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Paleontology and Stratigraphy of the Rochester, New York, Area

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From studying the paleontology and stratigraphy of the Rochester, New York, area, what can be learned about its geologic history? I collected invertebrate fossils in the Finger Lakes region of New York State at Fall Brook (42° 47'N, 77° 48'W), two miles south of Geneseo, and the Barge Canal (43° 010'N, 77° 48'W), near Greece, New York. By studying these fossils I will be learning about the climate and environment in the Silurian and Devonian Periods of the Paleozoic Era.

A fossil is any remains, trace, or imprint of a plant or an animal naturally preserved in sediments or rocks from past geologic times. Typically, for fossils to form, the material must be buried rapidly, usually by waterborne sediment. Burial is followed by chemical alteration where minerals may be removed or added (Fossils, p. 10). Paleontology is the study of fossils to help learn about ancient life.

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Fall Brook, near Geneseo, has a 100-foot ravine and waterfall that cuts through 380-million-year-old Middle Devonian shale and limestone that is located in the Moscow Formation of the Hamilton group. The fossils I uncovered--trilobites, brachiopods, pelecypods, crinoid stems, and bryozoans--were in the Windom Shale Member, which is 45 to 48 feet thick. The rock strata of Fall Brook, a tributary of the Genesee River, which is one of the few rivers in the world to flow north, are now exposed due to the erosive force of [Laurentian] water when the ice cap melted 14,000 years ago.

Fall Brook is a very well-known place in the Geneseo area. One of the local stories states that in 1779, during Sullivan's expedition, a band of Seneca Indians was driven over the bank of the falls (Genesee Country, p. 58). The area of Fall Brook, which was located close to the equator during the Middle Devonian Period, was covered by warm, shallow seas that teemed with life. New York was covered by inland seas during that time, and there is evidence that suggests the region occasionally rose above sea level and was subjected to erosion.

To find fossils, I had to search for the correct layers of shale that were formed when climatic conditions included long sunlit days, warm temperatures, low turbidity, and an environment that had a good food supply and was conducive to life. Most of the fossils I found were in the gray shale rather than the thin interbedded layers of limestone. While working in various stratigraphic layers, I found the layers of limestone to be harder to dig in but easy to identify. They stood out because of their resistance to weathering, but they lacked fossils and evidence of abundant life. I discovered that the lighter gray shale was more fossiliferous.

I enjoyed digging through the five-inch-thick, rust-stained layer of Leicester marcasite, previously known as the Tully Pyrite. The pyrite, which is a sulfide of iron, indicates that the waters that covered this area carried iron in solution, and decaying organic matter released hydrogen sulfide covering the sea floor (Genesee Country, 60). This layer acts as a key bed for identifying geologic time in this area. It was difficult to dig in and contained dwarfed fauna of mainly brachiopods and pelecypods. Above the strata where my fossils were found, there was an 83-foot deposit of Geneseo black shale. The waters of this time contained few animals but vegetation was abundant. The plants didn't fossilize well because of the lack of hard parts. The water was shallow and the source of sediment came from the east. The dark color of the shale was caused by lack of oxygen in the sediment. Few fossils occurred within it. (Genesee Country, 59).

Below the black shale in the lighter gray shale strata, I found trilobites, bryozoa, brachiopods, pelecypods, horn corals, and crinoid stems. Since these fossils were found at a lower elevation they were older than any found above. The lowest layer, which was located in the rapids, is where I uncovered trilobites ( Phacops rana ). It was very easy digging, because over the winter the water between the layers of shale expanded due to freezing, making it easy to separate fossils from the shale.

On the trip home I saw the well-known Retsof salt mines (42° 46' 30"N, 77° 53' W), located a few miles from Fall Brook. The mine is over a thousand feet deep and has been of major economic importance to the area. It has recently been shut down due to a collapse, but a new mine is being built a few miles away. Retsof got its name from a man named Foster who started the salt-mining town but didn't want it named after him, so he spelled his name backward. The rock salt, of the Sallna group, was formed in the Late Silurian age when the ocean water evaporated, leaving the salt. The Silurian rock salt covers over 10,000 square miles of western and central New York. The abundant salt deposit at Retsof and the type and diversity of fossils at Fall Brook help us to understand the paleoecology of the area.

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The other location where I looked for fossils was the north bank of the Erie Canal, now known as the Barge Canal, one quarter of a mile east of where Long Pond Road crosses the canal, approximately five miles from downtown Rochester. The rock exposure at the canal is about thirty million years older than the rocks at Fall Brook. This is due to the slight southerly dip of the rock strata, which exposes younger Devonian rocks to the south and older Silurian rocks to the north (Guidebook to Field Trips, ii). The rock layers at the site are Rochester shales and limestones of the Upper Silurian in the Clinton Group. It is the most fossiliferous and forms the uppermost unit in the Clinton Group. Each year, the canal, which was hand-dug in 1825, is drained from December to May, exposing the layers. As a whole, the Rochester shale is a brownish-gray, calcareous, fossiliferous shale with interbedded argillaceous limestone layers (Gillette, 100). The upper thirty feet where I was digging is called the Gates Dolostone Member since the exposure exhibits dolomitic mudstone with thin interbedded dolostone and limestone seams (General Stratigraphic Profile). The Rochester Shale at the Canal indicates that, about 425 million years ago, life in the area was mostly composed of invertebrates living in a bottom-dwelling community. The diverse fauna indicates a relatively quiet, well-oxygenated, offshore environment of the Rochester Shale (Guidebook to Field Trips, 116). Because the fossils are found in pockets, it suggests they were swept along the sea bottom by currents (Barge Canal).

