Snapshot: Introduction to the glacial history of the Ithaca area, including the evolution of Cayuga Lake, Fall Creek, and Ithaca Falls.
Topics covered on this page: Our changing landscape; Evidence of a glacial past; Evolution of Cayuga Lake and the Fall Creek Watershed; How old is Fall Creek Gorge?; Ithaca Falls.
Credit: Dr. Alexandra Moore, Paleontological Research Institution, Ithaca, NY
Updates: Page last updated January 20, 2022.
Image above: Hemlock Gorge on Fall Creek, near Beebe Lake on the Cornell University Campus. Photograph by Alexandra Moore.
Our changing landscape
We often think of our landscape as unchanging and “solid as a rock.” But in the Finger Lakes region of New York, the landscape has undergone dramatic changes in the last 20,000 years due to the effects of glacial ice.
Glaciers sculpt the land surface both when the ice advances and when it retreats. The deep north-south-trending valleys now occupied by the Finger Lakes were scoured by advancing glacial ice. When the glaciers melted away, a lot of water was left behind, forming temporary lakes, fed by meltwater streams. As the ice retreated, the lakes followed the receding glacier, and the streams followed the lakes, carving Ithaca’s deeply-eroded gorges. While most streams don’t form gorges, many streams around the southern Finger Lakes do. How and why have the local streams carved these gorges here?
Two important processes control the behavior of rivers and streams. First, the amount of water, or discharge, of a stream is important. Rivers work much harder and alter the landscape much more when there is more water flow.
Ithaca Falls, Ithaca, New York. Variations in water flow cause changes in the rate of erosion of the river channel. Photographs by Alexandra Moore.
Second, the slope of the stream is important. One way to alter the slope is to change the location of the endpoint of the river, which is where it enters into a larger body of water. This is typically the ocean or a large lake. The elevation of this endpoint is called the base level of the stream, and it sets the behavior of the entire watershed. For example, a stream that flows into a lake basin with a very high water level has a low gradient because the elevation drop over the length of the stream is small. However, if the lake level were to quickly fall to a lower position, then the stream gradient would be very steep. The large vertical drop gives the water a lot of potential energy, and a lot of stream power to erode down to the new lake level. A melting glacier can do a lot to change the elevation of a lake and the base level of a stream that flows into it. The landscapes of the Finger Lakes region of New York reveal abundant evidence of past glaciation.
Evidence of a glacial past
Advancing glaciers carve valleys with steep sides and flat floors. The Cayuga Basin and other nearby U-shaped valleys are classic glacial landforms. The exotic rocks scoured by glaciers and dropped hundreds of miles away are also evidence of glacial movement. And in the Fall Creek watershed it is the creek itself that documents the glacial history of the region.
In Fall Creek one of the first clues to the presence of glaciers is the appearance of the rocks that we find when we look in the creek bed. The local bedrock is sandstone and shale, derived from sand and mud deposited on the floor of a shallow sea during the Devonian Period, about 380 million years ago. It is flat and layered and dark gray in color, and it carries abundant evidence of its marine origin in the ripple marks formed by the movement of submarine currents, and in the fossils of marine organisms that we find. But, there are other rocks nearby that are quite different. They are rounded, colorful, and some have unusual minerals that we donʻt see in the bedrock, So, these rocks are definitely not derived locally.
Sample of Devonian bedrock (left) and glacial erratics (right). Note the differences in the shapes and colors of the rocks. Photograph by Alexandra Moore.
If we were to look farther afield across the northeastern United States and southern Canada we would find bedrock outcrops that match these unusual rocks. Glacial ice detached fragments and carried them here to the Finger Lakes; strangers like these are called glacial erratics.
Below the land surface we also see thick glacial deposits, like the glacial till exposed at Varna High Bank.Till is sand and silt with embedded erratics deposited as the glaciers melted away. The glacial till is a geologically recent deposit, not as old and hard as the bedrock, so it is easily eroded by moving water, to form dramatic knife-edged ridges.
View of the modern Fall Creek stream bank that is eroded into glacial till at the Varna High Banks, near Varna, New York. Photograph by Alexandra Moore.
