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Discovering layers of past in CVNP

12/13/2012 - West Side Leader
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By Jennie Vasarhelyi

Shown is water flowing over bedrock in the Cuyahoga Valley National Park.
Photo: Tom Jones; courtesy of Cuyahoga Valley National Park

CVNP — In winter, the underpinnings of Cuyahoga Valley National Park’s (CVNP) landscape come more into view. Ice highlights the bedrock, making it a good time to come to the park to explore its hard rock geology.

The valley’s bedrock dates to the Paleozoic Era. This is the era when fish, amphibians and reptiles first appeared, but dinosaurs had not yet entered the scene. Local bedrock is comprised of layers of sedimentary rocks, formed from materials left by seas and streams as this area fluctuated between marine, coastal and river environments. The oldest layers rest at the bottom, with successively younger layers on top. In some places of the world, mountain building has melded and scrambled layers of rock, but here the layers remain in the order they were deposited millions of years ago.

The bottom layers of exposed bedrock are shale. They are not the natural gas-producing shales that are so much in the news lately: Marcellus Shale is found to the east of the park; Utica Shale does occur in this area, but is deeply buried below ground. The National Park Service owns most of the mineral rights to the Utica Shale underlying the park.

Shale is a mudstone, formed from silt and clay. You can recognize shale because it is a laminated rock with many thin layers that easily split apart. Shale can be found in many cuts created by creeks in the park. Geologists have divided the shales into Chagrin Shale, Cleveland Shale and the Bedford Formation. All three occur either at Brandywine Falls or the Brandywine Gorge Trail, a 1.5-mile loop along the creek below the falls.

One challenge that geologists face is determining the date of rocks. Current thinking is that all three shales date to the Devonian Period (360-420 million years ago), the time when amphibians appeared and fish became abundant. Chagrin Shale is the oldest; the Bedford Formation, the youngest. Showing how scientific understanding changes as new evidence comes to light, the Bedford Formation has recently been assigned to the Devonian Period; it had long thought to date to a later time period.

Trying to distinguish the three shales is challenging, especially since there is variation within each. Describing their subtle differences is outside the scope of the article. However, I hope I can get you to take a closer look at variation in the shale you encounter. Color is one type of variation. Darker shales had more organic material mixed in with the mud and silt. A color variation that I like is the greenish-gray shale that is part of the Chagrin Shale. You can find it by following the Brandywine Gorge Trail along the north side of the creek. The trail takes you down a long hill to near the edge of the creek, where you can find the shale.

The other rock layer to observe at Brandywine Falls is Berea Sandstone. This is the harder layer that forms the protective cap at the top of the waterfall. As with the Bedford Formation, geologists recently revised its age and now consider it to date to the Devonian Period. The Berea Sandstone was deposited in an environment that shifted between river and near-shore river delta. Its sand is primarily quartz intermixed with other minerals and rock fragments.

Sharon Conglomerate is the other bedrock layer that is easy to view in CVNP. This highly textured rock, formed from sand and quartz pebbles, crops out at the Ritchie Ledges, as well as other named ledges in Northeast Ohio. The sand and pebbles were deposited by ancient streams about 320 million years ago during a period of the Paleozoic known as the Pennsylvanian. These were high-energy streams that flowed from the rising Appalachian Mountains. Instead of having a main channel that meanders in a sinuous pattern like the Cuyahoga River, they had a network of diverging and converging shallow channels that resemble a braid.

The Ledges Trail provides up-close views of the Sharon Conglomerate. Look for individual grains of quartz sand and pebbles that have loosely cemented together to form the rock. You can notice how the grains have been smoothed and rounded, the result of natural polishing as they were carried by water. Within the thick layer of Sharon Conglomerate, you can find thin, angled layers that resulted from water action shaping how the grains were deposited. The water action also sorted the sand and pebbles by size. Pebbles are not evenly distributed throughout the rock; instead they occur where channels of faster moving water once ran.

The National Park Service is a preservation agency. The geologic features of the park are just one aspect of the nature of the Cuyahoga Valley that we protect. We ask that visitors help by having a low-impact visit. Please resist the temptation to break apart rocks; focus on what you can observe on the surface of the rocks.

Brandywine Falls is located at 8176 Brandywine Road in Sagamore Hills. Visitors can access Ritchie Ledges from the Ledges Trailhead, located on Truxell/Kendall Park Road in Peninsula.

 

Jennie Vasarhelyi is chief of interpretation, education and visitor services for Cuyahoga Valley National Park.

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