The Finger Lakes are a group of eleven long, narrow, and deep lakes situated in the west-central region of New York State. Appearing like the parallel marks of an outstretched hand, these bodies of water are oriented on a generally north-south axis. Their shape and depth are the result of massive geological forces acting upon the landscape. Understanding their formation requires looking back at the topography that existed before the Ice Age.
The Pre-Glacial Landscape
Before the Pleistocene Epoch, the topography of what is now the Finger Lakes region was part of an uplifted plateau. This high land was composed primarily of layered Devonian-age sedimentary rocks, such as shale and sandstone, which were relatively soft and easily eroded. Over millions of years, water carved a network of mature river valleys into this bedrock.
These ancient river systems were generally broad and shallow, characterized by the typical V-shape created by water erosion. Although the valleys trended north-to-south, the overall drainage system flowed predominantly southward. This water emptied into the Susquehanna River system, contributing to the Chesapeake Bay watershed. The deep, narrow troughs seen today bear little resemblance to the gentle landscape that defined the region prior to the arrival of the great ice sheets.
The Role of Continental Glaciation
The transformation of this landscape began with the advance of the Laurentide Ice Sheet from central Canada, a colossal mass of ice that covered much of North America. This ice sheet was miles thick and moved southward, following the path of least resistance offered by the pre-existing river valleys. The weight and movement of the ice acted as a powerful sculpting tool.
As the glaciers pushed through the river valleys, they employed two primary mechanisms of erosion to deepen and widen them significantly. One process, known as glacial abrasion or scouring, involved the ice carrying vast amounts of rock fragments and sediment embedded in its base. This material acted like sandpaper, grinding away the soft sedimentary rock along the valley floors and sides. This continuous grinding action dramatically lowered the elevation of the valley bottoms.
The second mechanism was glacial plucking, where meltwater seeped into fractures and joints within the bedrock, froze, and expanded. As the glacier flowed over the frozen area, the ice ripped out large chunks of rock, incorporating them into its load. The cumulative effect of this scouring and plucking transformed the original V-shaped river valleys into deep, steep-sided U-shaped troughs. For instance, the bedrock floor of Cayuga Lake was eroded to a depth of over 300 meters below sea level in some places, demonstrating the erosive power of the ice.
The Final Stages of Formation
The final phase of the lakes’ creation occurred as the climate warmed and the Laurentide Ice Sheet began its slow retreat northward, concluding roughly 10,000 to 14,000 years ago. As the ice margin receded, it deposited enormous volumes of rock debris, gravel, and sediment—a mixture known as till. This material was deposited in a broad arc across the landscape, forming massive ridges called recessional moraines.
The most significant of these deposits, the Valley Heads Moraine, accumulated at the southern ends of the newly carved glacial troughs. This moraine acted as a natural, impermeable dam, effectively blocking the southward flow of the former river valleys. Vast quantities of meltwater from the retreating ice were consequently trapped within the deep, U-shaped basins.
This impoundment of water created the long, narrow Finger Lakes, which filled the overdeepened troughs. The moraine reversed the primary drainage pattern; water that once flowed south now flows north, with most modern lakes draining into the Lake Ontario basin. A secondary consequence of this deep erosion was the creation of hydrological features along the valley walls. Tributary streams, which were not subjected to the same deep glacial erosion, were left hanging high above the new valley floors. These “hanging valleys” result in numerous, steep waterfalls and gorges, such as those found at Watkins Glen and Taughannock Falls, where the side streams plunge down to meet the lake level.