INTERNAL STRATIGRAPHY OF DRUMLINS IN SOUTHERN MAINE FROM RESISTIVITY (ERM) PROFILES

Southernmost Maine was glaciated at the last glacial maximum (LGM) 23-18 ka, and deglaciated between 15 and 13 ka. Hundreds of streamlined hills including drumlins were emplaced at that time, and pre-LGM glacial sediments were over-ridden and preserved, producing two tills. We applied the medium-depth geophysical technique of Electrical Resistivity Measurement (ERM) to drumlins and other streamlined hills along 9 km of survey lines, yielding interpreted longitudinal profiles and cross-sections to about 20 m depth. Especially significant is the ability of the ERM survey to reveal the inflection point (where the slope of the streamlined hill begins to rise from a subhorizontal plane), a feature that is routinely hidden beneath surficial sediments.

These longitudinal profiles commonly reveal masses of sorted sands at the initiation point of drumlinization. In ascending stratigraphic order, the ERM surveys show: 1) a bedrock protuberance located beneath the surface, ice-distal relative to the inflection point; 2) a body of sorted sand, ice-proximal relative to the bedrock protuberance and coincident with the inflection point, extensively deformed, draped over the bedrock, accumulated in the lee of the bedrock, and entrained thereafter for some distance; 3) a lodgment till; and 4) various shallow surficial sediments.

Southern Maine’s coast-parallel and glacial-flow-normal bedrock ridges present an ideal geometry for pinning retreating ice on parallel ridges and accumulating proximal and distal sorted sands in the topographic lows between ridges. Remnant concentrations of sand, backed by bedrock topography, can initiate the construction of streamlined hills during subsequent glaciation. True drumlins are particularly well preserved in the lee of the Agamenticus uplands, with their stoss ends arrayed in ranks normal to regional ice flow and coincident with lines of minor bedrock ridges. In these drumlins, late-glacial marine erosion (50-70 m above present sea level), as well as modern commercial excavation, commonly expose abundant sands. Our ERM surveys trace these sands beneath the surface to the point of initial drumlinization. In our interpretation, the drumlins preserve within them extensive pre-Late Wisconsinan moraines in arcuate bands oriented roughly SW-NE across southernmost Maine.