When the Ohio River is low, a 386-million-year-old fossil coral reef lies bare across acres of riverbed at Clarksville, Indiana, one of the largest naturally exposed Devonian reefs anywhere. You walk on it. Late summer and autumn give the best exposure. LOOK ONLY; it is a state park and a national wildlife conservation area.
Devonian coral reef · Devonian, ~387 mya
See it on the map
Other places on the map within a couple of hours.
