A REVISED CORRELATION OF GLACIAL LACUSTRINE STRANDLINES BETWEEN THE CHAMPLAIN AND HUDSON VALLEYS HELPS PINPOINT A MISSING THRESHOLD

The proglacial lake that fronted the receding Hudson-Champlain ice lobe is most commonly known as glacial Lake Albany in the Hudson Valley & Lake Vermont in the Champlain Valley, even though the lakes were coeval for some time. Recognized levels from highest to lowest are Albany I & II, Quaker Springs, and Coveville in the Hudson Valley; Quaker Springs, Coveville, Upper & Lower Fort Ann in the Champlain Valley. Data from deltas and beaches indicate these levels transitioned to the next lower level abruptly or gradually through erosion of a dam or incision of a threshold. Location of the bedrock threshold in Fort Ann, NY that stabilized Lower Fort Ann has long been known. However, there have not been satisfactory thresholds identified for the higher levels. Hypotheses for lake level controls include the uplifting lake bottom and discrete moraines or sediment plugs. Other bedrock thresholds have been sought but never found.

Recent mapping of 4 quadrangles for the National Park Service and field investigation of channels in the northern Hudson Valley coupled with Schock’s surficial map of the Troy North quad (1963) indicates that there has been a strandline mis-correlation between the two valleys. A profile of strandline data suggests Quaker Springs in the southern Champlain Valley corresponds to Albany II in the northern Hudson Valley. A consequence of this is that the large Halfmoon-Speigletown kame moraine complex mapped by Schock would have been at a sufficient elevation to act as a dam for Lake Coveville. A newly recognized bedrock shelf in north Troy near the southeastern margin of the moraine is at the projected Coveville elevation and may be the threshold. We conclude that the transition from Quaker Springs to Coveville dammed Lake Coveville behind this moraine. This would have dropped base level at Cohoes to the lake bed and exposed the drainage divide between Ballston Lake and Round Lake. Forcing the Ballston channels to this base level change would have incised lake sediments and non-resistant shale/melange bedrock, rapidly down-cut the channel between Eastline and Round Lake, and migrated the Cohoes Falls nick point. Within a few hundred years, a catastrophic flood from Lake Iroquois into northern Lake Vermont would have breached the dam and forced the threshold to migrate northward up the lake bed until it stabilized at Fort Ann.