Environmental Engineering Reference
In-Depth Information
short-wave radiation. Some is swept from the glacier surface by wind or water.
The eventual character of sediments, facies and landforms 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 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
