Rapid soft bed sliding of the Puget Glacial Lobe

The Puget lobe of the late Pleistocene Cordilleran ice sheet advanced into western Washington over a thick sequence of unlithified sediments. The basal drag due to sliding over this surface was calculated by (1) idealizing the sediment bed as a rigid planar surface scattered with roughness elements corresponding to individual sedimentary particles and (2) taking the minimum reconstructed sliding velocity to be 500 m yr −1 . The calculated basal drag, minimized by choices of parameters, is 400 kPa, much higher than the reconstructed gravitational driving stress of 40 kPa, indicating that a rigid bed and low water pressure are not consistent with the glacier's rapid motion. High subglacial water pressure, averaging 90% of the ice overburden (10 MPa) is inferred from overconsolidation of subglacial clays. The occurrence and deformation of water‐deposited sediments within the till and the requirement to drain large quantities of water from the interface suggest that water pressure reached 99% of the ice overburden. Ploughing of till by ice‐entrained clasts, pervasive shearing, and, finally, ice‐bed separation become possible as water pressure increases. At the low inferred effective normal stress the basal drag is reduced from the rigid bed case by water layers which decouple the smallest particles from the glacier sole and ploughing which reduces the resistance offered by the largest particles. The limited shear strain observed in much of the till implies pervasive shearing did not contribute significantly to ice motion; basal motion was confined to the ice‐bed interface or to distinct faults within the substrate.