Field Trip to the Mohawk Valley, New York

Adriane here-

Students collecting fossils in the Lower Devonian (~410 million year old) rocks.

Every Spring, UMass takes its Historical Geology class on a weekend field trip to the Mohawk Valley in New York. The purpose of the trip is to show students the formation of an ancient ocean that once covered North America ~450 million years ago in the Ordovician and talk about the ancient environments that animals lived in. We also take the students to some locations on the second day of the trip to rocks that are a bit younger (~410 million years old) so they can collect fossils. In addition to talking about tectonics and fossils, we also show the students some really great sedimentary structures preserved in the rock record.

Some fossils that were collected on the trip. Red circle is around two brachiopods found on the first day; blue circles are around two trilobite heads found in the Utica Shale.

The first stop of the day involved looking at older limestones of early Middle Ordovician age in an abandoned rock quarry. At the second stop, we looked at finer-grained shales that were deposited in a deeper portion of the ancient basin. At the third stop of the day, we investigated the Utica Shale, one of the largest shale deposits on the east coast, and one that is commonly targeted by oil and gas companies because it produces these products at the right depths in the Earth. Here, we all found trilobite pieces (and a few whole specimens). The last stop of the day involved walking down to a waterfall on Plotter Kill Preserve (not really a welcoming name for a park) and investigate the shallowing or infilling of the ancient basin. In addition to beautiful scenery, there are also some really great sedimentary structures preserved at this location.

Raquel (left) and I (right) demonstrating the Taconic Unconformity. The younger Devonian rocks on the right are tilted, but the older Ordovician rocks on the left are standing straight up. Where the two formations meet, there is about 50 million years missing.

The second day of the trip is always the highlight of the weekend, at least for me! Our first stop was the Lower Devonian limestones of Schoharie, NY, which are highly fossiliferous. After we spent a few hours at this locality, we took the class to a locally famous locality called the Taconic Unconformity. Here, rocks of Middle Ordovician age (~460 million years old) are against Lower Devonian rocks (~410 million years old), meaning that a huge chunk of time (the entire Silurian Period) is totally missing. The unconformity is really apparent because the older Ordovician rocks are standing almost straight up, whereas the Devonian rocks are less tilted.

Flute casts preserved on the bottom of a rock formation. From these markings, you can interpret which direction water was flowing (indicated by the pink arrow).

The last stop of the field trip was to another locally famous and well-loved locality called ‘Dinosaur Skin’. This name is wildly misleading, as the rocks here were formed about 250 million years before the first dinosaurs roamed the Earth, although the sedimentary features preserved in the rock do resemble dinosaur skin. Instead, the features preserved in the rocks at this location are called flute casts, which are a type of sole mark, a sedimentary feature that is formed and preserved on the bottom of sedimentary beds. In the Middle Ordovician, there were intense storms as well as earthquakes as other pieces of continents and island arcs collided with the east coast of ancestral North America. These caused sediment instability in the form of turbidites, which can be thought of as underwater landslides. So, the Dinosaur Skin is actually scoured sediment that was preserved in the rock record.

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