photo: the Ordovician sea
by Rick Forrester and Janet Slater
We are so enamored with the “bike” trail, we sometimes forget about the Park areas that aren’t manmade. If you meander off the asphalt/berm and head down into one of the stream crossings, you will find a variety of historical adventures awaiting you in the rocks.
Southwest Ohio’s surface rocks are the oldest in the state. Most of the rock you see in Warren County is from the Ordovician (440-500 million years ago) period of the Paleozoic Era. Older Cambrian rock (mostly sandstone) lies deep below it, but no “modern” rock strata for the Mesozoic Era (65-225 million years ago) or Cenozoic era (2-65 million years ago) are present as they have all been abraded and weathered away. Our Ordovician strata are mostly limestone and shale, rocks that form on the earth’s crust. But this bedrock was far from here when it was formed, because what is now Ohio was about 20 degrees south of the equator at that time.
What is now southwest Ohio was once a vast but shallow saltwater sea, and our Ordovician rocks were formed from minerals, older rock sediment and shell fragments settling to the bottom. The tropical climate was ideal for marine life, so we now find a wide variety of fossils in this part of Ohio. Trilobites, our official state fossil, were some of the first animals with complex exoskeletons (outer shell-like skeletons). Sponges, jellyfish, and brachiopods (similar to clams) were abundant. The Caesar Creek area is an excellent area to hunt for fossils, especially the spillway area of the dam.
Take some time and check out the stream beds, and travel back to an ancient time with tropical temperatures and a saltwater sea. You never know what ancient history awaits you.
Photos above: Trilobite, brachiopods
Below: Geologic map of Ohio and Geologic Time Column, from Ohio’s Natural Heritage, Ohio Academy of Science
