St. Lawrence University Geology Club

Letchworth State Park

by Maureen Jones and Melisa Jones

Letchworth Gorge, also deemed “the Grand Canyon of the East”, offers a prime example of the power of the mighty Genesee River. The gorge is approximately 22 miles long and up to 550 feet deep. Three major waterfalls (cataracts) can be seen within the gorge (Boyd, 1996). If one looks into the walls of the canyon, the past 400 million years of the region’s geologic history can be unfolded (Van Diver, 1980). After one does this, it can be seen that the fossil record for the Silurian and Devonian in this area are the most diverse compared to anywhere in the world (Van Diver, 1980).

The Genessee River is the only river to completely cross New York State. Its current northward flow has been consistent since the Ice Age. As a result of this, one can see a stratigraphic unit which represents the environments and marine life in which inhabited this area through geologic history.

The rocks seen in the gorge are sedimentary in origin (Figure ). These sediments represent the depositional environments of the Ordovician, Silurian, and Devonian Periods. On top of this bedrock, lie poorly consolidated rocks and glacial sediments of Pleistocene age. Even though this seems to be a great deal of time, only 2% of the Earth’s geologic history is represented in the gorge (Figure )(Van Diver, 1980).

During the Ordovician, this region was flooded by a shallow inland sea. As a result, a great deal of sand, silt, and carbonate mud were deposited which later became the sandstone, shale, and limestone we see today (Figure ). Also taking place during this time was the beginning stages of the Taconic Orogeny. As the Taconic Mountains rose in the East, the sediments of erosion came to be deposited in the West, which eventually formed the Queenston Delta (Figure ). In the gorge, these sediments are seen as the Queenston Formation. This formation is comprised of red shales and siltstones, which contain no fossils. At the base of the lower falls, approximately 55 feet of the Queenstone Formation can be seen. The thickness of this unit reaches close to 1000 feet (Van Diver, 1980)!

The end of the Taconic Orogeny and the beginning of the Silurian can be seen in the Medina Group (Figure ). These sandstones are well-known for their oil and gas reservoirs. Directly overlying the Queenston is the Grimsby Sandstone which represents the dying stages of the Taconic Orogeny. The Grimsby is very similar to the Queenston and with both lacking fossils, the contact between the two is uncertain (Van Diver, 1980).

The red color seen in these rocks is due to iron oxide resulting from deposition in a highly oxidizing environment.

The Lower and Middle Silurian Age are represented by the Clinton Group. These sediments are mainly shales and thin limestones that represent a quiet time without and mountain building (Figure ). The aquatic environment represented by the Clinton Group is quite contrasting to the Medina Group (Figure ). Even though a shallow sea environment still persists, life began to flourish at this time. The Rochester Shale, at the top of the Clinton Group, is quite fossiliferous (Figure ). The organisms represented here include brachiopods, bryozoans, trilobites and ostracodes (Figure ). The limestones interbedded with this group formed in clear water where reefs usually flourished. The near complete record of the Clinton Group can be seen in all three falls within the gorge.

Lying above this group is the Lockport Group. It is comprised of gray, coarse textured massive dolomite. As a result of the dolomite being highly resistant to erosion, it forms the caprock to the falls at Rochester and Niagara Falls. In other regions of the Lockport Group, massive coral reefs with vast amounts of fossils can be seen, representing a warm, shallow sea conducive to life. But, as seen at the gorge, very few fossils can be seen. At the upper part of the Lockport Group, a diversity of minerals found in solution cavities can be seen. Some of these minerals being “variously colored fluorite cubes, very clear gypsum, reddish brown sphalerite, celestite, dolomite, and calcite” (Van Diver, 1980).

The Late Silurian is represented by the Salina Group. This unit consists of thick deposits of shale and dolostone interbedded with salt and gypsum. These sediments represent an environment with a generally arid climate and a coastline with shallow bays and lagoons. This type of environment was almost certainly inhospitable to marine life in that very few fossils can be seen. One organism that did adopt well to this type of environment was the eurypteratid (sea scorpion)(Figure ). Some eurypteratids have been found to have grown to close to nine feet in length. The Salina Group is not very well represented within the gorge due to the sediment not being very well resistant to erosion. The Devonian is not very well represented in this area due to erosion of most of the sediments (Van Diver, 1980).

The Onondaga Formation represents the Middle Devonian. These limestones are at the base of the Catskill delta. The sediments were deposited in a clear, warm, shallow sea where sea life again flourished. But, again, these sediments are represented further south at a prominent escarpment in Syracuse (Van Diver, 1980).

Overlying are black to blue-gray shales with thin interbedded limestone which forms the Hamilton Group. These deposits represent the far western edge of the Catskill delta. The lower unit shales represent a depositional environment that had a stagnant, poorly oxygenated waters that contained only sparse fauna. The upper Hamilton is one of contrast to the lower part. The upper sections are seen as calcareous and richly fossiliferous with abundant corals, bryzoans, brachiopods, trilobites, crinoids, mollusks, ostracodes, and a great deal of plant material (Figure )(Van Diver, 1980).

Following are the Sonyea and West Falls Groups. This is the main section exposed at the gorge. The sediments seen are mainly black shales at the base that grade into grayish interbedded shales, siltstones, and sandstones at the top. This sequence is part of a prograding delta (Van Diver, 1980).

At the southern parts of the gorge, progressively younger formations are seen in the walls due to the gentle, southward dip of the beds. All three falls within the gorge are capped by resistant sandstone beds within the West Falls Group. Only the lower part of this group is seen within the walls. The Nunda sandstone is seen to cap the upper falls and many of the smaller falls to the south. This region was mainly mined for its building stone, bluestone, which some old abandoned quarries may be seen (Van Diver, 1980).

The falls and gorge at Letchworth are also noted around the area by legends. “The legend of Mon-a-sha-sha is haunting, tragic and beautiful. It is a story of a young Seneca bride who perished with her child, in the great cataract (Letchworth Gorge). Their spirits are said to live today in the elusive white deer of Letchworth State Park...” (Clarion Pub, 1996). Two other people as legends within the gorge are Mary Jemison and William Pryor Letchworth. “Mary Jemison, the White Woman of Genesee, was captures by Indians from her Pennsylvania home at age 15 and brought to the Genesee Country. Here, she married, raised her children and lived to age 90 among her adopted Seneca people. Her cabin on the Garden Flats, just north of the falls, was her home for many years. A cabin she built for her daughter, Nancy, along with the Caneadea Seneca Council House have been moved to Letchworth Park and open to visitors. Like many modern initiates to the gorge’s splendor, William P. Letchworth fell in love with the land around the canyon an his first visit and wished to make in his home. First, he acquired 100 acres and began a lifetime of restoring the cut-over forests and building the estate he called Glen Iris, now a restaurant and an inn. His holdings had increased to 1000 acres when, in 1859, he deeded his property to the State of New York as a park. “Letchworth Park’s many legends echo in the thunder of their falls. Their voices whisper in the canyon winds. They are the voices of the Red Jacket and Cornplanter. Of ghost towns like St. Helena and Gibsonville. They are heard in the voices of men who built impossible canals, towering bridges, and mighty dams” (Clarion Pub., 1996).

Return to 1997 Geology Club Field Trip page Return to Geology Club Field Trip page

Return to The Geology Club page

Sarah Zimmerman
last updated April 24, 1997