Bedrock control of ice-marginal positions in central New York

Abstract

Although ice-marginal features have commonly been held to mark a period of ice stillstand and stagnation, this is not always so. Bedrock topography controlled the position of proglacial meltwater channels, kame deltas, and till deposition in tributaries to the southward-flowing Chenango River during late Wisconsinan (Woodfordian) ice retreat. Bedrock divides in these eastward-flowing streams served as dams for melt-water and permitted deposition of kame deltas along valley walls between the retreating ice and the east-west divides. Drainage adjusted to lower elevations when ice retreated away from the valley walls, and meltwater then established a channel parallel to the ice margin, abandoning the outflow channel. The sedimentary structures of resultant kame terraces suggest braided-stream depositional environments. Continued ice recession permitted meltwater to flow near the present stream course, eroding and dissecting the kame deltas and kame terraces. A veneer of dead ice remained on the south-facing slopes as the active ice front retreated northward. This dead ice deposited till on bedrock or previously deposited (lodgement?) till. Stratified drift in places mantles the till downstream from bedrock outflow channels. Marginal positions are topographically controlled rather than climatically induced stillstands.