Reflections on soft subglacial beds as a mosaic of deforming and stable spots

Abstract

Much discussion regarding subglacial processes is centred around the question of how soft beds respond to ice overriding, in particular how widespread and penetrative is their deformation, how does it affect the ice stability, and what are the consequences for large-scale sediment redistribution patterns. Contrasting views have been proposed, which are difficult to verify and reconcile in the light of scarcity of agreed criteria to identify deforming beds. Here, we suggest that soft beds under warm-based glaciers can be envisaged as a mosaic of deforming and stable spots transient in time and space according to changes in the control parameters at the ice–bed interface. The subglacial mosaic model is consistent with field data showing heavily deformed beds interspersed with largely undisturbed beds. It is suggested that deformation found in tills is a cumulative effect of strain within a relatively thin, ephemeral deforming spots under the ice sole affecting the till as it accretes during ice overriding. In accord with the Coulomb-plastic rheology, the role of clast ploughing in bed deformation is emphasised.