the building materials they used later on: Imported brick 'n' tabby! Tabby was a concrete made of burned oyster shells, water, sand and whatever else was handy...which sounds pretty sketchy, but here it still is.

When settlers arrived at what would become Fort Frederica, they found piles of oyster shells, left there by the locals who had consumed the edible parts and understandably discarded the impossible-to-digest bits (which were the shells). James Oglethorpe himself, having seen tabby utilized as a building material for Spanish forts, suggested its use at Fort Frederica. Thanks for the trash, Indians!

A not-so-nearby sign describes all of this much better than I just tried to do: Strong Walls From Refuse.