Cuyahoga Valley National Park has a rich geologic history. It wasn’t always the picturesque valley that can be seen today. It was formed over many millions of years, with the help of oceans, glaciers, and rivers. The oldest exposed rocks in the park formed about 400 million years ago, and the valley is still being shaped and changed today by the Cuyahoga River and its tributary streams.
Picture this: It is 400 million years ago. The state of Ohio, now in the north eastern United States, sits just south of the Equator, covered in seawater. The waters covering the state are still, and there isn’t any mixing of deep and shallow waters. This means that the shallow waters, which were rich in oxygen, never brought this oxygen to the deeper waters. Because of this, shallow-water marine organisms that died and sunk to the ocean bottom were preserved as fossils. The carbon-rich organic material that sank to the bottom was compressed and formed rocks known as the Ohio Shale. This is the lowest layer of rock in the park and lower layers in flat-lying, undistorted rocks are always older than the rocks on top of them.
The time is now about 360 million years ago. The waters covering Ohio are starting to retreat... Meanwhile, streams are flowing towards Ohio, bringing with them sand and mud from eroding mountains. These streams empty into the shallow sea, carrying their sediment with them, and this is the environment that the Bedford Shale and the two subsequent rock layers formed in. The Bedford Shale is about 95 feet in height, mostly gray sandy shale, with a redder layer in the middle, which can be seen on the west side of the valley in Cuyahoga Valley National Parks, in the Chippewa Creek gorge. The Bedford Shale layer formed slightly out to sea, at an off-shore river delta.
The rock layer directly on top of the Bedford Shale is Called the Berea Sandstone. There are some fascinating structures that are preserved in this layer. Namely, in the Berea Sandstone, there is something called cross bedding, which looks like distinct layers of almost-diagonal parallel lines. Crossbedding is formed by sand dunes or ripples, which erode and allow new dunes to form on top of them, which erode again, continually eroding and redepositing over and over again. There are also ripple marks in the rocks, which are preserved ancient ripples that were formed from water movement. Depending on if the ripples are facing a certain way, you can tell if there was a current, and which way it flowed. The deeper rocks in this layer also have something called channel-form features, which look like semi-circular pieces of the Berea which break the boundary between the rock layer below them and seem to dip into the Bedford Shale. These were created when streams flowing across river deltas brought and left sand which slumped into the still-soft Bedford shale underneath it.
On top of the Berea Sandstone is the Cuyahoga Formation, which has the easiest name to remember! This layer is shale, sandstone, and siltstone, which is a rock that has a grain size between that of shale and sandstone. The Cuyahoga Formation was deposited when the area was a shallow marine shelf, and parts of it show the same features as the Berea.
After the Cuyahoga Formation, there is a slice of time missing from the rocks! Where did it go, you may ask? The rock is missing because the sea covering Ohio receded dramatically. That means that there was no more deposition of marine sediment to form rocks, and in fact some of the rock that used to be on top of the Cuyahoga Formation was eroded away.
Starting about 320 million years ago, streams flowing from the north covered Ohio, and brought with them quartz-rich sand and gravel. At the same time, oceans once again rose to cover tropical Ohio. These seas carried lots of sand and mud, which eroded from the Appalachian Mountains to the south east and from the north in Canada, and which was carried to the Ohioan seas by rivers and streams. These streams formed deltas that were covered in coal swamps. All the sediment that was laid down became the Sharon Conglomerate, which is the youngest rock layer in the park. A conglomerate is a sedimentary rock that is composed of several different grain sizes. For example, a conglomerate can have both pebbles and mud in it and in fact the Sharon Conglomerate contains abundant, beautiful white quartz pebbles. The Sharon Conglomerate tops the highest ridges and hills in the park, and like the Berea Sandstone, has crossbedding and remnants of old surface stream channels. After the formation of the Sharon Conglomerate, there was extensive erosion for about 245 million years, so if there were any rock deposits after the Sharon, they are unknown. This is another gap in geologic time.
