so-called America's Stonehenge has also been dated; part of the main, central structure was excavated by archaeologists from Pennsylvania (one of the universities, I just don't recall which one). A part of the structure used as a hearth was discovered, and the ash dated to 2000 BC. The thing that impressed me about the site was its size -- the hilltop is ringed by rough walls of piled stone (no mortar, or at least none left). And the core structure may very well been used as foundation for a recent colonial structure, but it's obvious from the construction that it wasn't "root cellar" or even just built as a house foundation. Anyone who would take the time to build like that in stone would just make the entire house of out of stone.
A big continent and lots of sea faring peoples on other continents and lots and lots of time.
Imo, it’d be a true wonder if there weren’t artifacts of ancient peoples from other places here.