THE SHATTUCK MOUNTAIN CHANNELS AND POTHOLES: EVIDENCE FOR FOCUSED HIGH-VELOCITY SUBGLACIAL STREAMFLOW ACROSS THE NORTHERN GREEN MOUNTAINS, VERMONT, USA

Subglacial drainage systems in northern New England are typically manifest by esker systems preserved in valleys where ice flow and hydraulic gradients were topographically controlled. Less commonly, water-eroded bedrock records older subglacial drainage systems that extend across the mountains. A nearly continuous 2.7 km long system of deep bedrock channels, large-scale potholes, and fluvially-scoured and fluted bedrock surfaces occurs across a notch in Shattuck Mountain in the northern Green Mountains of Vermont, USA. These landforms are developed on either side of the drainage divide (320 m asl) between a northwest-flowing tributary to the Missisquoi River (The Branch) and a southeast-flowing tributary to the Lamoille River (Streeter Brook) in Waterville, Vermont. The channels are narrow (3-4 m) and deep (10-17 m) where they cut across bedrock ridges and wider (10-15 m) but still deep where they parallel those ridges. Potholes within and adjacent to the channels are 5 to 8 m in diameter and often extend the full height of the deepest channels. The scale of these bedrock erosional features is maintained across the drainage divide. Bedrock NW and SE of the study area is buried by younger lacustrine sediments.

These large-scale erosional features are the product of high-velocity subglacial (confined) stream flow up and over the drainage divide. Glacial striations in the adjacent mountains are oriented NW-SE, parallel to the alignment of the channels. The described drainage network developed when the ice sheet was thick enough to flow obliquely NW to SE across the N-S mountain ranges and the hydraulic gradient within the waning ice sheet was similarly sloped to the SE. Despite good exposure, fluvially-eroded features of this scale and extent are unknown across the northern Green Mountains suggesting that the processes producing them were relatively rare. One such process might be the catastrophic drainage, perhaps multiple times, of a supraglacial lake through this subglacial conduit system. However, large drainage events initiated by a combination of rapidly melting ice and a large rain storm might also be the source of the water.