Students from the GeoEnvironmental Science Program ascend a local highway road to cut in search of Ames limestone. This distinctive rock layer marks the last time seas flooded Ohio. This is a significant event in Ohio's geologic history and is one that's observed in the Historical Geology course at Hocking, which focuses on the geologic origin of the North American continent in particular.
The Ames limestone (pictured above) is an important rock layer in the approximately 1,500 feet-thick sequence of layers that accumulated in Ohio during the Pennsylvanian Period (approx. 320 - 300 million years ago). It contains distinctive marine invertebrate fossils, such as brachiopods and crinoid, and at this location, a rare shark tooth. These fossils are the skeletal remains of sea creatures that once lived in this area. This durable skeletal material accumulated on the sea floor along with the sediment that eventually hardened to make the Ames limestone.
While out, students examined a rock layer widely known in the central Appalachian region as the Pittsburgh reds. This reddish claystone colored by iron-oxide contains features indicating it formed as an ancient soil on a broad plain. In this outcrop, the Ames limestone lies about 10-feet above the Pittsburg reds. The reddish claystone is an ancient soil formed on the land. The Ames limestone is limey mud with invertebrate skeletal debris that accumulated on the floor of a shallow ancient sea. Together they are an excellent field example of the seas flooding the land, a marine transgression.
Students are often puzzled by finding evidence of the sea in rock layers high up on the local hill sides. Regarding this apparent complexity, it is important to understand that the hills of this region formed long after the seas flooded the land and deposited the sediment that would eventually become the Ames limestone.
The image below depicts the position, in the Pennsylvanian time period, of the landmass that would grow to become North American. Note that Ohio lies near to and south of the equator. To the west of Ohio lies a shallow sea flooding most of the Midwest. East of the shallow sea, Ohio occupies a broad low-lying plain. Throughout the approximately 20 million year-long Pennsylvanian Period, this shallow sea frequently rose and fell in response to the repeated partial melting and refreezing of a massive ice cap situated in the southern polar region. Sea-level rises of a few tens of feet covered a broad swath of the low-lying plain repeatedly inundating Ohio and even parts of western Maryland and Pennsylvania.
It's not hard to imagine a sea level rise of similar magnitude covering much of Louisiana and other low-lying coastal states. In contrast, imagine what a sea level drop of several hundred feet would have on a flat lowing-lying state like Louisiana. Many gentle-winding streams flowing across this swampy region would carve deep valleys to keep up with falling sea-level. As a result, flat, low-lying Louisiana would be shaped into a landscape similar to the Appalachian foothills of southeastern Ohio. These newly-formed Louisiana hills would expose, high up on their sides, sea floor deposits like the Ames limestone of our region.