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Fall Creek is the largest tributary to Cayuga Lake but it earlier had a much larger drainage, which included that of the Tioughnioga River above Cortland. This beheading must have occurred before the Erie Interstade. Several earlier synopses of the Fall Creek history called for a proglacial Freeville-Dryden Lake trapped behind ice in the Cayuga trough, draining either south of Dryden or into Cascadilla Valley. Recent LiDar imagery and field work have produced a very different post Valley Heads (VH) history. There is no evidence for Lake Freeville-Dryden but a large proglacial lake Freeville that drained into the VH ice front in South Cortland did exist between the peak of the VH readvance and a subsequent Brooktondale readvance, which marked the end of the VH period. This minor readvance was a glacial surge that resulted in eskers, a field of crevasse squeeze moraine and pitted outwash in the area around Freeville. The Fall Creek drainage after the Brooktondale readvance was subglacial into the Cayuga trough, but rapid ice retreat during that time led to the uncovering of the stream channel to the lake and its incision into the Quaternary infill, leading to the formation of the Holocene Fall Creek Gorge. The pre -VH history of Fall Creek is little known, but the earliest recognized element is a broad flat-floored bedrock valley that is probably pre-Wisconsin in age. A bedrock channel is incised into the floor of this valley at least as far upstream as Varna and is probably of Sangamon age. The oldest Quaternary sediment in the upstream high banks along the Holocene gorge is a lacustrine clay that has been mobilized into till, possibly during the middle Wisconsin. The clay is overlain by fluvial sand with an OSL age of about 28 ka, and thus pre LGM (Nissouri Stade). In these upstream high banks the sand is sharply overlain by glacial outwash, itself mobilized to till, most probably during the LGM ice advance. Another till, clearly of VH age, lies with either a sharp or gradational contact over the outwash/till. Downstream exposures along Fall Creek show very different Quaternary stratigraphies, with sediment sections between the Nissouri and VH tills, which contributes an argument for the limited extent of the Erie Interstade ice front retreat. The more downstream of the exposures reveal a fluvial to lacustrine section with low to moderate energy environments of deposition. Two OSL ages of a fine sand at this exposure gave ages of 20 and 22ka. The other, more upstream exposure is of an ice proximal varved lacustrine section, paleomagnetic declination measurements on which suggest an age of somewhat less than 18 ka. The ages and environments of deposition of these two exposures indicate that the Erie Interstade was oscillatory, a conclusion also reached in the Cayuga Inlet Valley. The varved section was likely deposited in the same lake as the coarse clastic foresets at the Varna high bank, not far upstream. The VH readvance covered the entire Fall Creek drainage. In addition to a surficial till there appeared to be a till-filled channel, expressed at high bank 1 that conveyed subglacial and englacial water to the ice front.

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Quaternary Geology, Fall Creek, Deglaciation of Tompkins County


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