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

A system of deep bedrock channels, large-scale potholes, and fluvially scoured and fluted bedrock surfaces occurs northeast of Shattuck Mountain in the Green Mountains of northern Vermont. These large-scale products of stream erosion are developed at 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 pronounced bedrock erosional features extend at least 40 m below the drainage divide both to the northwest and to the southeast over a distance of approximately 0.5 km. The channel network across the top of the divide in places bifurcates as it cuts through schist and greenstone that is completely devoid of till. Potholes within the channels are 5 to 8 m in diameter and often extend up the entire ~15 m height of the deepest channels. Where the channels are shallow (1–2 m deep), the potholes are also 5 to 8 m in diameter. The channels appear structurally to be a product of coalesced potholes. A small ephemeral stream enters and drains southeast through a portion of the channel network, but otherwise the channels do not host any moving water.

It is unlikely that this channel network developed as an outlet to Glacial Lake Winooski, the large glacial lake that occupied the Winooski and Lamoille river valleys when ice in the Champlain valley blocked the outlets to these valleys. While the drainage divide where the channel network is developed lies below the local elevation of Glacial Lake Winooski (~330 m asl), reconstruction of the Laurentide ice sheet in the Champlain valley indicates that during the entire history of Lake Winooski, the ice sheet at the latitude of the channels was thick enough to flow across the drainage divide. Furthermore, the local NW to SE hydraulic gradient within the ice sheet would have prevented lake water from draining across the divide to the northwest.

A more likely explanation that accounts for the channel system and potholes developed on both sides of the drainage divide is that the channels developed in a subglacial (confined) drainage system similar to that responsible for the Labyrinth in Antarctica. The large scale of the potholes suggests that very high velocity water moved through this system, possibly during a single catastrophic drainage event.