Late Wisconsin ice extent in Maine : evidence from Mount Desert Island and the Saint John River area

Lowell, Thomas

'The areal extent and nature of glacial ice in Maine at the late Wisconsin maximum are the subjects of a lively debate, as is the pattern of late Wisconsin deglaciation. One school holds that thick Laurentide ice covered Maine and extended close to the edge of the continental shelf; during deglaciation a local dome may have been isolated in northern Maine due to downdrawn through a Laurentian Channel ice stream. In contrast, the other school postulates a restricted ice dome of Appalachian origin that at its maximum extended only several kilometers beyond the present-day Maine coast. Two widely separated field areas, one on Mount Desert Island on the coast and one in the Saint John River area in the northern interior, were studied to test these two working hypotheses. Detailed field mapping on Mount Desert Island showed: 1. basal till on valley floors, 2. plucked bedrock and erosional cirques on ridges, 3. subglacial meltwater channels and P-forms on ridges, 4. striated and polished bedrock surfaces on ridge crests and high mountain tops, 5. regional full-glacial southeastward ice flow yielding to late-glacial ice flow controlled by local topography, 6. interbedded glacial and marine deposits, and 7. lack of high-level tors or weathering zone development with increasing altitude. The ice sheet that produced these terrestrial features overran the highest summits on Mount Desert Island. This glaciation is assigned a late Wisconsin age as a result of studies of bedrock weathering and radiocarbon dates of 13,320 +- 360 (SI-4042) and 11,355 +- 125 (SI-4043) from two lake cores. Further, minimum late Wisconsin ice thickness on this portion of the Maine coast exceeded 600 m. Ice conditions inferred from the field data are: 1. basal ice melting (active till deposition) below 90 m in altitude, 2. basal ice freezing (active bedrock erosion) above 90 m in altitude, 3) meltwater saturation of the ice-bed interface. Deglaciation occurred largely by calving into marine embayments.' 'The simplest interpretation of the combined field evidence from both areas is that thick Laurentide ice of late Wisconsin age covered Maine and extended well out onto the continental shelf; marine processes controlled ice recession in coastal regions. During deglaciation, ice-stream downdraw caused flow reversal and formation of a local dome in northern Maine.' Provides a summary of previous geological work conducted on Mount Desert Island. /STUDY LOCATIONS:/ /MOUNT DESERT ISLAND