Shale, also called mudstone, is a sedimentary rock formed from fine particles such as mud, silt and clay. Shale is formed by finely bedded material that splits into thin layers. The pieces commonly found in Southwestern Ohio were formed about 450 million years ago during the Late Ordovician Period, when a warm shallow sea covered much of North America. Shale typically formed in the deeper parts of this sea, where fine-grained sediments were deposited in quiet waters. These sediments originally came from an active volcanic mountain chain near the present-day Appalachians, and were brought here by hurricane-sized storms. Although local shales typically contain fewer fossils than the fossil-rich limestone with which they are interlayered, they nevertheless preserve valuable information about where the sediments came from as well as clues to sea level changes over time. Mudrocks, mudstone, and shale comprise about 65% of all sedimentary rocks. In the roll over image, the penny helps to show the size of the shale specimen.

Find more information at these sites:
Ordovician Geology (PDF)
Shale at Wikipedia.org