Formation of Cuyahoga Valley
Over the course of the last 245-million-years, large river systems were developing on top of the no-longer-underwater Ohio. These eroded any bedrock layers after the Sharon Conglomerate and cut deep valleys into the rock. One of these was the Cuyahoga Valley. The Cuyahoga River, which flows through this valley, has a strange v-shaped course, which it acquired about 10,000 years ago, which is a blink of an eye compared to the hundreds-of-millions of years old rocks.
On top of the bedrock that forms Cuyahoga Valley, sediments were delivered by glaciers that existed 70,000 to 14,000 years ago. As glaciers move, they erode and pick up the rocks in their path like bulldozers. When the glaciers later start to melt and recede, they leave behind the sediments and rocks that they were carrying. Glacial sediment can also come from icy streams that flowed away from the glacier.
Some of the glacial sediment in the park forms ground moraines. These occur when glaciers melt and retreat, leaving behind the sediment and rocks they were carrying. Ground moraines are usually a relatively thin and even layer of sediment that covers the bedrock closely, like a blanket. They are unstratified and unsorted, meaning that there are no distinct layers within the moraine, and the sediments can be of multiple different sizes and compositions.
End moraines are very different-looking sediment deposits. Instead of forming from a retreating glacier, they form at the end of a stationary glacier, and are composed of sediment that has been pushed ahead of the glacier. They are thicker deposits than ground moraines, and not as widespread.
Glacial erratics are large, boulder-sized rocks that were picked up by glaciers and carried to new locations. They are recognizable because they don’t match the region’s bedrock. In Cuyahoga Valley National Park, if you look in the streambeds, you might see large boulders that are igneous pink granites and banded gneisses. These were carried down from Canada by glaciers. They often have striations or beveled edges. Striations are linear scratches or grooves on the surface of rocks, marks gouged into rocks from the bulldozer effect of glaciers moving over them. Beveled edges are flat surfaces of an erratic that were created by jostling against glacial ice or other rocks, which cause the erratics to break. Striations are often found along these beveled edges.
Kames, Kame Terraces, and Kame Deltas
Each of these features are formed by water from melting glaciers. They are stratified, meaning that they are sorted in layers by sediment size. Kames look like elongated hills of gravel and sand. They are created on the side of a glacier when it starts to melt. Kame terraces form when sediment and melt water are trapped between a glacier and a valley that the glacier is moving through. They are made of clay, silt, and sand-sized sediments, and look like flat-topped kames. Kame deltas form when water from glaciers flows into ponds or lakes, creating a triangular deposit.
Outwash is yet another kind of sediment deposit that acts as a “glacier was here” signature. Outwash is sediment that was carried by streams which were created by a melting glacier. Outwash features are similar to kames, but they do not have as distinctive of a structure. It can be challenging to determine whether a specific sediment deposit was created by a glacier, but there are traces of glacial activity all over the park.
A Buried Valley
When the glaciers covering Ohio retreated for the last time, significant amounts of sediment were left behind; enough to change the topography of the valley. The valley floor that we see today in Cuyahoga is relatively new. The real valley base, created before glaciers advanced into the area, is about 500 feet below the surface. This makes Cuyahoga Valley what is called a buried valley, or a valley that has been partially filled with glacial sediments.
Some very interesting fossils have been found in the bedrock of Cuyahoga Valley National Park, particularly in the Ohio Shale. Shale is a sedimentary rock that forms in deep water that are often poorly oxygenated. When organisms die and sink to the bottom in poorly oxygenated waters, they decompose more slowly and are often preserved in greater detail as the mud transforms into rock. These include terrestrial plants and animals that floated out to sea, and creatures that lived in the more oxygen-rich upper waters. Some of the most notable discoveries are the bones of twenty-two species of arthrodires, or jawed, heavily armored fishes. The most well-known among these is called Dunkleosteus, the largest known predatory fish of its time. This formidable fish could reach lengths of almost 40 feet! Dunkleosteus can be seen at the Cleveland Museum of Natural History, alongside Cladoselache, another marine creature excavated from the Ohio Shale. Cladoselache is one of the earliest known sharks, and its preservation is exquisite because its soft parts are preserved alongside the hard parts like bones. Smaller-scale fossils can also be found in the Cuyahoga Formation. This formation is moderately fossiliferous and has produced some beautiful fossils of shelled marine creatures such as brachiopods, mollusks, and crinoids. The hard, segmented, tube-shaped stems of crinoids, commonly known as sea lilies, commonly break apart during decomposition and are preserved in the rock record resembling singular or stacked o-shaped cereal.
