Hocking Hills State Park, located southwest of Logan, Ohio, offers some of the most beautiful geology in the state. The geologic features in the park are all thanks to the weathering of the Black Hand Sandstone, part of the Cuyahoga Formation that was deposited early in the Mississippian Period (359-318 million years ago). Once thought to have been formed by a river delta, this sandstone has since been shown to be more consistent with incised valley fill. The area was near a coast at that time, with much of Michigan and Ohio inundated with a shallow inland sea. The ancient Appalachian Mountains to the east fed rivers which flowed into the basin. This location was situated in one such river valley, where sand and gravel were deposited by the ancient river flow.
Long after being deposited by the action of moving water, the sandstone is now being worn down by it. The area features seven points of geologic interest, six of which I visited this past weekend.
The largest recess cave in Ohio, Ash Cave is roughly 700 feet across, 100 feet deep, and 100 feet high. As with other features in the area, the cave formed from the differential weathering of the Black Hand Sandstone. This member is several hundred feet thick here. The sand and pebbles in the upper and lower parts are highly indurated (cemented tightly together and resistant to erosion). The middle part is less indurated, and more susceptible to erosion.
Old Man’s Cave
Similar to Ash Cave, Old Man’s Cave is also a recess cave formed from the erosion of the variably resistant sandstone. Looking at the two caves you can see the running water that has caused this erosion over the last few tens of thousands of years. While the top and bottom of the rock unit are resistant, the middle section is more easily worn down by the steady and unrelenting force of water.
The impressively loud flow at Cedar Falls gives a more dynamic example of the erosive power of running water. Here Queer Creek found a joint (fracture) in the resistant upper sandstone and subsequently wore into the weaker middle part. I was lucky to visit the area in late winter, when water flow from snow melt is at its peak.
At the bottom of the falls the creek encounters the more resistant lower section of the sandstone and continues on its way.
Here’s a better view of the joints that form in the resistant upper part of the sandstone, which give water an entry point to reach the less resistant middle part. Once water has permeated the tough upper layer, it’s free to wreak havoc on the middle:
This differential weathering also works on a smaller scale, in the form of honeycomb weathering:
Here small-scale weaknesses in the sandstone allow water to wear down localized spots, creating these intricate patterns.
The running water has over the millenia created more than just recess caves and waterfalls; it’s also carved steep cliffs from the sandstone. This 200-foot cliff at Conkle’s Hollow State Nature Preserve is an impressive example of river downcutting. Conkle’s Hollow is a narrow, steep-walled gorge formed by the relentless action of the creek’s flow over time.
Even the resistant upper part of the Black Hand isn’t immune to the power of water. Here a massive slump block of the sandstone has collapsed from undercutting, falling into the creek bed below.
Flowing water can also act more subtly, but still create impressive structures. At the Rock House, water has slowly trickled through joints in the sandstone to carve out a proper cave. The main cavern was created from the erosion of a main joint, while perpendicular joints were eroded into the windows that allow light to filter inside:
In addition to providing material for beautiful rock formations, sandstones like this can also serve as economically important reservoirs for oil and natural gas. These fossil fuels form from the decay of ancient organisms, and they can ascend into relatively porous sandstone. When sandstone is capped by less permeable sediments and the bedding is faulted or folded into an anticline, traps can form that keep the valuable fossil fuels conveniently in place for human extraction. This area has all of these ingredients, and is littered with several oil wells and scores of natural gas wells:
Despite the straightforward appearance of the Black Hand Sandstone, this relatively simple rock unit provides some interesting examples of ancient river valley deposition, differential weathering, variable resistance and induration, downcutting, undercutting, mass wasting, and fossil fuel reservoirs and traps. But even if you don’t care for the geology, the beautiful scenery and challenging hiking make it worth the trip.