Nick Eyles, Drumlins & Glacial Megalineations

Large continental-scale ice sheets have covered nearly all of Canada and parts of the US and large parts of northern Europe many times (50+?) in the last 2.5 million years.  They have moved large volumes of sediment and eroded out the Great Lake basins and dramatically altered Canada’s coastlines. Common landforms left behind are elongate ridges called drumlins and much longer more narrow landforms called megaridges that together cover as much as 40% of the area of former glaciation. Despite their wide extent, and more than 100 years of study, their origin(s) are still poorly understood and yet are key to reconstructing the footprints and flow paths of ancient ice sheets.

Some work suggests that drumlins are related to systematic Raleigh-Taylor instabilities at the ice bed interface where ice rests on soft thick till creating incipient ‘bumps’ that ‘grow up’ but this it at odds with observations that drumlin cores are made of other materials that were not deformable.

The erodent layer hypothesis (ELH) proposes that drumlinization is primarily an erosional process that cuts an unconformity across pre-existing bed materials. Drumlins most commonly have autochthonous cores of antecedent till(s), other stiff and coarse-grained sediment and rock or any combination thereof, and are also found closely juxtaposed with rock drumlins within the same flow sets (‘mixed beds’). ELH argues that drumlins ‘grow down’ by erosional carving of pre-existing stiff till, sediment and/or rock by a thin (< 1 m) layer of deforming subglacial debris which abrades its substrate. This process is well known to the science of tribology (the study of wearing surfaces) where remnant micro-drumlins, ridges and grooves comparable to drumlins and megaridges are cut by debris (‘erodent layers’) between surfaces in relative motion. In the subglacial setting the erodent layer comprises deforming diamict containing harder ‘erodents’ such as boulders, clast-rich zones or frozen rafts. Similar, till-like erodent layers (cataclasites) cut streamlined surfaces below gravity-driven mass flows such as rock avalanches, landslides and slumps, pyroclastic flows and debris flows; streamlined surfaces including drumlin-like ‘ellipsoidal bumps’ and ridges are also common on the surfaces of faults.

This talk will explore an erosional model for drumlins and will outline an evolutionary succession from megadrumlins, drumlins to long highly elongate megaridges. This appears to record the progressive dissection of large streamlined bedforms to form successively more elongate daughter drumlins and megaridges (‘clones’) as the bed is lowered to create a low-slip surface that allows fast ice flow and ice ‘streaming’ within large ice sheets. Clones are the ‘missing links’ in the continuum.

ELH suggests that there is a fundamental commonality of all forms of erosional wear and streamlining on sliding interfaces on geological and man-made surfaces from the microscopic scale to the macroscopic scale of ice sheet beds.