The Ohio Shale underneath the park is rich in hydrocarbons, including oil and natural gas. In fact, the Ohio Shale is a minor source of natural gas and produces about 20-25 gallons of oil per ton of rock. The reason for the presence of these natural resources can be linked to the amount of organic material in the rock, which is about 1/3 of the volume. If you find a dark shale in the park’s deepest rock layer, it is likely that you are holding a rock with oil in it. A telltale sign is the distinctly oil-like odor the rocks give off when broken.
Deep Lock Quarry
In the 1800s, Deep Lock Quarry opened in the park to cut and extract Berea Sandstone, which is useful for structural and decorative stone because of its high quartz content and its ability to be easily cut and shaped. The quarried stone was used to make foundations, curbing, sidewalks, and grindstones. Several of these grindstones can be found abandoned along the trail to the quarry. In order to begin quarrying, the glacial sediment that was covering the Berea Sandstone layer was partially removed. Underneath these removed sections, glacial striations on the rock show places where the rock-filled ice and carried scraped across the bedrock, leaving marks behind.
One of the most striking features of the park is Brandywine Falls, a picturesque waterfall that drops 65 feet before flowing onward. Starting in 1810, this waterfall was used to power lumber, wood, cider, and distillery industries. Today, it is one of the more popular attractions in the park and is appreciated by visitors for its natural beauty. The rushing water exposes and erodes the Bedford Shale, which is capped by a 15-foot layer of the Berea Sandstone. Behind the falls, at the connection between these two layers, natural springs leak out of the rocks. These springs carry iron which stains the rocks orange. At the base of the falls, large boulders of the Berea Sandstone that caps the waterfall. These have fallen because the Bedford Shale underneath the Berea is softer, and the rushing water erodes the less resistant shale more quickly and easily than the sandstone above it. Eventually, the latter breaks under its own weight and falls.
The Ritchie Ledges, also known as the Virginia Kendall Ledges, are another popular attraction. They are comprised of the Sharon Conglomerate. At several places along the ledges, cross-bedding and remnants of old stream and river channels, which are traces of the rock’s formation in a marine environment can be found. Also, honeycomb weathering is an easy-to-find weathering feature that forms when some parts of a rock are cemented together better than others, leading to different rates of erosion. Honeycomb weathering creates hollowed-out pits that look like honeycomb or swiss cheese. At the ledges you can also see joints, which are places where the rocks have fractured and extended away from each other. Some of these joints are wide enough that you can walk between them like small canyons. The ledges also boast Ice Box Cave, which is a 50-foot deep cave, aptly named for its cool ice-box-like temperatures year-round.
Mass Wasting Features
Mass wasting is the downslope movement of material due to gravity. and seasonal freeze-thaw cycles. Examples of mass wasting in Cuyahoga Valley National Park include debris slides and slumps, which involve the movement of sediments on the surface. Often, the sediment involved was left behind by glaciers. Mudflows and earthflows, which involve rapid downslope movement of water-carrying debris, also occur. The difference between the two is that mudflows usually contain more water than earthflows. Creep is the slowest of the mass wasting features and is nearly imperceptible. When creep occurs in the upper soil layers, evidence can be seen in exposed tree roots that grow along slopes and which seem to sprout from the rock itself. Creep also occurs with entire blocks of the Sharon Conglomerate which makes up the ledges, although they move downhill so slowly that you would never know it. Sometimes, these mass wasting features can be dangerous, especially fast-moving features like mudflows and earthflows.