Geological History and Glacial Formation of the Finger Lakes

Writers and artists have long tried to capture in words, paintings and photographs the scenic beauty of the eleven long and narrow Finger Lakes of west-central New York State. From above, these lakes appear as the fingers of a pair of outstretched hands. One legend says it was the hand of the Great Spirit that created the six largest lakes when he laid his hand on the land to bless it.

According to geological history, the formation of these scenic lakes begins millions of years ago and is written in the rock formations of the state. Geology, or the evolution of the earth and its inhabitants as revealed in the rocks, uses a geologic time scale that divides the known history of the earth into great eras, and into lesser periods and epochs. The principal events in the history of the state follow the scale.

For 325 million years, during the Paleozoic Era of geologic time, the Finger Lakes region was part of a vast inland sea. Gradually, bed after bed of sand, mud, lime and salt accumulated and were compressed into rocks reaching a total depth of about 8,000 feet. Then, about 200 million years ago, the land heaved upward and at this time drainage flowed south through the Susquehanna system. Over the next 100 million years the uplifted land was eroded into a plain which was then disrupted by additional uplifting. Continued erosion produced deep north-south gaps in the landscape.

Approximately 10 million years ago, during the last of what is called the Tertiary Period of geological history, climates grew colder around the world for reasons that are not fully known. The Ice Age, or Pleistocene Epoch of the geologic time scale in North America, began about 2 million years ago. Glaciers covered as much as one-third of the present land surface before the Ice Age ended with the melting northward of the last ice sheets 10,000 years ago. This long time period was characterized by advances and retreats of huge sheets of ice, caused by changes in the climate.

According to the New York State Education Department’s publication, Geology of New York (Educational Leaflet No. 28, 1991), glaciation reached its peak in the state about 21,750 years ago. Ice sheets extended from Canada south and covered all of New York, with the exception of a small area in the Allegheny State Park region. Based on the knowledge that some modern glaciers advance a meter each day, geologists believe the Pleistocene glacier advanced at a similar speed. When the ice sheet retreated from New York about 10,000 years ago, it melted, releasing huge volumes of meltwater. It melted completely in Canada approximately 7,000 years ago.

The slow glacial advances and retreats met a barrier in the Finger Lakes region with steep north-facing slopes. The glacier, filled with mud, sand, gravel and boulders, had a rough underside that ground, polished and scoured the rock surface in its path. Glacial action included erosion, or the scraping away of soil and loose sediments, wearing away bedrock and gouging river valleys into deep troughs. Where glaciers flowed parallel to V-shaped river valleys, they gouged the valleys into deep troughs with U-shaped cross-sections. The Finger Lakes lie in former river valleys carved into U-shaped troughs of this type. Water flowing into the deepened valleys became trapped between the debris at the north and south ends of the valleys. Eventually, water filled in the valleys and the Finger Lakes resulted, in much the same form as they appear today.

The process of dropping rock debris that the glacier carried is called deposition. Chunks of underlying bedrock were deposited in distinctive landforms. Rocks called Erratics were transported far from their places of origin and were left when the ice melted. These may range in size from small pebbles to boulders of many tons. Till, the most widespread glacial debris, is a dense, unsorted mixture of clay, sand, gravel and boulders. Glacial debris created other distinctive landforms that include the following:

• Drumlins are cigar-shaped hills of till. These hills are steeper on the upstream end — that is the direction from which the ice flowed. In height, they rarely exceed 200 feet. There exist thousands of drumlins from Oswego and Syracuse to west of Rochester. One well-known example is Hill Cumorah, near Palmyra, where Joseph Smith reported seeing a vision that led him to found the Mormon Church.

• Eskers are long narrow ridges formed by a stream flowing in an ice tunnel or on the surface of a glacier.

• Kames are long, low steep-sided mounds of sand and gravel, deposited by meltwater streams from the glacier.

• Kettle lakes were formed when blocks of ice, buried in the outwash in front of the glacier, melted. As the glacier retreated northward, the stagnant ice broke off and was buried in the accumulating sediment. Dryden Lake on Route 38, south of Cayuga Lake is a good example of a kettle lake. Mendon Ponds Park near Pittsford has eskers, kames and kettle lakes among its features.

• Moraines are ridges of till or rock debris which piled up or were dumped along the edge of the ice. They show where the ice front remained in a fixed position long enough for a ridge of glacial debris to accumulate. An end moraine marks the farthest advance of an ice sheet.

