Glacier-Flow Patterns and the Origin of Late Wisconsinan Till in the Southern Rocky Mountain Trench, British Columbia

Abstract

Late Wisconsinan glacier-flow patterns and till genesis in the southern Rocky Mountain Trench, British Columbia, have been determined through a study of glacial landforms, till fabric, and till composition. A major shift in the pattern of glacier flow occurred near the end of glaciation; tributary glaciers coalescent with the trunk glacier in the Rocky Mountain Trench receded, and side valleys were invaded by the trunk glacier. Final recession of the trunk glacier occurred with no major halts and without stagnation of the terminus. Till associated with drumlins in the Rocky Mountain Trench accumulated by lodgement of particles due to subglacial pressure melting against a planar substratum. Fabric results from the lower part of the younger till sheet suggest either complex patterns of ice flow similar to those at junctions of many modern valley glaciers, or mass movement of supraglacial till associated with an interstadial interval within the late Wisconsinan glaciation. Compositional analysis is relatively insensitive as an indicator of short-lived ice-flow fluctuations in the southern Rocky Mountain Trench. Rather, till composition reflects the dominant pattern of ice flow during glaciation: a south- to southeast-flowing trunk glacier augmented by coalescent tributary glaciers. Most pebbles and fine-grained sand in the tills of the Rocky Mountain Trench are of local origin. Local constituents decrease in relative amounts away from the mouths of tributary valleys whence they are derived. These decreasing gradients result largely from progressive deposition and dilution. Compositional differences in till are pronounced off tributary valleys but become less distinct in a down-glacier direction. This probably results from lateral mixing of sediment due to shifts in the zone of coalescence of trunk and tributary glaciers with changes in relative ice flux from the two.