More than 400 million years ago, a warm, shallow sea covered Ohio. Sand, rocks, silt, mud, and minerals formed the seafloor, and small shelled animals lived in the water. As these animals died and new sediments washed into the sea, layers built up on the seafloor, with newer sediments covering older ones. Over time, the growing mass compressed the deeper layers into sedimentary rock. The park’s bedrock includes two kinds of sedimentary rock: dolomite from the Silurian Period and Columbus limestone from the Devonian Period. The dolomite forms a thicker, stronger layer of rock, while the Columbus limestone preserves evidence of animals that lived on the ocean floor, including seashell-shaped fossils called brachiopods, one kind of shelled animal from the ancient sea.