Erosional Origin for Drumlins by Subglacial Erodent Layers: A Tribological Model

English Abstract:

We explore the hypothesis that drumlins and megaflutes (regardless of their cores) are residual immobile landforms formed by erosion of pre-existing sediment (or rock) below a mobile abrasive carpet of deforming subglacial debris; we draw on the industrial science of tribology where comparable remnant ridges and grooves (‘wear tracks’) are formed by abrasion below an ‘erodent layer’ at the interface of surfaces in relative motion to each other. Lineated surfaces are not unique to glacial environments and are produced by erodent layers at the base of gravity-driven mass flow deposits such as rock avalanches, landslides and debris flows, and are also very common along fault traces. Correspondingly, mega-scale glacially lineated surfaces are identified as wear tracks cut by a thin erodent layer consisting of deforming subglacial diamictic debris with erodents in the form of clast-rich debris or stiffer (frozen?) rafts entrained from the underlying substrate. Swales and megagrooves are cut into older pre-existing till sheets, sediment or rock leaving remnant high-standing drumlins and megaflute ridges with autochthonous cores of till, other sediment or rock. The common erosional origin of rock drumlins and ‘sediment’ drumlins explains their close juxtaposition in many drumlin fields. Many drumlins have till cores: the model predicts widespread deposition of thick subglacial till earlier in the glacial cycle followed by erosion and drumlinization accompanying the onset of ice streaming late in the glacial cycle. This implies widespread transient storage of till below ice sheets.