Subglacial bed forms (drumlins, ribbed moraines, and megascale glacial lineations) are enigmatic repetitive flow‐parallel and flow‐transverse landforms common in glaciated landscapes. Their evolution and morphology are a potentially powerful constraint for ice sheet modeling, but there is little consensus on bed form dynamics or formative mechanisms. Here we explore shallow sediment bed form dynamics via a simple model that iterates (i) down‐flow till flux, (ii) pressure gradient‐driven till flux, and (iii) entrainment and deposition of sediment. Under various boundary conditions, replicas of subglacial bed forms readily emerge. Bed form dynamics mirror those in subaqueous and aeolian domains. Transitions between ribbed moraines and elongate flow‐parallel bed forms are associated with increasing ice speeds and declining sediment thickness. These simulations provide quantitative flux estimates and suggest that widely observed transitions in shallow sediment subglacial bed forms (e.g., ribbed moraines to drumlinoids to megascale glacial lineations) are manifestations of subtle variations in ice velocity and sediment thickness.
A contemporary trend in glacial geomorphology is the quest for a unifying theory of drumlin and ribbed moraine formation. Three Swedish example areas of each are here newly mapped, using high-resolution Light Detecting and Ranging (LiDAR) data. From previous ribbed moraine studies in these areas, we know that they formed differently, one by melt-out of stagnant debris-zonated ice, another by remoulding of preexisting landforms and the third by active subglacial stacking and folding of sediment into proto ridges, contemporaneously with lee-side cavity sediment infill. Through previous work on streamlined terrain in Sweden we know that rock-cored drumlins dominate, while soft-cored drumlins are much less common. The morphology of both types varies along a continuum, whereas their processes of formation differ. Our field examples from ribbed moraine tracts and streamlined terrain suggest that the subglacial environment contains a suite of different ice-sediment physics that, irrespective of this, leads to an equifinality of shape of some subglacial landforms (ribbed moraine and streamlined terrain) despite the differences in the detail of their process of formation. We thus argue that different processes, on their own or in combination, lead to a similar form of morphometric expression, that is, equifinality, and the quest for a unifying process of formation is a misleading one. Furthermore, we argue that large morphological data-sets of subglacial landforms can only be used to classify features as part of a formation continuum -or not -when they are used in conjunction with the sedimentology of said landforms.
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