Page 101 - Fundamentals of Geomorphology

84 INTRODUCING LANDFORMS AND LANDSCAPES

Supraglacial
debris

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