An instability theory for the formation of ribbed moraine, drumlins and mega-scale glacial lineations

Abstract

We present a theory for the coupled ow of ice, subglacial water, and sub- glacial sediment, which is designed to represent the processes which occur at the bed of an ice sheet. The ice is assumed to ow as a Newtonian viscous

uid, the water can ow between the till and the ice as a thin lm, which may thicken to form streams or cavities, and the till is assumed to be transported, either through shearing by the ice, squeezing by pressure gradients in the till, or by uvial sediment transport processes in streams or cavities. In previous studies, it was shown that the dependence of ice sliding veloc- ity on e ective pressure provided a mechanism for the generation of bedforms resembling ribbed moraine, while the dependence of uvial sediment transport on water lm depth provides a mechanism for the generation of bedforms re- sembling mega-scale glacial lineations (MSGL). Here we combine these two processes in a single model, and show that, depending largely on the granulom- etry of the till, instability can occur in a range of types which range from ribbed moraine through three-dimensional drumlins to mega-scale glacial lineations.