The Thornton Quarry located near Chicago, Illinois is the site of a 420 million year old Silurian reef.
The reef was formed in a shallow shelf, epicontinental sea environment, so the rocks of the quarry are filled with fossils!
Many of the beds in the quarry are naturally gently tilted, and the structure has an overall dome shape, which is a depositional feature and not related to tectonics.
Some of the fossils commonly found at the quarry include rugose corals, crinoids, sponges, and cephalopods, along with other common Silurian life. The photos above show some top views and cross-sections of some portions with abundant coral fossils!
All images by author