Cryospheric systems - Fundamentals of the Physical Environment - page 360

Deformation of the glacier bed

APPLICATIONS

Glacier velocity is linked with erosive power but not in the simple relationship applicable to flowing water or wind.
The deformability of ice and its bed is worth exploring as the prelude to glacier erosion and deposition, which could
be seen as discrete events in a continuum of processes. Glaciers were formerly thought to move over a rigid bed
but we now appreciate that the glacier bed is a composite interface between ice, rock, water, debris (sediment) and
even air. The bed can therefore also be fluid, not only in the presence of atmospheric or supercooled meltwater but
also as fluidized debris. Any one of these materials may locally form the 'bed' over which all or part of the ice moves.
Not only is the boundary deformable but the zone of deformation can move from one locus to another and is
susceptible to changes in any of the material properties. Changes in ice thickness or velocity alter basal shear stress.
Pressure-melting inevitably pumps up pore water and rock discontinuity water pressures. Changes in the character
of granular debris, pressure-melting conditions and pore water pressure alter the strength and deformability of basally
lodged or entrained sediments. General relationships between the zone of deformation and glacial geomorphic
processes are shown in Figure 15.9 .

Internal shear

tidewater-glacier deposition. Each environment stamps its
mark on the character of individual particles and facies.
Subglacial debris entrainment by regelation can build
up alternating bands of dirty and cleaner ice several metres
thick ( Plate 15.9 , upper). In this way, debris assumes an
englacial position. It experiences little attrition compared
with material either in traction or moving frequently in
and out of basal ice as pressure-melting conditions
change. Further englacial incorporation occurs where
basal debris is squeezed into crevasses which remain open
to the bed in thinner ice towards the glacier terminus. In
the same zone, moving ice may shear over stationary ice
or along debris-rich bands, thrusting debris along the
deformation plane to the glacier surface. Supraglacial
debris is sourced primarily by glacier destabilization of
adjacent rock walls and their subsequent mass wasting on
to the glacier surface and, to a lesser extent, by thrusting
and melt-out of englacial debris ( Plate 15.10 ). Glaciers
may also receive airborne dust from a variety of extra-
glacial sources. Debris enters the glacier via crevasses,
entrained in meltwater and by pressure- and thermal
melting through its mass or absorption of short-wave
radiation. Some is swept from the glacier surface by wind
or water.
The eventual character of sediments, facies and land-
forms evolves through the extent of clast attrition and
winnowing of fines from bulk materials in transit. The
degree of debris concentration by mass wasting or
glaciotectonic processes (the development of shear planes,
thrusting, etc.) and mode of deposition apply the finishing
touches. Poorly sorted, clast-rounded lodgement till and
well sorted, coarser and angular ablation till represent the

Pressure melting

FROZEN TO BED BASAL SLIDING

Pressure
melting Regelation

ABRASION

QUARRYING

Regelation

ENTRAINMENT LODGEMENT

Debris
cone Ablation till

SHEARING

DRUMLINIZATION

Ice Bedrock Meltwater

Debris
(abrading
tools)

Regelated
(refrozen water)

Till

Deformation
zone

Iceflow Shear plane

Figure 15.9 Zones of deformation in glacial geomorphic
processes: the ice-bedrock-water-debris interface.