Contrary to the colonial dismissal of Indian civilization, which has endured to the present, the ancestors of modern American Indians were highly sophisticated. As this diagram shows, the Hopewell people built massive and precise structures. These were mounds of earth, topped in many cases with timbers. There were also hilltop fortifications.
The dimensions of these structures signify precise measurements for specific reasons, but scientists can only speculate on the purpose and meaning of these impressive works.