A Late Silurian circular coral-strome patch reef just south of Chicago.
The surrounding water was about 200 feet deep.
Images modified from J.J.C. Ingels, Geometry, paleontology, and petrography of Thornton Reef Complex, Silurian of northeastern Illinois, AAPG Bulletin; March 1963; v. 47; no. 3; p. 405-440.