Published in Trumansburg Free Press January 11, 2006
|Land & People
by Bill Chaisson
In the present day all of the Finger Lakes drain northward, ultimately emptying into Lake Ontario, reaching the Atlantic Ocean through the St. Lawrence River. It was not always so. If you stare at a map of the central part of upstate New York, you will notice that the Finger Lakes are divided into two groups, the western and the eastern lakes. The dividing line is between Seneca and Keuka Lakes. If you draw a straight line along the long axis of Seneca, Cayuga, Owasco, Skaneatles, and Otisco from their southern end northward, these lines all cross somewhere in the middle of Lake Ontario. If you do the same exercise with Keuka, Canandaigua, Honeoye, Canadice, Hemlock and Conesus, but going from their northern ends southwards, most of the lines will cross at a point somewhere south of the New York-Pennsylvania border. These groups of long, thin lakes were once dendritic networks of tributaries leading to different “trunk streams”. The northern trunk stream ran through what is now Lake Ontario. In older literature it is referred to as “the Ontarian River” and it actually flowed westward into the continental interior.
Most people are aware that the Finger Lakes are “glacially carved”, but fewer people seem to realize that there were pre-existing river valleys that were greatly deepened by the ice sheets. Traditionally, it is thought that four “ice ages”—the Kansan (ending ~1.5 million years ago), the Nebraskan (ending ~425,000 years ago), the Illinoian (ending ~125 years ago) and the Wisconsinan (ending ~10,000 years ago)—expanded and retreated out of the Hudson Bay area down into the northern United States during the Pleistocene Epoch, which began 1.77 million years ago. Each ice age received its name because significant evidence of that advance was found in a particular state. Because the ice sheets advance in enormous lobes, each ice sheet is a slightly different shape and size. Therefore some peripheral deposits of the older ice ages escape eradication by subsequent events.
The glacial debris that is scattered across the New York landscape is largely that of the Wisconsinan icesheet, but the erosive work, like the excavation of the Finger Lakes, was likely done by more than one advancing icesheet. The existing river valleys are difficult to date with any certainty, but the northward drainage of the eastern group of Finger Lakes is not the oldest for which there is evidence. At an earlier date they drained southward off of the higher Canadian Shield down toward the Atlantic coastal plain.
|Some evidence for this can be found on the eastern side of
Cayuga Lake in the town of Lansing. Salmon Creek drains southward to
the lake’s eastern shore, where it has built a delta out into the lake.
Within the delta the creek’s course bends sharply to the west, giving
it a “hooked” appearance. The general pattern of the Salmon Creek
drainage consists of its tributaries, the Little and Big Salmon in
Genoa and Locke Creek in Lansing, all joining at acute angles pointing
southward, making a dendritic pattern that converges toward the Cayuga
valley. The angle between Salmon Creek and the Cayuga valley is also an
acute one, pointing southward, indicating that the creek was a
tributary to a larger, south-running river in the Cayuga valley. Its
trunk stream was the Susquehanna River.
At some point in the geologic past, perhaps between previous ice ages, but probably earlier, the river in the Cayuga valley (and those in the other eastern Finger Lake valleys) began to flow north instead of south. The Appalachian Mountains are made of rock that is hundreds of millions of years old, but the mountains that we look at today have been uplifted much more recently, probably in the last 20 million years. (Why and how is, I’m afraid, the subject of an entirely different column.) The gradual uplift of the Allegheny Plateau (upon which we live) and the mountains to the south has been one factor in producing a divide between the modern Susquehanna drainage and the rivers that once flowed through the Finger Lakes valleys. Tributaries of the Tioghnioga River, which flows into the Chenango, a tributary of the Susquehanna, head up only a few miles from the southern ends of Skaneatles and Otisco Lakes.
The western group of Finger Lakes was likely cut off from the aboriginal Susquehanna drainage system much more recently. Only glacial till separates their southern ends from the Cohocton River tributaries that reach north and eastward. The Cohocton meets the Chemung River at Corning, and the Chemung joins the Susquehanna in Sayre, Pennsylvania, just south of the New York border. Perhaps the most obvious evidence of the riverine origins of the Finger Lakes and their former direction of flow can be seen in the shape and orientation of Keuka Lake. The east and west branches come together at Bluff Point, making an acute angle pointing south. If you have a topographic map on hand you can see the same shape and orientation at Canandaigua Lake. At the south end of Canandaigua, however, the eastern tributary valley is occupied by the (counter-intuitively- named) West River, instead of a lake.
For more information on this topic, please see The Finger Lakes Region: Its Origin and Nature, by O.D. von Engeln (Cornell University Press, 1961).
Hibernian Weather Channel Productions