Evolution of Cayuga Lake and the Fall Creek Watershed
While the advancing ice sheet scoured and enlarged pre-existing valleys that ran north-south, the east-west-trending valleys were filled with glacial sediment. The trough that contains modern Cayuga Lake was deepened while Fall Creek valley was buried.
The ice melted when the climate warmed, exposing this glacially-modified landscape. The meltwater formed new creeks and began eroding and re-creating new stream valleys. Some of the water became trapped between ridges of glacial debris and the retreating ice sheet itself. This impounded meltwater formed a lake at the edge of the ice. Initially the surface elevation of the lake was more than 1000 feet above sea level, which is hundreds of feet higher than the surface of modern Cayuga Lake.
As the glacier continued to retreat the proglacial lake drained and formed again at a lower elevation. Lowering the lake surface rapidly lowered the base level of ancient Fall Creek, which meant changes for the watershed upstream of the lake. With a steeper slope and more stream power the creek eroded down faster into the underlying glacial till, leaving behind abandoned terraces and old channels formed when the proglacial lake stood at a higher elevation.
Episodically dropping lake levels leave behind terraces and abandoned stream channels that are still visible in the modern landscape. Image by Alexandra Moore.
This lake level dropped repeatedly. Each time the proglacial lake drained, the creek was forced to drop to a lower base level, leaving its old meandering channels behind. When Fall Creek encountered the Devonian bedrock underlying the glacial till, erosion of the tough sedimentary rocks became much more difficult.
The Devonian bedrock is cut, however, by numerous vertical fractures, and the creek took advantage these ancient zones of weakness. Instead of meandering from side to side, as it did higher up in the watershed, the creek now began to follow the fractures—many of which are at 90 degree angles to each other—downward, cutting a narrow gorge near its base level at the lake.
Joints in the bedrock facilitate erosion and the formation of narrow gorges. Photograph by Alexandra Moore.
Old, abandoned terraces and channels are visible all over the landscape in the Fall Creek valley. The sinuous looping channels are easy to see because they create semi-circular bowls with steeply-sloping sides. These features formed in the same way as the Varna cliffs, but they’re much older. The creek cut these banks thousands of years ago, and they remain in the landscape now, marking the former position of ancient Fall Creek.
In the Cornell Botanic Gardens the curved basin that surrounds the artificial ponds is an old river meander, abandoned when the base level changed. There is even a second terrace and channel, inset within and lower than the first. These abandoned meanders were left by a creek headed toward a proglacial lake that no longer exists.
We see many more abandoned stream banks as we walk downstream toward Beebe Lake. For example, the stairs to the Wildflower Garden descend another ancient bank. So the fingerprint of the old proglacial lakes is still here today along the banks of Fall Creek.
Topographic map showing the location of some of Fall Creek’s abandoned stream banks. Base map by USGS (public domain), modified by Alexandra Moore.
How old is Fall Creek Gorge?
At Flat Rock, near the Cornell campus, we see that Fall Creek has eroded through the overlying blanket of glacial debris and has exposed the Devonian bedrock underneath. We can even see places where the glacial debris sits directly on top of the Devonian bedrock. A surface that juxtaposes two strata of different age is called an unconformity, and it represents missing time. In this case the length of missing time is enormous: the difference between 20,000 year-old glacial deposits and 380 million year-old bedrock.
What else can we infer about the timing of events in the Fall Creek watershed? The Varna cliffs are a modern stream bank, actively eroded by Fall Creek. The banks at the Botanic Gardens aren’t modern, because the creek has meandered away and left them behind as it followed the draining proglacial lake. But the banks are younger than the glacial deposits that form them. And what about the bedrock banks farther downstream? By the time we get to the bridge at Beebe Lake on the Cornell campus, we see that the creek has eroded quite deeply into the bedrock and the stream banks are no longer glacial debris, but layered Devonian sandstone and shale.
Three stream banks along Fall Creek Gorge, each a different age. Beebe Lake, on the near side of the stone bridge (center), occupies a broad bedrock gorge eroded before the last glacial period. Photographs by Alexandra Moore.