What was the effect of this glacial debris? Where glacial debris had dammed valleys, vast temporary lakes were formed. Many of these glacial lakes have long since drained. The Valley Heads Moraine, a major moraine across central New York, closed the southern ends of several formerly south-flowing river valleys. The Finger Lakes were produced with the damming of these valleys. The moraine, or pile of glacial material, created a drainage divide across the middle of the state and changed the state’s drainage.

North of the moraine the streams and rivers generally flow north and eventually run into Lake Ontario, then into the St. Lawrence River and out to the Atlantic Ocean. Streams and rivers south of the moraine flow into south-flowing rivers. One exception is the Genesee River, which crosses the moraine. The most extensive segment of the Valley Heads is between Ithaca and Spencer where boulders and kettles cover the area along the southern portion of the Cayuga Lake trough.

For the Finger Lakes “west” (all the lakes west of Seneca: Keuka, Canandaigua, Honeoye, Canadice, Hemlock and Conesus), the northward drainage is a reversal of the preglacial drainage direction. In the Finger Lakes “east”: Seneca, Cayuga, Owasco, Skaneateles, and Otisco, the apparent preglacial drainage was northward as it is now, although modified by glaciation. The basins of the western Finger Lakes are smaller, shorter, and shallower than those of the eastern lakes, perhaps because the northern ends of the western Finger Lakes were higher.

Of all the lakes, Keuka Lake is distinctive because it is the only Finger Lake that drains into one of the other lakes. Keuka outlet passes through Penn Yan at the end of the east branch and flows eastward into Seneca Lake at Dresden.  Keuka Lake is also the only Finger Lake that has branches. The popular author of upstate New York stories, Arch Merrill, described the lake’s irregular outline in his books on the Finger Lakes. “When the great glacial blanket was lifted from this land after it had dammed the primitive rivers and transformed them into lakes, it left a Y-shaped sheet of water and 60 miles of curving shoreline, dotted with bays and coves.” At Bluff Point, a promontory between the two arms of Keuka Lake, lies one of the best panoramic views of the Finger Lakes region.

Canandaigua Lake would be similar to Keuka Lake if its water level were high enough to fill the branching West River trough, according to Bradford Van Diver in Roadside Geology of New York (1985).

Many unique features of the natural scenery of the Finger Lakes area owe their existence to the glacial invasion of the Pleistocene Epoch. Some of New York’s most spectacular gorges and waterfalls are secondary products of the Ice Age. The tributary streams are the sites of the many famous glens. East-west tributaries, lying athwart or across the direction of the ice flows, were excavated to lesser depths. As a result they were left hanging far above the main valleys and now cascade into them through a series of scenically spectacular waterfalls.

Watkins Glen State Park, at the end of Seneca Lake, is the best known of all the Finger Lakes glens and considered one of the most beautiful places in eastern North America. It is several miles long, often very narrow, with a maximum depth of over 300 feet. Here Glen Creek has cut a deep and unusually narrow gorge into the siltstones and shale, but because of the relatively uniform resistance of the rocks to erosion, the gorge does not have large falls. Watkins Glen is also a showpiece of potholes where, in time of flood, numerous swirling eddies drive sand and gravel that grind hollows in the bedrock.

Taughannock Gorge, on the west side of Cayuga Lake is one and a quarter miles long and has Taughannock Falls at a height of 215 feet, which is a higher vertical drop than Niagara Falls. The falls were formed when a tributary stream perpendicular to the direction of glacial flow remained unmodified while the main valley, parallel to ice movement, was considerably widened and deepened.

The Finger Lakes region’s glacial past did not always result in hilly and rocky terrain. Cayuga Lake is the lowest in elevation of the Finger Lakes, and this has resulted in the formation of extensive marshes around the northern end. It is also the only one of the Finger Lakes that extends northward as marshland. All of the others square off against moraines. The Native Americans called the lake “Tiohero,” the lake of flags or rushes, or lake of the marsh. The marshes are now the Montezuma National Wildlife Refuge.

To explore the Finger Lakes region firsthand is to avail oneself of millions of years of geologic change. It is possible to see a great deal while traveling the highways of New York. The book, Roadside Geology of New York by Bradford Van Diver is especially useful in leading the reader throughout the state with easy-to-understand geologic maps and text.

by Laurel Wemett
Laurel Wemett is a correspondent for the Daily Messenger newspaper in Canandaigua. She owns a gift shop named Cat’s in the Kitchen and lives in Canandaigua.

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