On December 11, a very chilly morning, retired geology professor David Hutchison led Susquehanna Chapter hikers on an informative walk and talk about the bedrock exposed in our area and the environmental conditions that were responsible for the deposition of the sediments that formed these layers. All the bedrock that we walk on in Oneonta is composed of sedimentary rock formed about 390 to 360 million years ago during the Middle and the beginning of the Late Devonian Period of the Paleozoic Era. Devonian Period rocks cover the southern tier, including the Catskills, southwest into Pennsylvania, and just south of Buffalo. This area is known as the Allegheny Plateau.
The bedrock in our area is almost horizontal (flat) with a slight dip to the southwest. Geologists infer that lower-level rocks are older and that the rock layers get progressively younger as you move up the hillsides surrounding Oneonta. The Devonian environment was not at all like what we see today. For one thing, the land that would form North American and New York State was on the equator, which meant that the climate was much warmer than it is now. During Devonian times much of the state, including Oneonta, was often covered by a warm, shallow sea. It was not that the ocean levels were over a 1,000 feet higher (there is not that much water on Earth to cover the land to that depth). Rather, the area was a low-lying basin, caused by down-warping of the land.
During Middle Devonian, the collision of tectonic plates resulted in the Acadian Orogeny, the formation of a huge mountain chain along the eastern margin of North America. Erosion from these extremely high mountains carried sediments downslope to the Catskill Delta and into the inland sea basin to the west of the Acadian mountains. How do geologists know that the area was once a sea? Along with the uniform bedding (layering) of the sediments, the fossils of marine organisms provide evidence. Devonian Period bedrock in the inland basin has a wealth of marine fossils including brachiopods, trilobites, crinoids, and corals. If you have picked up rocks in Oneonta filled with imprints or casts of small shells, you are looking at evidence that salt-water brachiopods lived in this area millions of years ago.
During these millions of years (a concept that we cannot even imagine) the land at times was uplifted and erosion of sediments increased. At other times the land subsided and deposition into the inland sea increased. Consequently the shoreline advanced and retreated. There were even volcanoes about 100 miles to the east. (Stark’s Knob, a scientific reservation northeast of Albany managed by the New York State Museum, features the eroded remains of an ancient volcano) The Devonian is called the Age of Fishes, although fossil evidence shows the beginnings of colonization of the land by early amphibians and plant life. At some point, the entire Allegheny Plateau was uplifted to its present height, well above sea level.
The first stop on our geological tour was the layered rock by Nick’s Diner (elevation 1120 feet). The gray rock beds show uniform layering, with some marine brachiopod fossils. This suggests an offshore sea of quiet waters without wave action, perhaps 50 feet in depth, where fine clay and fine sediments could settle to form the layers that would eventually become shales and siltstones. Because oxygen is not available deep down, the rocks have a dark gray or even black color. There are some joints in the layers due to internal stress during uplifting, although rock layers in our area have not been folded or faulted.
The next stop was up the hill to the rock outcrop behind the Hartwick science building (elevation 1420 feet). The outcrop consists of dark gray shales, thin siltstones, and thick lightercolored sandstone. David explained that the massive sandstone filled in a Devonian stream channel. An erosional surface under the sandstone marks the bottom of the channel, which sits on shale. Differential weathering is evident here, with the softer, easily weathered shale eroding out and undermining the stronger blocks of sandstone, which slide downslope.
More uphill walking brought us to the top of Oyaron Hill by the practice field and observatory (elevation 1620 feet). Exposed here are the Devonian “red beds,” an outcrop of reddish shales, mudstones, and thin conglomerates that were deposited at or near the surface when the area uplifted and/or sea levels fell. These red rocks were thus formed in a terrestrial environment, where oxygen in the air chemically combined with a small amount of iron in the sediments, to produce hematite, a mineral that gives a red color to the rocks. David pointed out casts of plant roots (rhizomorphs) that had grown in these sediments. The same red beds, but younger still, can be seen across the valley along the road cuts on Route 28 heading up Franklin Mountain.
A short walk through the woods brought us to Table Rocks, the last stop on our tour. Here we looked over the glacially carved Susquehanna Valley southwest of Oneonta. We looked at striations on the flat surface of Table Rock. From the orientation of the linear marks, we could infer the direction of ice flow, as rocks embedded at the bottom of the glacier carved and scratched the Devonian bedrock. We tried to imagine a sheet of ice filling the valley and over-riding the hillsides, and then, as the glacier melts, the valley filling with water to form a huge lake that extends from Oneonta to a glacial moraine impoundment at Wells Bridge.
We wondered: Here were rocks older than 300 million years that had been marked by rocks carried by glacial ice only 10-15,000 years ago. What happened to the rock history of the past millions of years? We do not know for certain, said David. Perhaps all later rock layers have eroded away; perhaps there were millions of years when nothing was deposited. And when did this land, which was once the bottom of an ocean, get to be over 1600 feet in elevation? Interesting puzzles to contemplate as we thanked Dr. Hutchison for this very informative geological journey.
I would like to thank Dr. David Hutchison for his help in writing this article. — Aleda Koehn
Photographs (left to right) are: 1-Dave Hutchison by Nick’s Diner, pointing out layered shales and siltstones formed by deposition in a marine environment. (L.S.) 2-Blocky sandstone filled in an ancient stream channel as shown in the back of the science building (D.F) 3-Contact between the stronger sandstone and the more easily weathered shale layer in back of the science building (L.S.) 4-Outcrop showing cross layering (J.S.) 5-Red oxidized layers of shale, siltstone, and some conglomerate as seen near the observatory on top of Oyaron Hill (L.S.) 6-Root casts, rhizomorphs, in the red rocks at the top of the hill (L.S.) 7-On Table Rocks (J.S.) 8-View down the Susquehanna Valley which was filled with ice during the recent ice age (D.F.) 9-Diagrams of Brachiopods (J.S.) 10-Map of Devonian area to become New York (J.S.) 11-Map showing the Catskill Delta (J.S.) 12-The Allegheny Plateau (J.S.) — Photograph credits designated by abbreviations – (D.F.) Doug Fielder, (L.S.) Linda Seifried and (J.S.) Julia Smith.