So how old is the gorge? It's definitely younger than the Devonian bedrock, but that only narrows it down to the last 400 million years. Is there a way to tell if the gorges formed after the last glacial period – OR – is it possible that they were already here when the glaciers advanced across Ithaca? What if we could find a gorge filled with glacial sediment? What would that look like?
If we examine Fall Creek closely between the unconformity and the waterfall at Forest Home, we see that the steam moves across broad, flat bedrock slabs. Below the waterfall the bedrock disappears and the modern channel is full of glacial debris. So the glacial debris and bedrock are adjacent to one another (similar to the unconformity), but in the creek bed they are side-by-side. That’s different, and it's significant; it tells us that something much more interesting happened here.
At the unconformity the glacial debris sits on top of the Devonian bedrock. This is what we expect to see: the youngest layers are deposited on top of the oldest layers. The glacial deposits at the waterfall are lower than the elevation of the bedrock surface. This tells us that there was already a stream channel eroded into the bedrock before the glacial debris filled it in.
Two views of glacial deposits adjacent to Devonian bedrock. At left, the younger debris sits on top of the older bedrock, along a surface called an unconformity. At right, the glacial debris lies along a bedrock ledge and below the top of the bedrock surface. This geometry is evidence of a pre-existing gorge eroded into the bedrock *prior to* the most recent glacial advance. Image by Alexandra Moore.
When the glacier left behind a blanket of debris, the new streams that formed didn’t always follow the path of the ancestral creeks buried beneath them. As modern Fall Creek eroded downward it eventually re-exposed the buried bedrock stream bank. But now Fall Creek flows diagonally across the older precursor stream. The waterfall itself is the edge of the pre-glacial stream bank, formed prior to the most recent glacial advance. So does this mean that ice sheets have advanced across New York more than once? Yes, in fact, although each new glacier tends to erase the evidence of older ice advances, here and there we can see the remains of previous glacial periods. Indeed, global, long term climate data from ice cores and microfossils in sea floor sediment confirm that there have been many ice ages over the past million years.
We can look at those temperature records, for example, at the climate exhibit at the Museum of the Earth in Ithaca. On the timeline below, the pink curve is the temperature variation from 800,000 years ago to the present day. Clearly there are multiple glacial and interglacial cycles recorded in these data.
Timeline of glacial and interglacial periods over the last 800,000 years. The temperature and CO2 data are derived from ice cores drilled in Antarctica, and record the temperature and composition of the atmosphere at the time the ice formed. Learn more here.
There are multiple glacial and interglacial cycles recorded by the stream banks of Fall Creek. In addition to the waterfall at Flat Rock, the ledge that holds up the Beebe Lake footbridge is the bank of an older interglacial stream. The narrow channel of Hemlock gorge is post-glacial, while the broad debris-filled basin occupied by Beebe Lake is part of the older interglacial gorge. Once we know what to look for we can see that our landscape actually preserves a record of *two* glacial and interglacial cycles.
To complete our tour of Fall Creek, we’ll head back downstream toward Ithaca Falls, and on the way we might ask, “Why is the waterfall so far downstream and so near to Cayuga Lake?”
It is much more common to find waterfalls in a riverʻs headwaters, up near the mountains and hills where the river begins, not down at the mouth where it ends. Again, this is because of glaciation. Each time a proglacial lake drains and the base level drops, the creek is left stranded at a higher elevation that is out of equilibrium with the new mouth of the creek. A common consequence of that rapid base level drop is the creation of waterfalls. A stream valley that is stranded up too high for the new base level is called a hanging valley. Hanging valleys are often marked by waterfalls, and we see them all over the world where there has been recent glaciation. Cayuga Lake is surrounded by hanging valleys. Like the abandoned river banks upstream, Ithaca Falls is another example of the handiwork of retreating glaciers.
Ithaca Falls at very high flow! Compare this aerial view with the close-ups above from different seasons. Photograph by Alexandra Moore.
There are so many things to see once you know what the shape of the landscape is telling you. For one, this is a really young landscape that is incredibly dynamic. And it's that dynamic landscape that makes Ithaca Gorges!