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Directly above the Rochester Shale is the Lockport Dolomite, which makes up the caprock of Niagara Falls found sixty miles west. Digging at the canal site was more difficult than at Fall Brook because the dolomitic limestone was much more compact. I wasn't as successful at finding fossils at the Barge Canal site but on my next trip I plan to bring a larger hammer and chisel.

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The fossils I was most trying to find were trilobites because they are good index fossils. An index fossil is a fossil that can be found over a large geographic area but that only existed for a brief period of geologic time and is easily identifiable. Trilobite Phacops is one of the most common trilobites of the Middle Devonian of North America. There are close relatives 4,000 miles away in northwest Africa that suggests that Africa was very close to North America during the Devonian. The collision between Africa and North America was actually the origin of the Appalachian Mountains (www.englib.cornell.edu). Since Phacops is found in both fossil-hunting locations, we can infer that they were deposited in similar saltwater conditions at approximately the same geologic time. I was able to find a complete  Phacops rana but I mainly found individual elements, such as the cephalon (head), thorax (body), and pygidium (tail), which were probably separated by water currents. At the Barge Canal I found many specimens of Trimerus, another trilobite genus. The most common complete fossils I was able to find were Mucrospirifer and Atrypa (brachiopods) and Heliophyllum (horn coral).

To improve my fossil hunting in the future, I plan to go when it's warmer and have better collecting materials so the fossils won't get damaged. It was difficult to stop collecting because I was finding so many fossils, and I would have liked to have more time. When I uncovered these fossils it was the first time they had seen light in 400 million years, and they were mine to keep. I was disappointed that I was unable to find a eurypterid, which is the New York state fossil. It is my goal in life to find a complete eurypterid, also known as a sea scorpion. Eurypterids are arthropods, and are the ancestors of the modern king crab and lived in Siliurian seas. The Bertie limestone, located not far from Rochester, is well-known for yielding eurypterids, and I plan to collect there during the summer.

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Collecting fossils is an enjoyable and interesting hobby. Permission is needed to enter the Fall Brook collecting site, but it is a beautiful quarter-mile walk through glacial till cut by the creek into the site and the 90-foot waterfall. The waterfall formed because the Genundewa Limestone is harder and more resistant to weathering than the underlying shale. During the twenty-first century the waterfall may recede slightly due to the water cascading over it. Only a small volume of water flows over the falls, mainly in the spring, which will account for the small amount of recession. At both Fall Brook and the Barge Canal site, the physical weathering caused by the freezing and thawing of the water between the rock layers will cause the strata to loosen and fall due to gravity. The amount of talus will increase at both areas, but especially at Fall Brook, due to its 100-foot-high sedimentary walls. I hope that the beauty of the areas will remain the same as it is today.

I learned that fossils are a good way to study the Earth's past climate and environment. The fossils I found in Devonian and Silurian rocks represent less than 1 percent of the Earth's history. If a 24-hour film representing the 4.6 billion years of geologic history were made, the areas I studied would appear in the film for less than 10 minutes. When I visit the diorama at the Rochester Museum and Science Center, I can visualize how my fossils would have lived [as organisms] on the ocean floor approximately 400 million years ago. I will continue to add to my fossil collection and knowledge of the paleontology and stratigraphy of the sedimentary deposits of the Rochester area.

 

References

Cornell University, Engineering Library. http://www.englib.cornell.edu/

Fairchild, Herman. Geologic Story of the Genesee Valley and Western New York.  Rochester, N. Y.: Fairchild, Herman, 1928.

Fenton, Carroll. Tales Told By Fossils. Garden City, N.Y: Doubleday and Co. Inc, 1966.

General Stratigraphic Profile. Rochester, N.Y: H and A of New York, 1991.

Guidebook for Field Trips. Rochester, N.Y: University of Rochester, 1956.

Hewitt, Phillip. Guidebook to Field Trips. State University of New York College at Brockport, 1973.

Lambert, Mark. Fossils. New York: Arco Publishing, Inc., 1978.

Payne, Thomas. The Genesee Country. Rochester, N.Y: Rochester Museum of Arts and Science, 1938.

Van Diver, Bradford. Roadside Geology of New York. Missoula, Mont.: Mountain Press Publishing Co. 1985.

Walker, Cyril, and David Ward. Fossils. New York: Dorling Kindersley, Inc. 1992.

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