STATISTICAL APPROACH TO THE RECONSTRUCTION OF A DYNAMIC DEGLACIAL MARGIN IN THE GULF OF MAINE FROM THE ORIENTATIONS OF STREAMLINED HILLS

A method for statistical analysis of the orientations of glacially streamlined hills allows reconstruction of late-Wisconsinan deglacial conditions in a critical area of the Gulf of Maine and its coastal lowlands. We performed an analysis of means using Student's t-statistic on ~400 down-ice azimuths. This approach permits the identification of small but significant changes in flow direction, and is not dependent on potentially biasing visual clues. Independently sampled azimuths suggest well preserved but geographically scattered groups of flow directions indicative of distinct ice regimes, characterized as: 1) regionally uniform flow; 2) flowline flow; 3) compartmentalized reorientation flow; and 4) sublobe flow. Fabric analysis of local drumlin till provides independent confirmation of these results. Streamlined hills a moderate distance upstream on a flowline serve as proxies for conditions at the glacial front.

Ice in a 150 km² area of the coastal lowlands and near-shore waters was undergoing dynamic reorganization between 17 ka and 14 ka, from topographically independent thick marine ice with a SE orientation to topographically influenced terrestrial thin ice with a S to SW orientation. Regionally uniform flow at 138° (True) was essentially independent of all but the largest topographic or bathymetric influences. Confined to a narrow geographical area, flow at 128° was structured as a flowline, exploiting pre-glacial drainage of the Piscataqua River, actively influencing offshore morphology, and probably remaining as a residual mass as nearby thinner ice retreated. Geographically separated areas sharing a common flow of 142° signal the onset of influence from ice masses to the north, and demonstrate that ice began to rotate to a more northerly retreat direction from a terminal position near the present-day coast, aligning gradually with the larger regional pattern of retreat in Maine. Boundaries of sublobate flow in southern Maine and adjacent New Hampshire are quantitatively identified. We also suggest that deposition of the late-glacial sediments of Jeffrey’s Ledge, a prominent submerged curvilinear feature beneath the Gulf of Maine, can be correlated to these events.