Wednesday, September 24, 2008

Dirt on Ohio Shale...

Location: 40.2676N 82.9517W

See lake shore shale cliffs and an erosion exposed preglacial ravine filled with glacial sediments. Glacial till and sand layers filled the ancient small ravine as the Laurentide Ice Sheet advanced through central Ohio during the Pleistocene Epoch.

Cliffs of Ohio Shale eroding under lapping waves along the banks of Alum Creek Reservoir.

These shale cliffs are visible from the Route 36 bridge over Alum Creek Reservoir, Alum Creek State Park, Delaware County, Ohio. The Devonian Ohio Shale bedrock formation forms the cliffs seen in several directions from the bridge. The inlet exposing a channel filled with glacial deposits is visible along the east shoreline in the distance from the parking area at the southwest end of the bridge. The deposits can be observed closely on foot from the parking area at the southeast end of bridge or by boat (my favorite approach is by kayak).

Views of the contact between Pleistocene glacial deposits and a buried Ohio Shale erosion surface. The contact between Devonian Ohio Shale (below keys) and Pleistocene glacial till (above keys) is found near eye-level along the bank of Alum Creek Reservoir. The Ohio Shale is 365+ million years old. The glacial deposits were left by the Wisconsin Ice Sheet
between 20,000 and 100,000 years ago*.

Glacial till: Rocks locked in mud dampened for better contrast. The center rock is shale oriented flat-side toward the camera. The large rock is crystalline, probably carried by ice from upper Ontario. Texture seen in this till suggests it was squashed and pressed into the small ravine by heavy ice.

A white oak tree's roots cling to Ohio Shale while softer ground moraine erodes out from under them. Soon, this tree will fall into the lake. The geological contact seen in the photos above is located under the tree's root web.

Glacial till is composed of rocks and gravel in a mud matrix deposited by melt-release from ice absent flowing water. Mud (mostly clay with some silt and sand) supports the clasts (rocks) in near-random orientations. Water deposition would have sorted the clasts by grain-size and density. The small inlet pictured above exposes ground moraine interleaved with small deformed sand lenses filling a narrow, steep-walled ravine. A cross-section of the ravine-fill is exposed.

Picturesque Alum Creek Reservoir floods the principle valley and side ravines of Alum Creek in central Ohio. The north-south axis of the valley follows the sub-surface exposure of the Ohio Shale Formation.

Central Ohio bedrock formations are mapped on the ODNR Geological Survey's "Geologic Map and Cross Section of Ohio". Glacial deposits shroud most of central Ohio's bedrock under thin deposits of clayey till or gravel and sand. Highway-cuts, railway-cuts, quarries, shorelines, and natural gorges offer uncommon views of exposed bedrock.

*The age of the channel fill deposits is not dated using calibrated methods. The young relative date range estimate assumes the possibility that the immediate area was deglaciated during middle Wisconsin time. The older date applies if the area remained covered by the Wisconsin Ice Sheet throughout Early, Middle, and Late Wisconsin time.

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