Excerpts from Books and Wikipedia
"By Leif Ericsson's day, the Hopewell culture had spread across what is now Ohio and Illinois, reached its height, and dwindled away. The Hopewell builders left behind enormous tomb mounds, each a geometric set of circles and squares, and a winding mysterious earthwork . . . In their place, and slightly more to the south, the Mississippian peoples built cities. The largest of these was Cahokia, spreading across almost five square mile, with perhaps thirty thousand people living in it. They had left behind them tremendous earthen mounds, the remains of foundations and well-planned streets, tools and figurines, burial grounds and sacrificial pits-- one containing fifty young women, all apparently executed at the same time to accompany a great nobleman into death. Farther west, the Anasazi people built complexes from adobe bricks of baked clay and sand; rows of linked dwellings some with as many as seven hundred rooms, housing thousands. The Anasazi were hunters, farmers, and turquoise-miners; their civilization reached its height just before AD 1100." [Bauer; Medieval World, p. 577]