The woodland period is the period the American Indians were in when we discovered them. They used to have cities and functional civilization some time before our discovery of them, but somehow that collapsed and they entered a long dark ages, which is how we discovered them.
What do you think caused civilization to collapse for so long, and what kept it from re-igniting?
CGP Grey had an interesting take on it, suggesting that domesticated animals were a huge pressure to create cities. North America severely lacked in domesticable animals.