Page 99 - Fundamentals of Geomorphology

82 INTRODUCING LANDFORMS AND LANDSCAPES

Glacial erosion

Glacial erosion is achieved by three chief processes:
quarrying or plucking (the crushing and fracturing of
structurally uniform rock and of jointed rock), abrasion,
and meltwater erosion (p. 266).The bottom of the glacier
entrains the material eroded by abrasion and fracturing.

1 Quarrying or plucking. This involves two sepa-
rate processes: the fracturing of bedrock beneath
a glacier, and the entrainment of the fractured or
crushed bedrock. Thin and fast-flowing ice favours
quarrying because it encourages extensive separation
of the ice from its bed to create subglacial cav-
ities and because it focuses stresses at sites, such
as bedrock ledges, where ice touches the bed. In
uniform rocks, the force of large clasts in moving
ice may crush and fracture structurally homoge-
neous bedrock at the glacier bed. The process creates
crescent-shaped features, sheared boulders, and
chattermarks (p. 259). Bedrock may also fracture
by pressure release once the ice has melted. With
the weight of ice gone, the bedrock is in a stressed
state and joints may develop, which often leads to
exfoliation of large sheets of rock on steep valley
sides. Rocks particularly prone to glacial fracture are
those that possess joint systems before the advent
of ice, and those that are stratified, foliated, and Plate 3.3 Striations on Tertiary gabbro with erratics,
faulted are prone to erosion. The joints may not Loch Coruisk, Isle of Skye, Scotland.
have been weathered before the arrival of the ice; (Photograph by Mike Hambrey)
but, with an ice cover present, freeze–thaw action
at the glacier bed may loosen blocks and subglacial
meltwater may erode the joint lines. The loosening surfaces, commonly carrying striations, testify to the
and erosion facilitate the quarrying of large blocks of efficacy of glacial abrasion. Rock flour (silt-sized
rock by the sliding ice to form rafts. Block removal and clay-sized particles), which finds its way into
is common on the down-glacier sides of roches glacial meltwater streams, is a product of glacial abra-
moutonnées (p. 259). sion. The effectiveness of glacial abrasion depends
2 Glacial abrasion. This is the scoring of bedrock upon at least eight factors (cf. Hambrey 1994, 81).
by subglacial sediment or individual rock frag- (1) The presence and concentration of basal-ice
ments (clasts) sliding over bedrock. The clasts debris. (2) The velocity at which the glacier slides.
scratch, groove, and polish the bedrock to pro- (3) The rate at which fresh debris is carried towards
duce striations (fine grooves) and other features the glacier base to keep a keen abrading surface.
(Plates 3.3 and 3.4), as well as grinding the (4) The ice thickness, which defines the normal
bedrock to mill fine-grained materials (less than stress at the contact between entrained glacial debris
100 micrometres diameter). Smoothed bedrock and substrate at the glacier bed. All other factors