DRAINAGE FROM GLACIAL LAKE DELANSON AND STRUCTURALLY CONTROLLED MELTWATER ROUTING IN EASTERN NEW YORK STATE

In eastern New York State near the confluence of the Mohawk Valley and the Schoharie Valley the westward flowing Mohawk lobe of the Laurentide Ice Sheet diverged from the southward flowing Hudson lobe. Glacial ice within the Mohawk Valley overflowed and intruded southward into the Schoharie Valley and blocked northward drainage impounding a series of extensive, deep proglacial lakes. New geologic mapping and analysis with high resolution lidar terrain data provides a context to evaluate the third stage of proglacial lake development within the Schoharie Valley after the abandonment of the Franklinton outlet that maintained a threshold elevation of 360 meters.

As Mohawk lobe ice retreated eastward a col in the Devonian Schenectady Formation sandstones was gradually exposed near the Hamlet of Delanson and proglacial lake levels in the Schoharie Valley lowered, possibly as an outburst flood draining eastward toward the Hudson Valley. An extensive wave cut shoreline and outlet channel provide evidence for a stabilized threshold elevation of 268 meters to form glacial Lake Delanson. Subsequent outlet channel incision at thresholds of 261 meters and then 250 meters suggest other large pulses of meltwater may have released eastward from Lake Delanson.

Escaping meltwater floods into the Hudson Valley flowed between the western edge of the Hudson lobe and the edge of the Helderberg Escarpment scouring large tracts and exposing broad areas of folded and faulted bedrock along the Hudson Valley fold and thrust belt. As Hudson lobe ice contracted and retreated eastward meltwater flow paths became increasingly controlled by the tortuous topography of plunging anticlines and synclines between Clarksville and Climax. As high intensity meltwater abandoned bedrock spillways in favor of increasingly lower elevation outlets, ponding and siltation commenced in the structurally controlled bedrock spillways. Abundant tundra plant macrofossils preserved in the basal silt layers of the bedrock channels provide a chronologic timeline for spillway abandonment. Knickpoint incision and a large bedrock gorge along Hannacrois Creek two kilometers southwest of Ravena provide additional supporting evidence for large scale meltwater floods most likely originating from glacial Lake Delanson.