GEOLOGIC AND GEOMORPHIC RELATIONS BETWEEN DRUMLIN FIELDS AND TUNNEL VALLEY NETWORKS: CONSTRAINTS ON FORMATIVE SUBGLACIAL PROCESSES

Subglacial flow-field bedforms, including drumlin fields, and tunnel valley networks form common and overlapping elements within landscape records of deglaciated Pleistocene ice sheets, yet the origins of these enigmatic landforms and associated glacial geologic frameworks are not well understood. Analysis of geologic maps, subsurface framework, and surface morphology from North American glacial records show relations between forms and geology that (along with geophysics and continuity) constrain processes relevant to the subglacial origin of these features. For example, the morphologically concordant occurrence of bedrock-cored drumlins within drumlin fields provides evidence that subglacial erosional processes are necessary to their development. Aligned with similar orientations as bedrock-cored forms, drumlins and fluted forms with cores of unconsolidated sediment are a combination of erosional remnants of preexisting substrate (similar to bedrock cores) and later secondary accretionary mantle. Such a mantle is not essential to the drumlin form, but was accreted from debris-rich basal ice and/or advected subglacial sediment after the erosion of the swales that surround and created the core of the drumlins. In many areas, inter-drumlin networks of swales have a more deeply incised tunnel valley network superimposed upon them. Morphologic relations provide evidence that tunnel valley formation occurred during relatively late stages of glaciations and after drumlin formation. Drumlin form and dimensions generally do not vary in relation to proximity to tunnel valley margins, except for individual drumlins that adjoin and appear to be truncated at the margins of larger tunnel valleys. These relations indicate a transformation of erosional subglacial hydraulic conditions from a partially but not entirely distributed system (possibly linked cavity; but not sheetflow) to a more channelized regime of the tunnel valley network.