The oldest rocks exposed at the surface in Kentucky are Ordovician in age and are exposed in the Blue Grass Region. Rocks deposited during the first half of the Ordovician Period occur entirely below the surface throughout Kentucky. Some of these deep rocks contain oil, so some oil wells have been drilled down to them.
During most of the Ordovician, Kentucky was covered by shallow tropical seas. Accordingly, the fossils found in Kentucky's Ordovician rocks are marine (sea-dwelling) invertebrates. Common Ordovician fossils found in Kentucky include sponges (Porifera), corals (Cnidaria), bryozoans, brachiopods, trilobites, snails (gastropods), clams (pelecypods), squid-like animals (cephalopods), crinoids (Echinodermata), and microscopic animals like ostracodes and conodonts.
One of the most unusual sponge fossils that occurs in Kentucky is the Brachiospongia digitata, a silica sponge.
Perhaps the most common sponge fossils found in Kentucky are the stromatoporoids, or stroms for short. Stroms are calcareous sponges that form mounds 2 to 3 feet across on the sea floor. Stroms still exist today in moderately deep water. Ordovician fossil stromatoporoids can be seen along roadcuts in the Blue Grass Region, especially near Lexington, where the white mounds contrast against the gray limestones.
One Ordovician bryozoan fossil has numerous star-shaped patterns on its surface and is therefore called Constellaria.
Fossils of the large Isotelus gigas trilobite (up to 1 foot long) have also been found in the Ordovician rocks in the Blue Grass Region. A trilobite is being captured by a cephalopod in the Ordovician scene.
One giant ostracode, Leperditia, about the size of a fingernail, can sometimes be found in the Ordovician rocks in the central part of the Blue Grass Region.
Giant water scorpions, as much as 5 feet long, may have lived in Kentucky during the Ordovician, but fossils have not been found.
A variety of stemmed crinoids are shown in the background of the Ordovician scene.