Turtle Mound in Florida which is a massive oyster-shell midden is being re-investigated. Archaeologists have found 1,200 year old pottery and other artifacts for radio-carbon analysis. Turtle Mound is the highest shell midden in the country being 35 feet tall at this point but may have been 54 feet tall before erosion. There are 35,000 cubic feet of oyster shells.
The mound may be as old as 1000 BCE and the new radio-carbon dates may confirm that.
Orlando Sentinel has the story here with a video;
Here is a tiny URL;
Mike Ruggeri’s Mississippians and Mound Builders including the Adena and Hopewell