Geology Reference
In-Depth Information
Supraglacial
debris
Subglacial
debris
Englacial
debris
Supraglacial
debris
Subglacial
debris
Figure 3.15
Transport by ice: supraglacial, englacial, and subglacial paths.
Source:
Adapted from Summerfield (1991, 271)
Glacial deposition
deposited in braided river channels and proglacial lakes.
The breaching of glacial lakes may lay down glacial
sediments over vast areas (pp. 246-7).
A host of processes bring about the deposition of glacial
sediments. The mechanisms involved may be classified
according to location relative to a glacier - subglacial,
supraglacial, and marginal.
Subglacial deposition
is
effected by at least three mechanisms: (1) undermelt,
which is the deposition of sediments from melting basal
ice; (2) basal lodgement, which is the plastering of fine
sediments on to a glacier bed; and (3) basal flowage,
which is in part an erosional process and involves the
pushing of unconsolidated water-soaked sediments into
basal ice concavities and the streamlining of till by over-
riding ice.
Supraglacial deposition
is caused by two
processes: melt-out and flowage. Melt-out, which is the
deposition of sediments by the melting of the ice sur-
face, is most active in the snout of warm glaciers, where
ablation may reduce the ice surface by 20 m in one
summer. Flowage is the movement of debris down the
ice surface. It is especially common near the glacier
snout and ranges from a slow creep to rapid liquid
flow.
Marginal deposition
arises from several processes.
Saturated till may be squeezed from under the ice, and
some supraglacial and englacial debris may be dumped
by melt-out.
Proglacial sediments form in front of an ice sheet
or glacier. The sediments are borne by meltwater and
AEOLIAN PROCESSES
Air is a dusty gas. It moves in three ways: (1) as
stream-
lines
, which are parallel layers of moving air; (2) as
turbulent flow
, which is irregular movements of air
involving up-and-down and side-to-side currents; and
as (3)
vortices
, which are helical or spiral flows, com-
monly around a vertical central axis. Streamlined objects,
such as aircraft wings, split streamlines without creating
much turbulence. Blunt objects, such as rock outcrops
and buildings, split streamlines and stir up turbulent flow,
the zones of turbulence depending on the shape of the
object.
Air moving in the lower 1,000 m of the atmosphere
(the boundary layer) is affected by the frictional drag
associated with the ground surface. The drag ham-
pers motion near the ground and greatly lessens the
mean wind speed. In consequence, the wind-speed pro-
file looks much like the velocity profile of water in an
open channel and increases at a declining rate with
height, as established in wind-tunnel experiments by
the English engineer and professional soldier Brigadier
