There is a common misconception that the Native Americans were primitive or only a step or two removed from barbarians. In reality they built sophisticated societies across the Western Hemisphere. The cities of the Aztecs, Mayans and Incas are a testament contrary to the myth. So are the sophisticated irrigation works and brick towns of the Southwest. In the Ohio and Mississippi Valleys natives built thousands of mounds across the region. Many were complex shapes, effigies of animals or were part of sprawling earthworks. Mounds often hid burials, some with underground tombs. Places like Cahokia in East St. Louis, Illinois, were the centers of massive populations. This city centered around an earthen flat-topped mound that resembled the pyramids in Mesoamerica in shape and Egypt’s in size. It was no small feat for pre-industrial peoples to build such structures that seem only mounds of dirt to us.
Sadly, the vast majority were destroyed as settlers moved into these areas. Where they stood out in the relatively flat lands of Ohio, town-builders and farmers were quick to level them. In hillier lands, what mounds existed often blended into the terrain. Farmers, who had to farm the rolling land as it was, were more apt to leave mounds alone. At times they farmed around and over the mounds. I suspect that some have survived to this very day hiding in plain sight.
The Mound Builder cultures (Adena, Hopewell and Mississippian) were gone by the time the colonials arrived. The Indians then living in the lands knew little about those who had lived here before. The colonials couldn’t believe that the natives were sophisticated enough to build such things which gave rise to the fantastic tales of Josiah Priest, and some say the books of Joseph Smith, founder of the Mormon Church. What archaeology has uncovered, and some native legends confirm, is that after centuries of civilization the Mound Builders succumbed to war, assimilation and other factors.
There are those, however, who have uncovered evidence that they believe points to outside influence on these Mound Builder cultures. It’s not proof of the spinners of fables (whose stories bare scant resemblance to known history), but, in particular, the suggestion is of pre-Columbus explorers and settlers from across the oceans. Controversial to say the least, but not in the way one may think. These visitors assimilated into existing cultures and added to them from their own. Sounds innocent enough, but this diffusionism is not so simple. More on this later…
(Adapted from here. See book for references.)
And on 
Darrick Dean 