Researchers tested sediment core samples from Mound B at Louisiana State University in the U.S. They found ash from reed and cane plants, as well as burned osteons (microscopic pieces of charred animal bones), which date the start of the mound’s construction to roughly 8000 BCE. I believe I’m correct in saying that this pushes the date of mound building in North America back by about 4,500 years (Watson Brake is from 3500 BCE). The mound was then apparently abandoned 8,200 years ago at a time when temperatures dropped substantially in the northern hemisphere, and was then reconstructed 6,000 years ago. The crest of the final mound may align with the star known as Arcturus. And the plants that I mentioned were likely burned ceremonially, possibly for animal cremations. I think it’s important to mention here that the vast majority of mounds (I’ve heard as high as 90%) in North America have been plowed or otherwise destroyed over the years, so there are huge holes in the data set.