Since the people who built this city left no written records, we don’t know why these mounds were built, though some were clearly used for ceremonial and religious purposes. The largest mound at Cahokia is as tall as a ten-story building (it’s estimated that it took 300 years and 15 million baskets of earth to create it). It’s the largest earthen structure in North America, with a base that covers more than 14 acres. A huge building once stood on its summit where the chief of the city lived, conducted ceremonies, and governed.
There’s even a little bit of Stonehenge at Cahokia—a circle of wood posts that align with the sun. This huge calendar has been rebuilt and bears the name Woodhenge. It’s one of five circular sun calendars that have been excavated at Cahokia. Of differing sizes, they were likely used to determine the change of the seasons and ceremonial dates. Each had a number of large, evenly spaced red cedar posts, including ones that aligned with the rising sun at the spring and fall equinoxes and the summer and winter solstices. The reconstructed Woodhenge at Cahokia is the site of seasonal sunrise celebrations at these dates; see the Cahokia Mounds Interpretive Center for information.
We know so little of what went on here, including why the mounds were built, what they were used for, and why this civilization declined and disappeared around 1400. But we know that humans have an almost instinctual urge to create structures that evoke awe. Cahokia Mounds still does so.
Cahokia Mounds have been designated a World Heritage Site by the United Nations. They are indeed a treasure, one that whispers to us across many centuries.