Geoscience Reference
In-Depth Information
(a)
Rockwall
source
(b)
Bedrock
source
Iceflow and debris pathways
(c)
Rockfall
debris
Iceberg
rafting
Ablation
till
Sediment
plume
Fines winnowed
out - no clast
attrition
Melt-out
till
Pond and
lacustrine
Silts and clays
Flow
till
Moraine
Englacial
transfer
Debris
rainout
Mud
drape
Subglacial
erosion
Lodgement
till
Buried
ice
Glaciofluvial
sediments (sands, gravels)
Till
Delta
Debris comminuted
and abraded
Debris-rich
shear plane
Glaciofluvial
sediments
Figure 15.10
Glacier transport pathways and their depositional environment: (a) and (b) numbered pathways, 1 subglacial, 2
englacial, 3 supraglacial, 4 glacier-marginal, 5 extraglacial, 6 glacio-lacustrine/marine; (c) principal environments and processes
typically found in environments 2, 4, 5 and 6.
subglacial and overlying supraglacial deposits of the same
ice advance. Together, they demonstrate a degree of
bedding. Till is a glacial
diamicton
or poorly sorted
sediment in which clasts are embedded in a finer matrix,
usually of clay, silt and occasionally sand. It replaces the
term
boulder clay
. Lodgement occurs through net debris
release from moving basal ice and forms till sheets or
plains where pressure-melting is widespread or ice flow
diminishes and basal shear increases. Both are increasingly
common below the ELA and lead to lodgement rates of
10
1-3
mm yr
-1
. In effect, the basal deformation zone has
shifted from the debris-rock to the debris-ice boundary.
It can shift again through changes in the geotechnical
properties of till, including
dilatancy
or increase in volume
and void ratio, or as a result of ice readvance or surge. Both
processes may induce deformation
within
the till as upper
layers adhere to basal ice, leading to large-scale till block
thrusting and streamlining. This is thought to be a
principal mechanism in the formation of drumlins, 10
1-2
m high and 10
2-3
m long, and fluted moraine bed forms
an order of magnitude smaller.
Debris is also deposited by
melt-out
from active and,
especially, stagnant ice when atmospheric or geothermal
heat fluxes are sufficient to melt surface or basal ice
respectively. Melt progressively uncovers englacial debris.
It is enhanced initially at the glacier surface, where the
lower albedo of debris induces greater heat conduction
into the ice. However, the build-up of supraglacial debris
eventually insulates the ice, leading to delays in final melt-
out and the formation of ice-cored ablation till. In this
increasingly water-charged environment, debris slumps
and flows off the glacier to form an irregular assemblage
of ice-contact and waterlain landforms. The role of
meltwater in creating distinct glaciofluvial facies is
developed below.
Glacier meltwater
Glacier meltwater is traditionally associated with the
distinct roles of erosion of meltwater channels and
deposition of a suite of fluvial landforms, fed by the water
and sediment fluxes of the glacial environment. Meltwater
processes may occur anywhere in the glacial system
but become increasingly important in the ablation zone
below the ELA, sourced primarily by surface melting
and augmented by rainfall. Water proceeds via a glacial
plumbing system of surface channels, vertical
moulins
drawing water into the englacial environment and
