The erodent layer hypothesis (ELH) proposes that d
rumlinization leaves no substantial stratigraphic record because it is primarily an erosional process that cuts an unconformity across pre-existing bed materials. D
rumlins 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 d
rumlins within the same flow sets (‘mixed beds’). This is at odds with the suggested growth of d
rumlins by vertical accretion (‘emergence’) from deforming subglacial till (‘soft beds’). ELH argues that d
rumlins ‘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-d
rumlins, ridges and grooves comparable to d
rumlins 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 d
rumlin-like ‘ellipsoidal bumps’ and ridges are also common on the surfaces of faults.
Megadrumlins, drumlins and megaridges comprise an erosional continuum in many flow sets. This records 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. Clones are the ‘missing links’ in the continuum. ELH predicts preservation within drumlins of antecedent remnant tills and stratigraphies deposited earlier in the glacial cycle under sluggish or steady-state ice flows that were then streamlined by erosion under streaming ice flows. The erodent layer may be preserved as a relatively thin, loosely-consolidated surficial till that drapes the streamlined bedform (the ‘upper till’, ‘cap till’, ‘till veneer’, ‘till mantle’, ‘retreat till’, or ‘englacial debris’ of many previous reports). ELH suggests that there is a fundamental commonality of all forms of erosional wear and streamlining on sliding interfaces from the microscopic scale to the macroscopic scale of ice sheet beds.