Thornton Reef Complex

The Thornton Reef Complex, located outside Chicago, is a geological feature from the Silurian Period, approximately 443.8 to 419.2 million years ago. It is an example of a well-zoned reef mound, or bioherm, over 1 km in diameter. These reef mounds were organically constructed microtopographies on the Silurian seafloor, featuring distinct zones of invertebrates like brachiopods, gastropods, crinoids, and trilobites. Reef development likely peaked during the Wenlock epoch of the Silurian period, with thousands of bioherms recognized around the Great Lakes region of North America. Unlike the Great Barrier Reef, which is a modern coral reef complex in the Pacific Ocean off Australia, the Thornton Reef Complex represents ancient reef-building organisms.12