From: new York State Museum "Surficial Geological map of New York, Finger Lakes Sheet"
Compiled and Edited by: Ernest Muller and Donald Cadwell
1986
The Finger Lakes Sheet includes part of two physiographic provinces - the Appalachian Uplands and the Erie-Ontario Lowlands. The Tug Hill Upland in the northeast corner of the sheet is an outlier of the Appalachian Uplands, similar in structure and topography, though isolated from them by the eastward projecting Oneida Basin, a part of the Erie-Ontario Lowland.
The region is underlain by several thouund feet of Paleozoic strata ranging in age from Otdovician to Devonlan. Geologic structure is simple. Involving only very mild local deformation and regional dip at a fraction of a degree to the south-southwest. Because long erosion has beveled the tilted strata, progressively younger rocks are preserved to the south. Except in areas of steep slope, however, bedrock is generally mantled by a cover of drift which ranges from a few feet thick ever the uplands to several hundred feet thick in away valley bottoms.
Landscapes of central New York bear a dominantly glacial imprint. Only vestiges remain of land-forms that existed prior to Pleistocene glaciation. Flood plain. and valley walls have been reshaped only very incompletely by postglacial processes.
The record of continential glaciation, however, is far from uniform. Glacial erosion and deposition are most extensively expressed in the Erie-Ontario Lowland and in the east-west arctaate belt of the uplands that includes the northern portions of the Finger Lake basins. To the south, in a zone that includes the southern halves of the Finger Lake drainage basins, relief is moderate to high and the plateau is cut by a plexus of glacial troughs or through valleys. In the southern tier of counties along the Pennsylvania border, relief is likewise moderate to high. In this zone, however uplands bear only superficial evidence of glacial erosion and valleys follow courses minimally deranged from preglacial patterns. This southward decrease in intensity of glacial modification of topography results not only from the relatively high relief and elevation of the plateau prior to glaciation, but also from diminishing duration and frequency of glacial expansion southward across the state.
Clearly, on the basis of evidence in adjacent areas, New York has experienced several glaciations. Yet, because each glaciation tends to destroy the geologic record of previous events, evidence of multiple glaciation is only preserved in unusual situations. The deflection of the Chemung River from Its broad valley into the narrow canyon between Big Flats and Elmira is one such instance, for glacial deposits are inset within the canyon in a manner that was only possible because stream derangement had occurred previously. In protected gullies transverse to the main flow of glacier ice, older drift may escape erosion by later glaciation. Such is the stratigrapbic record for instance, in valleys of Six milc Creek and Great Gully, east-flank tributaries to Cayuga Trough In which organic materials in stratified sediment between till sheets have been radiocarbon-dated at more than 30,000 years. At Fernbank, on the west shore of Cayuga Lake a few miles north of Ithaca. sediments contain plant debris and freshwater shells that in- dicate glacial impondment of Cayuga Trough more than 50,000 years ago.
In spite of such evidence of prior glaciation, surface deposits in central New York date almost entirely from late Wisconsinan time, an expansion of the ice sheet that began some 27,000 years ago and culminated with maximum extent perhaps 20,000 years ago. In areas with as great topographic relief as southern New York, the wasting of an ice sheet typically leaves isolated areas of debris-covered and stagnating ice. The final melting out of such "dead" ice resulted in kames, eskers and gravel terrace deposits such as those in the Susquebanna Valley near Waverly and Elmira.
As long as the glacier margin lay in the hcadwaters of the Susquehanna, meltwater escaped freely, distributing outwash gravels in the valleys to the south. As the ice margin retreated north of the bedrock divide, however, meltwater was Imponded in many valleys. lit subsequent minor fluctuations, the ice margin tended to reach just about the same position without being able to expand across the divide. The result of such ice-margin fluctuation was the building of a massive complex of valley-blocking, ice-marginal deposits. This complex, the Valley Heads Moraine, comprises the present watershed between Susquehanna and St. Lawrence drainage basins across the breadth of the Finger Lakes Sheet.
Recession of the ice margin from the Valley Heads Moraine imponded small trough lakes in each valley between glacier terminus and moraine dam. Most of these "primitive lakes" initially had their outflows southward across the moraine barrier. Continued recession, although interrupted by minor readvances, uncovered alternative outlets at lower elevations or permitted coalescence of the northward expanding marginal lakes. Thus, ancestral Lake Ithaca in she Cayuga Basin joined Lake Watkins in the Seneca Basin to form Lake Newberry with Its outflow south past Horseheads into the Chemung Valley. At maximum extent, Lake Newberry extended into the Keuka Basin as well and received drainage from the other Finger Lake troughs as far east as Marcellus.
When the receding ice margin uncovered the Onondaga bench at the north margin of the plateau, glacial meltwater from as far west as the Erie and Huron Basins found outflow eastward toward the Mohawk Valley, carving deep channelways parallel to the edge of the plateau. Noteworthy among these meltwater scourways are the "Syracuse Channels" (Smoky Hollow, Clark Reservation, Rock Cut, Nottingham, Meadowbrook and Erie Channels) as well as Cedarvate Channel east of Marcellus and Pools Brook and Green Lake Channels east of Fayetteville. Several of these channels show clear evidence of repeated erosion as the ice sheet readvanced during the Port Huron Stadial and then retreated again from the Auburn-Waterloo-Skaneateles Moraine.
Ice recession across the Erie-Ontario Lowlands uncovered a landscape dominated by drumlins and areas of kame and kettle topography. It also brought into existence Lake Iroquois, a more extensive predecessor of Lake Ontario with outlet east to the Mohawk across the col near Rome. East of Rochester the Iroquois shoreline lies along Ridge Road with well-developed barrier-beach deposits. Farther east the Iroquois shoreline was island-studded and with deep embayments into the Seneca and Cayuga Basins.
Ice-sheet recession north of the Adirondacks uncovered a lower drainageway in the St. Lawrence Valley initiating a low water stage of Lake Ontario. Unloading of the earth's crust by melting of the continental ice sheet resulted in rebound or rise of the land which was greater at the east than at the west end of the lake. The result of such differential rebound has been a rise of lake level and submergence of river mouths along the south shore of Lake Ontario.
The relatively brief duration of postglacial time, approximately 11,000 years, has been enough only for limited carving of shore bluffs, partial filling of small basins and incomplete stream excavation of valley fill.