Southwestern Ohio's limestone is saturated with life -- life from about 450 million years ago, that is. Our local limestone layers reflect times of relative prosperity for the wide variety of marine invertebrates living in a warm, shallow sea that covered much of North America. However, this prosperity was interrupted time and time again by large storms. Records of these conditions have been left behind in the layers of limestone and mudstone in local bedrock. The fossils that these layers contain are world-famous for the details that they record about life on Earth during the Late Ordovician Period. Besides preserving pieces of Earth's history, limestone has many modern uses, such as road surfacing material, building stone, and concrete. Pure limestones are white or almost white, but impurities, such as clay, sand, organic remains, iron oxide, and other materials, cause them to exhibit different colors, especially on weathered surfaces.

Find more information at these sites:
Rocks at the Ohio Dept. of Natural Resources (PDF)
Limestone at Wikipedia.org