The Finger Lakes consist of 11 long, narrow, roughly parallel lakes, oriented north-south as fingers of a pair of outstretched hands. The southern ends have high walls, cut by steep gorges. Two of the lakes (Seneca and Cayuga) are among the deepest in North America and have bottoms below sea level. These lakes all formed over the last two million years by glacial carving of old stream valleys. Ithaca is located at the south end of Cayuga Lake, the longest and the second deepest of the Finger Lakes. Cayuga is 38.1 miles long and 435 feet deep (53 feet below sea level) at its deepest spot. The actual depth of carved rock is well over twice as deep, but it has been filled with sediments; there may be as much as 1000 feet of glacial sediment in the deep rock trough below the lakebed.

The Finger Lakes originated as a series of northward-flowing rivers that existed in what is now central New York State. Around two million years ago the first of numerous continental glaciers moved southward from the Hudson Bay area, initiating the Pleistocene glaciation, commonly known as the "Ice age."

The Athabasca Glacier in Alberta, Canada, is an example of a present-day glacier.

The "Ice age" was really a series of many advances and retreats of glaciers. The Finger Lakes were probably carved by several of these episodes. Ice sheets more than two miles thick flowed southward, parallel but opposite to the flow of the rivers, gouging deep trenches into these river valleys. Traces of most of the earlier glacial events have vanished, but much evidence remains of the last one or two glaciers that covered New York.

The latest glacial episode was most extensive around 21,000 years ago, when glaciers covered almost the entire state. Around 19,000 years ago, the climate warmed, and the glacier began to retreat, disappearing entirely from New York for the last time around 11,000 years ago.

The most obvious evidence left by the glaciers are the gravel deposits at the south ends of the Finger Lakes called moraines and streamlined elongated hills of glacial sediment called drumlins. Moraines are visible south of Ithaca at North Spencer, along Route 13 west of Newfield, and near Willseyville. Drumlins are visible northeast of Ithaca at the northern end of Cayuga and Seneca lakes in a broad band from Rochester to Syracuse.

The yellow ovals highlight drumlins, the elongated hills of glacial sediment.