"Ithaca is gorges." Why?

The famous gorges of the Finger Lakes are a result of the interaction between the south-to-north running river valleys, which were gouged by glaciers numerous times over the last 2 million years, and streams running obliquely to the glaciers, which merely filled with glacial sediment.

The story of the gorges began when the Finger Lakes were river valleys with small streams flowing in from the east and west. The valleys were repeatedly filled with hundreds of feet of glacial ice that originated from glaciers advancing south out of Canada, eroding the valleys deeper.

As the ice melted and glacial sediment dammed the river valleys, lakes much deeper than today's formed and streams plunged as waterfalls from the glacially steepened hills. As the lake levels dropped, a series of steps were left on the hillsides, like at the overlook at Taughannock Falls.

Both pictures show the Fall Creek gorge near the Cornell University campus.

The streams eroded downward, forming the gorges so characteristic of the Finger Lakes region, in a process that continues today. The rocks that are now so solid along the edges of Ithaca streams were once soft sand and mud. Sand and mud are easily sculpted by moving water, and the patterns left behind can be informative about ancient water movement. Certain kinds, like flute casts, adorn the surfaces of siltstones that Ithacans call "Llenroc" and rocks in some of the gorges.

The material that the streams eroded was deposited in large deltas at the mouths of the creeks. These deltas are clearly visible today at such places as Taughannock Falls State Park and Myers Point, near the south end of Cayuga Lake. Some of the gorges were likely cut during earlier interglacial times, filled with glacial sediment during ice advances, and then re-cut since the last glacial retreat.

Erosion of the gorges appears today to be slow and gradual. There are rounded pebbles worn smooth by the water and occasionally rounded holes in the stream beds (plunge pools and potholes) that have been scoured out by the water. But things are not always as they seem. The flow of streams in upstate New York is highly seasonal, with high volume in the spring from snowmelt and low volume in the summer and fall. More erosion is likely to happen when there is more water flowing in the stream. Look carefully at the rocks themselves and you will see other signs that erosion is not always constant and gradual. The rocks around Ithaca are cut by thousands of ruler-straight cracks, which look like they have been cut with a saw. These are natural fractures called joints. They are caused by stress of rocks on an enormous geographic scale, due to the collision of the continents more than 250 million years ago. These joints form weaknesses in the rocks of the gorge walls. Water flows into the cracks, freezes and expands. Eventually, catastrophic failure occurs and a rockslide happens. The broken rocks are then moved downstream by spring floods and eventually out into the main valley or lake. Look at the fracture patterns in the walls of the gorges. Look at the piles of rocks at the bases of the walls. The gorges have formed by this system of small catastrophes and variation of flow in the streams.

The high-walled, flat-bottomed gorge formed by Taughannock Creek.