Geologic Formation of the Finger Lakes

Approximately 600 million years ago, during the Neoproterozoic Era, the Earth’s supercontinent began to break apart, and an ocean formed between land masses that roughly correlate to Europe and North America today. For the next 100 million years, this ocean continued to widen until the continents changed course and began to move back towards each other.

The eventual collision of these land masses formed a tall mountain range now known as North America’s Appalachians.

For several hundred million years, the land mass to the west of the mountains was low and the ocean regularly flooded the region, creating a broad shallow sea. The shallow water resulted in a high evaporation rate that left behind thick salt deposits that are mined today across Central New York.

During the Paleozoic Era, which began 542 million years ago and ended about 251 million years ago, the earth’s sea level rose. The area now known as the Finger Lakes region was then near the equator, and marine life thrived on the ocean floor. Layers of skeletal remains of these bottom-dwellers piled up on the seafloor, forming a layer of mud that eventually became limestone.

The Appalachian Mountains continued to erode, sending gravel, sand, and mud into the shallow sea and eventually forming the thick stack of sedimentary rocks of the Devonian age across Central New York. The more coarse-grained sediments piled in deltas and beaches closer to the land to form the sandstone layers found in the region today. The finer-grained sediments stayed suspended longer and moved farther west, eventually settling out and forming the siltstone and shale layers found in the region.

Around 360 million years ago, the sea level fell and the land in what is now the Finger Lakes region became dry. By about 240 million years ago in the Age of the Dinosaurs, the continents were again joined as one. Evidence of dinosaurs in the region is almost nonexistent as a result of weathering and bulldozing by the glacial ice sheets that followed.

The Finger Lakes themselves formed during the Pleistocene glaciation “Ice Age” when the first of numerous continental glaciers up to two miles thick moved southward from the Hudson Bay area. The advances and retreats of these massive glaciers grounded, polished, and scoured bedrock and widened and deepened existing river valleys that would one day become the Finger Lakes. Around 19,000 years ago, the climate started warming and the glaciers began retreating, disappearing entirely from the state around 11,000 years ago.

Deposition of glacial debris and sediment from erosion contributed to the region’s distinctive landforms including:

• erratics—rocks dropped far from their origin by melting glaciers;
• till—a dense, mix of clay, gravel, and boulders mixed by glacial movement and melt;
• drumlins—hills composed of till that are characteristically steeper in the direction that ice flowed; and
• eskers—narrow ridges created by streams flowing on the surface of a glacier;
• kettle lakes—formations created where blocks of ice that broke off from a glacier were buried by sediment and ultimately melted; and
• moraines—ridges created along the edge of ice by till or debris that mark the end of an ice sheet.

The Valley Heads Moraine crosses what is now central New York and is most visible as a boulder field and kettles between the cities of Ithaca (Tompkins County) and Spencer (Tioga County). This moraine plugged north-south flowing streams, allowing glacial melt to collect in the deep ravines carved by pre-glacial rivers and to create the unusually deep and narrow Finger Lakes.

The moraine also marks the divide between north-flowing rivers — remnants of the Ice Age stream and glacial paths — and south-flowing rivers that run through Pennsylvania and drain into the Chesapeake Bay.

Glacial movement also created the gorges and waterfalls near the southern ends of the lakes. As glaciers widened and deepened preexisting north-south river valleys by hundreds of feet, secondary east-west streams running across ice flows were left relatively unaltered. This created the dramatic cliffs and drops that are still associated with Watkins Glen (Schuyler County) and Ithaca (Tompkins County).

This essay is drawn from the National Park Service’s Finger Lakes National Heritage Area Feasibility Study. You can read about the study and the Finger Lakes National Heritage Area here.

Illustrations, from above: A late fall snowstorm frosted the hills of the Finger Lakes region of central New York in early December 2004; farthest extend of the glacial ice sheet; and Valley Heads Moraine illustration created by Vanelle.

Read more stories about the Finger Lakes here.