Time Scavengers

Kyle here –

Geology is the physical manifestation of time. The rocky foundations of our planet are the consequence of billions of years of natural processes, many of which continue today. The record of this extensive history is visible not only as layers of rock, but also in what is missing. Although often unnoticeable on human timescales, steady erosion by wind, water, and ice is a tremendous force over millennia. And across millions of years, entire mountain ranges can be uplifted, ground down to their roots, and the resulting sediments compacted into rock and uplifted into mountains anew.

By definition, gorges and canyons are among the best places to view the results of erosion, often combining exposed bedrock with more superficial—but no less interesting—features carved by running water. The bedrock may also record evidence for its own intriguing origin, adding more layers to the story (pun only half intended). The American West is well known for such exposures, the Grand Canyon foremost among them, but the East and Midwest also have their share, most cutting through Paleozoic strata: Letchworth Gorge and Niagara Gorge in New York, the Kentucky River Palisades and Red River Gorge in Kentucky, and many others.

Ohio has a number of notable gorges, many easily accessible to visitors within regional or national parks. Clifton Gorge, in John Bryan State Park near Dayton, cuts through Silurian strata that are age-equivalent to those at Niagara Falls. Numerous small gorges and valleys near Cleveland slice through Upper Devonian, Lower Mississippian, and Lower Pennsylvanian rocks, including the great Cuyahoga Valley (and its eponymous National Park). And south-central Ohio is home to the Hocking Hills, where great sandstone cliffs form ridges, gorges, and natural bridges within a lush, relatively undeveloped forest.

Situated near the western edge of the Allegheny Plateau, about 45 miles (~70 kilometers) southeast of Columbus, the Hocking Hills expose shales and sandstones of Late Paleozoic age. Unlike the northern and western regions of Ohio, this area was not beveled flat by glaciers during the Pleistocene and thus retains a rugged topography. Hocking Hills State Park, as well as a variety of other nearby nature preserves and local parks, is the iconic centerpiece of this scenic area, a popular destination for hikers and other nature enthusiasts. The park contains numerous gorges, waterfalls, “caves”, and cliffs, all worn out of a picturesque orange to tan sandstone.

This rock is the Black Hand Sandstone. Early Mississippian in age (roughly 355 million years old), the Black Hand is a coarse, sometimes conglomeratic quartz sandstone. It is massive in nature, without many discrete beds or major changes in its consistency. However, a number of features are visible at some localities, including cross-bedding, the angled bedding of ancient ripples or dunes, and graded beds, where layers of coarse pebbles transition upward into layers of smaller pebbles and then into sand, an indication of sorting by water.

Another common feature of the Black Hand is liesegang banding, concentric, sometimes twisty patterns of rusty staining. In contrast to cross-bedding and graded beds, which show evidence of what was going on at the time when the sand was deposited, liesegang banding formed much later, as groundwater percolated through the sandstone, carrying iron and other minerals with it. These minerals precipitated out of solution over time, forming the colorful bands. This can be seen as a form of weathering, rather than rock formation, though the distinction is rather blurred in this case as the bands can comprise lumps and stringers that are more resistant than the surrounding sandstone.

A number of waterfalls that cascade through the local gorges, including the Upper and Lower Falls near Old Man’s Cave as well as the nearby Cedar Falls. These falls have cut smooth channels into the Black Hand.

Geologists consider the Black Hand Sandstone a member of the Cuyahoga Formation. The sandstone’s lower contact is apparently erosional, with the sandstones of the Black Hand cutting down into the shales and siltstones of the Fairfield Member of the Cuyahoga Formation below. Meanwhile, the top of the Black Hand is capped by thin conglomerate, the Byrne Member of the Logan Formation. The Logan is also sandstone-rich, but less massive than the Black Hand below and may have been deposited in deeper water.

Several hypotheses have been put forward to explain the origin of the Black Hand. One suggests that it is a part of a great delta, deposited offshore in the shallow sea that blanketed the midcontinent during the Mississippian. Another proposes that the Black Hand is in fact a channel itself, formed in an estuary or river that carved its way through the underlying strata during a brief episode of low sea level. In either case, the relatively large and well-worn quartz pebbles and sand that make up the sandstone must have come from land to the east, near what are today the Appalachian Mountains. Research on this matter is ongoing at the Ohio Geological Survey and elsewhere.

While Hocking Hills may be the most famous exposure of Black Hand Sandstone, it is by no means the only one. The name was coined for prominent exposures of the rock along Black Hand Gorge on the Licking River east of Newark, Ohio. (The Gorge itself is so-named for a Native American petroglyph featuring a large black hand that was once emblazoned on one of its sandstone walls; sadly, this rock art was destroyed by 19th century construction in the Gorge. The name may also be spelled Blackhand, but the split version is preferred herein.)

Thus Black Hand Gorge is the type locality of the Black Hand Sandstone, the primary place that geologists should refer to when determining what the Black Hand Sandstone is, what it correlates to, and other questions. Although the process of naming rock units is now codified by the rules of the International Commission on Stratigraphy, rock units were less rigorously defined in the 19th and early 20th century. Additionally, some localities that once provided excellent exposures are now gone, naturally weathered away, covered by vegetation, flooded, or destroyed by later human development.

Fortunately, the Black Hand is still well exposed in its type area, easily accessible from a hike-bike trail that follows the Licking River through the gorge, passing sandstone cliffs, fallen boulders, and old quarries. In addition to the Gorge itself, nearby roadcuts afford excellent views of the sandstone cliffs to casual observers.

True Black Hand Sandstone is only exposed in Ohio. However, some other sandstones in nearby states are believed to be of a similar, perhaps even equivalent, age, including the Burgoon Sandstone of Pennsylvania and the Marshall Sandstone of Michigan. Elsewhere, such as in northern Kentucky, the same timespan is represented by shales and is much thinner. It is sobering to note that the time period that forms towering cliffs in central Ohio is elsewhere represented by just a meter or so of mud or, in others, by nothing at all.

Similarly scenic sandstone gorges are exposed throughout the Midwest, including the previously mentioned Red River Gorge in Kentucky and Turkey Run State Park in Indiana. However, these sandstones are typically younger in age than the Black Hand, often Pennsylvanian, deposited as the American midcontinent sea was shrinking into oblivion.