Effigy Mounds Natl Mon

North American woodland Indians built numerous earthen mounds from NY to MN and the Gulf from 400 to 1300 A.D. Some were burial and others ceremonial. As a fan of human accomplishments, what's most amazing to me is not that they built so many using woven baskets and deer bone diggers or made them unique like these shaped like bears and birds.
I am intrigued by the security and abundance of food they must have enjoyed to have the luxury to undertake such massive endeavors. Or, did they have extraterrestrial assistance?
I suspect the land's bounty and lack of cable TV were major contributions to this prehistoric marvel.

A nice view of the Mississippi River when you take a break from your mound building.