SUBGLACIAL MELTWATER CHANNELWAYS IN MID-COAST MAINE

Subglacial meltwater channelways are an important, if little appreciated, component of landscapes in mid-coast Maine that were inundated by the Laurentide Ice Sheet. Such channelways strongly influence modern drainage patterns, and also provide insight into former ice-flow dynamics. Subglacial meltwater channelways are eroded into the northeast/southwest-striking bedrock, typically as steep-sided, locally overdeepened, southeast-trending divide crossings (e.g., the Androscoggin channelway, and the Chops at Merrymeeting Bay) or as south- to southwest-trending troughs parallel to bedrock strike (e.g., the Kennebec channelway between Augusta and Richmond, and the Thomas Bay-New Meadows channelway which continues off the modern coast).

The 50-km-long Cathance meltwater channelway (CMC), which diverges southwestward from the Kennebec channelway near Gardiner, provides a good example of features associated with channelized subglacial meltwater erosion on a regional scale. From the Kennebec, the CMC rises to a divide comprising numerous dissected bedrock hills, and descends as two parallel bedrock troughs that merge into a broader and shallower trough down flow. The CMC crosses the Androscoggin channelway at a high angle, where it is partially filled by a buried, southwest-trending sand-and-gravel aquifer, and continues its rising-and-falling course parallel to the modern coast to Freeport.

Modeling of subglacial hydraulic heads and resulting meltwater flow patterns for simplified (planar) ice sheets of varied slope angle and direction indicate that southeast- and some south- to southwest-trending subglacial meltwater channelways likely formed under south-southeastward ice flow, consistent with the bulk of regional striae data. In contrast, formation of the CMC requires ice flow to the southwest and/or sufficient subglacial discharge to overwhelm the capacity of the Kennebec and Androscoggin channelways. The age of formation of the various channelways is unknown and may include more than one glacial cycle.