Glacier dynamics and the development of glacial landforms in the eastern Puget lowland, Washington

Abstract

In the Late Pleistocene, the Puget lobe of the Cordilleran ice sheet covered the Puget lowland in western Washington to an average depth of 1000 m in the Skykomish-Snoqualmie region and abutted against the western front of the Cascade Range.Reconstructions of the extent, altitude, and mass balance of the Puget lobe lead to an estimate of its sliding velocity and basal water flux. Lobe dimensions are inferred from ice limits and flow-direction indicators. An equilibrium-line altitude of 1220 m is calculated for the maximum of the Vashon advance. Resultant average sliding velocities range up to 600 m/a and average meltwater discharges up to 2500 m('3)/s.Deeply incised valleys in the eastern lowland reflect considerable erosion primarily by subglacial meltwater. Many of these channels were continuously occupied by water, whereas a broad, submarginal channel carried highly variable discharge as a result of episodic, catastrophic drainage of ice-marginal lakes. The analysis of subglacial water flow is expanded to include the effects of sliding ice, providing a physical model for the development of subglacial fluvial landforms.Morainal embankments of glacial, fluvial, and lacustrine sediments fill the mouth of each alpine valley emerging into the east-central lowland. They reflect subglacial sedimentation at and near the grounding line of the Puget lobe adjacent to ice-dammed lakes. Analysis of the physical behavior of ice and water, particularly by detailed reconstruction of the hydraulic potentials, provides geologic insights regarding the location and internal composition of these embankments.Basal stress conditions control both depositional processes and physical properties of subglacial sediments. Beneath rapidly sliding ice, till, which averages 5-10 m in thickness in the lowland, can accrete only by the accumulation of sediments released by basal meltout. High pore pressures are expected under the glacier except within a few km of the margin, irrespective of subglacial tunnels or till properties. These pore pressures are responsible for low till strength and consequent deformation of subglacial sediments by moving ice, and for overconsolidation values significantly less than the maximum overburden.