I'm well aware of the great civilizations of Central America, however I have learned very little about North America in the Pre-Columbian times. I've always wondered what was going on in the Medieval Period or Roman Era in North America. Doing my own research however brought a few interesting things to my attention.

The Mounds are well known crafts by Native Americans, but there's also strongly engrained cultural traditions and clearly defined/easily categorized language groups which I understand would normally require some kind of larger central habitation area or organized language through writing. Also evidence of large scale, organized metal working has apparently been found as far back as the bronze age. This has me wondering if there was some great civilization in North America and Columbus discovered the New World in the middle of what was, to the natives, a dark age, and what the Europeans encountered were fractured tribes and barbarian groups that broke away from this main culture. This pet theory of mine was more of a joke until I brought it up with a few friends who said there was some merit there, and a few native Americans even told me there may be evidence that many tribes fractured from some larger group, however I personally am not informed on that topic.

What do the experts think? Was there some kind of larger civilization that's just not taught about normally? Is there just a theory of some American Kingdom in the Pre-Columbian times? Or is this just a series of coincidences?