Equifinality in glacial geomorphology: instability theory examined via ribbed moraine and drumlins in Sweden

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 pre-existing 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.