GLACIERS AND SHORELINES
GLACIAL EROSION
Glaciers are powerful agents of erosion, because glacial ice
is heavily laden with sediment. This "dirty ice" acts like sandpaper
to abrade and erode bedrock.
Erosion by mountain glaciers creates several distinctive landforms,
including:
- cirques and tarns
- aretes and horns
- U-shaped valleys and paternoster lakes
- hanging valleys
- fjords
- Cirques:
- spoon-shaped, steep-walled depressions on upper slopes of mountains
where glacier ice begins to accumulate
- when abandoned, they are often flooded to form subcircular lakes called
tarns
Aretes:
- sharp jagged ridges formed by lines of cirques
Horns:
- isolated peaks formed by circling cirques
Continental ice sheets level the land surface as they grow and advance,
leaving behind a broad low-relief plain.
This low relief is only interrupted by:
- roche moutonnees: elongate, streamlined hills that have been
smoothed and rounded by glacial abrasion
- elongate lake basins formed by the widening and deepening of
stream valleys (e.g. Great Lakes)
GLACIAL DEPOSITION
Large volumes of sediment (till) are transported beneath glaciers
and along their margins toward the glacial front.
When glaciers melt, this sediment is left behind to form ground
and end moraine deposits.
Moraine deposits are often eroded by outwash (or meltwater)
streams. This sediment is redeposited to form valley trains,
outwash plains, and other stratified drift deposits.
- Ground moraines: poorly drained sheets of till that are
deposited over once-glaciated land surfaces
- End moraines: ridges of till deposited at glacial fronts:
- terminal moraine: an end moraine that marks the outermost limit
of glacial advance
- recessional moraine: an end moraine that marks a brief pause
in the retreat of a glacial front
Old moraine deposits are often reworked by readvancing glaciers to
form elongate, streamlined drumlin hills.
- Glacial outwash streams experience very high discharges and
flow velocities during spring melts. Their deposits are thus largely composed
of gravel and sand.
- Valley trains: elongate bodies of outwash stream deposits that
accumulate downdip from mountain glaciers.
- Outwash plains: widespread bodies of outwash deposits that accumulate
downdip from continental glaciers.
The surfaces of valley trains and outwash plains are often pitted with
shallow circular kettles and kettle lakes.
SHORELINE EROSION
Erosion usually occurs along shorelines where the rate of sediment
supply is low and waves and tides are strong.
Erosive or destructive shorelines are characterized by:
- wave-cut cliffs: undercut by waves
- wave-cut platforms: flat or gently sloping benches at the bases
of wave-cut cliffs
- wave-cut terraces: uplifted wave-cut platforms
- sea arches and stacks: remnants of wave-cut cliffs
- pocket beaches: steep, narrow, and coarse grained
- spits and baymouth bars
SHORELINE DEPOSITION
Three types of constructive (depositional) shorelines can be distinguished:
- fluvial dominated shorelines: characterized by the birdfoot
deltas that form at the mouths of large rivers
- wave dominated shorelines: characterized by wave-washed beaches,
barrier islands, spits, and baymouth bars
- tide dominated shorelines: characterized by tidal flats and
tidal creeks
Fluvial-dominated deltas consist of:
- distributary channels: radiating branches of the main stream
channel lined by levees
- distributary mouth bar: sandy bar deposits at the mouths of
distributary channels
- swamps: low-lying vegetation rich flood plains between distributary
channels
- interdistributary bays: shallow water bodies between distributary
channels
- crevasse splays: small fan-shaped deposits that enter bays through
crevasses in levees
Wave-dominated shorelines are characterized by:
- beaches: wave-washed strips of sand that are attached to the
mainland
- barrier islands: wave-washed strips of sand that are separated
from mainland by lagoons or bays
- foredune ridges: linear ridges of wind-blown sand that accumulate
landward of beach
- cats-eye ponds: linear depressions that form between foredune
ridges
- spits and baymouth bars: sandy longshore drift
deposits that stretch across mouths of bays
Tide-dominated shorelines are found:
- along macrotidal coasts, where the tidal range is greater
than 4 meters
- along the shores of the lagoons of some barrier islands
- in the mouths of some drowned river mouths (estuaries)
Tide-dominated shorelines are characterized by:
- low relief, seaward sloping tidal flats
- tidal creeks that dissect and drain the tidal flats
- highly vegetated coastal marshes
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