Processes & landforms of erosion (Intermediate landforms (…
Processes & landforms of erosion
Erosion creates many glacial landscapes
Erosion prints can be used to reconstruct past size (trimlines show the height) and character (e.g. thermal regime – there was erosion so the glacier bed must have been warm ice) of former ice masses.
What can erosion tell us?
Inception centres – where ice masses originated from eg cirque, corrie
Flow direction and (potentially) speed – direction and speed – drumlins need faster ice flow (100m+ per year flow) to form
Ice flows faster (deforms more easily) when it is warmer.
Warm ice entrains basal material – able to pick up clasts and debris and erode
Cold ice has less basal material, is much slower flowing, and does not slide over bedrock so readily in most cases.
Thermal regime can evolve with climate and ice thickness.
Abrasion is the wear of rock surfaces by the processes of striation and polishing, in which the roughness of rock surfaces is reduced by the removal or small protuberances (Benn and Evans, 2014)
Produces; smooth surfaces, striae and rock flour
Requires a warm thermal regime for warm ice to slide over it
Striae: a particle of asperity is dug into the bedrock and moved which causes localised fractures in the rock through small brittle failures (Dewry, 1986)
Localised areas freeze onto areas of bedrock, usually areas that are jointed and movement of the ice means failure which is most likely where there are large stress gradients.
Erosion beneath cold ice
Sliding is negligible beneath cold ice due to high-adhesive strengths of the ice and rock beneath the pressure melt point. Cold base ice is subject to some drag force due to internal deformation from the bed shearing which may be enough for small amounts of rotation and slip across the bed (Dewry, 2986)
scratches in the bedrock or clay surface from the scoring of particles embedded within the glacial ice
They're direct evidence of subglacial abrasion
Striae appear continuous to the naked eye, but comprise numerous crescent-shaped fractures.
Small, residual longitudinal ridges extending down from resistant rock nodules that are essentially miniature crag and tails. Created by the removal of less resistant materials to the other side.
Pressure of basal debris in overriding basal ice removes rock flakes (quarrying).
Fracture cracks in bedrock record the removal of rock flakes by subglacial quarrying.
Asymmetrical bedrock bumps with abraded up-ice (toss) faces and quarried down-ice (lee) faces.
Range in size from 1m to several hundred metres.
Crag and tails
Elongated streamlined hills consisting of resistant bedrock (crag) at the up-ice end and a tapering tail of less resistant rock extending down ice.
Subglacial nye channels
Nye channels are erosional features that cut down into the bedrock and consolidate sediments by subglacial drainage.