TWO LATE PLEISTOCENE FLOODS IN THE SCHENECTADY, NY, AREA AND THE DEMOTION OF THE DELMAR READVANCE

In the 1980s, we defined the “Delmar Readvance” based on exposures in the Schenectady-Albany, NY, area. We relied on the distribution and texture of the “Pink Till” (PT), a light yellowish brown, matrix-supported, massive silt-to-cobble diamict, and on a trellis-drainage system that is developed on gentle folds in lacustrine sand. The PT is two- to three-feet thick and overlies laminated silt deposits that fill troughs cut into the Guilderland Kame Terrace. The PT exposures lie above the 320-ft contour. The PT was emplaced during the Lake Albany I-Lake Albany II transition. The PT is re-interpreted as a mudflow initiated and deposited by floodwaters that overtopped and eroded the Schenectady delta. The mudflow swept south across the fore-delta slope, and impacted the Guilderland Kame terrace. The floodwaters also removed the glacial sediments in the preglacial Ballston Channel and deposited the Malta sand plain. The folds in the lake sands along the Normans Kill might be caused by differential compaction over a buried recessional moraine or by drag folding of the lake bottom by the flood currents.

Exposures of a second, near-surface diamict have been mapped along the Colonie Reservoir and in test borings drilled for a regional ground water study. The second, younger diamict can be traced from the Albany Airport to Round Lake. The younger flood eroded the Ballston-Round Lake-Drummond channels, scoured out the Round Lake and Saratoga Lake depressions, and deposited the Wilton subaqueous fan. It marks the transition from the Albany II to the Quaker Springs stages of Lake Albany, which occured approximately 11,900 ¹⁴C years ago.

The trace of the Delmar ‘readvance’ marks a still-stand of the glacier.