A third Paleozoic cratonic basin that underlies parts of Illinois, Indiana and Kentucky.
Experienced a pre-Silurian history similar to that of the Michigan and Northern Ohio basins.
Had a Silurian barrier reef that did not encircle it or restrict circulation, in contrast to the other basins.
Contained many scattered patch reefs as well, such as Thornton Reef.
Left-hand image courtesy of Ron Blakey, Northern Arizona University.
Right-hand image courtesy of The Field Museum, Chicago.