  Many fast-flowing glaciers and ice streams move over beds consisting of
reworked sediments and erosional products, commonly referred to as till. The
complex interplay between ice, meltwater, and till at the subglacial bed
connects several fundamental problems in glaciology including the debate about
rapid mass loss from the ice sheets, the formation and evolution of subglacial
landforms, and the storage and transport of subglacial water. In-situ
measurements have probed the subglacial bed, but provide surprisingly variable
and seemingly inconsistent evidence of the depth where deformation occurs, even
at a given field site. These observations suggest that subglacial beds are
inherently dynamic. The goal of this paper is to advance our understanding of
the physical processes that contribute to the dynamics of subglacial beds as
reflected in existing observations of basal deformation. We build on recent
advances in modeling dense, granular flows to derive a new numerical model for
water-saturated till. Our model demonstrates that changes in the force balance
or temporal variations in water pressure can shift slip away from the ice-bed
interface and far into the bed, causing episodes of significantly enhanced till
transport because the entire till layer above the deep slip interface becomes
mobilized. We compare our model results against observations of basal
deformation from both mountain glacier and ice-stream settings in the past and
present. We also present an analytical solution to assess the variability of
till transport for different glacial settings and hydraulic properties.
