Geology of Cascadilla Gorge

Cascadilla Gorge

Cascadilla Gorge is located in Ithaca, New York. Cascadilla Creek flows from the east, along the south side of the Cornell University campus, passing Collegetown, and tumbles 400 feet down Cascadilla Gorge to downtown Ithaca. During the warm weather months, you can follow a trail that hugs the creek and examine the spectacular cliff walls along the way.

Cascadilla Gorge exposes thousands of thin rock layers made of fine sand, silt, and mud. Because layers of sediment form one upon another, a process we can watch in water bodies today, we know that the oldest layers are at the bottom. This idea is known as the principle of superposition. The profound implications are that we can read the history of the Earth by investigating each successive layer, from bottom to top. The kind of sediment, fossils, and other features in each layer tell geologists about the environment in which that sediment settled.

The rocks of Cascadilla Gorge were deposited in the Late Devonian Period, about 370 million years ago, similar in age to the layers in other gorges around the southern edge of Cayuga Lake. The particular rocks exposed in Cascadilla Gorge are from rocks known as the Ithaca Formation, and rocks in the gorge were among those first used by geologists to define local rock units. In fact, the "Cascadilla Member" is a subsection of the Ithaca Formation that is defined by the rocks found in Cascadilla gorge between its base in downtown Ithaca and its top in Collegetown.

The silt and mud that make up these rocks eroded off of a mountain range to the east, which geologists call the Acadian Mountains (precursors to the modern Appalachian Mountains). West of those mountains the land was subtly buckled downward, forming a long narrow depression we call the Appalachian Basin. Sea level was high enough fill the Appalachian Basin with a shallow inland sea.

Many of the layers you can observe in Cascadilla Gorge were deposited quickly when sediment-laden water swept westward down the slope of the ancient Catskill Delta. These layers have coarser grains at the bottom, which settled first, followed by finer grains, including clay, on top.

The rocks made of coarser grains such as fine sand and silt tend to be more weathering-resistant and stick out of the cliff, relative to the muddy layers with substantial clay, which are softer and weathered back into the cliff.

Most of the rocks in Cascadilla Gorge do not have especially abundant fossils (some other layers in Central New York are famous for their fossils), but if you look carefully, you may notice fossils in blocks or slabs of rock that have fallen off the cliff. Many of these fossils are the shells of marine creatures known as brachiopods.

Brachiopods look like the clams you can find along beaches today, but in fact they are a very different kind of animal. We know they are different because we can find living brachiopods today, though they are not nearly as common as they once were. Brachiopods are one type of evidence by which we know that the Appalachian Basin was filled with sea water.

Like most topographic features, the age of the creek and gorge are very young compared to the rocks they cut through. The gorge has formed largely since the last time glacial ice sheets covered Ithaca, less than 20,000 years ago. The process was facilitated by weathering along pre-existing vertical cracks called "joints" that formed due to stresses during a mountain building event that occurred in the early Mesozoic Era.

Cascadilla Gorge Video

Learn more about the geology of Cascadilla Gorge in this video.

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