I visited the Cahokia Mounds in Collinsville, Illinois over the weekend. These mounds are the remnants of a city that existed on this continent long before the Europeans made contact with the native people. Some of the mounds are meant as burial mounds while some of the other ones served as the home of what the Visitor's Center refers to as kings. As many as 40,000 people were thought to have lived in or around the city. The entire city was surrounded by a fence composed of logs placed upright in the ground. The adobe structure in the last picture resembles the kind of material used in constructing some of the houses at the site.
The guides recommend the month of April as the best time to visit the site because of the cooler weather. It was particularly hot and humid when I visited. I suspect that it would be especially pretty in the autumn.