Transcript Slide 1

GLACIAL ENVIRONMENTS 2
Glacial processes and landforms:
• glacial erosion processes
• corries and associated landforms
• glacial troughs and associated landforms
• glacial deposition landforms
Ice is capable of transporting huge quantities of rock. Some rocks fall on to the
surface of the ice from the valley sides and are transported as supraglacial
debris. Some material finds its way into the ice via crevasses to be transported
as englacial debris. Where there is basal sliding, debris may also be picked up
below the ice and be transported as subglacial debris.
Glaciers that move relatively quickly and that transport large amounts of debris
at the base, are capable of powerful physical erosion which can drastically
alter the pre-glacial landscape. Chemical erosion, because of the low
temperatures is relatively ineffectual.
Weathering, in the form of frostshattering (freeze-thaw) aids the
erosion processes by providing a ready
supply of broken rock debris. If this
debris is incorporated into the sides and
base of the ice, abrasion becomes
active, sandpapering the rock surfaces
to produce smooth, gently sloping
landforms. Striations or grooves may
show the direction of ice flow.
Image copyright:
http://tvl1.geo.uc.edu/ice/other/use.html
Plucking is a process that is now regarded as only a minor erosion
process as only a small quantity of already fractured rock is capable of
being removed by ice which freezes to the rock surface and then moves
forward, pulling out the loose blocks. Plucking produces jagged slopes
to landforms.
Rates of erosion will vary considerable but are greatest where:
• temperatures fluctuate around freezing point
• where rocks are more jointed and faulted providing weaknesses
• where slopes are slightly steeper leading to more rapid glacier movement
(very steep slopes can lead to extended flow, a thinning of the ice and
reduced erosive power
• two or more glaciers meet and combine to give an increased depth of ice
• ice moves by rotational flow in corrie glaciers leading to over-deepening of
the hollow
Cirques (France), corries (Scotland) or
cwms (Wales) are glacial hollows with a
very steep backwall and a basin that may
contain a lake or tarn when the glacier
retreats and melts.
Snow collects in pre-glacial hollows, particularly on sheltered, north-facing
slopes and a combination of freeze-thaw weathering, solifluction flow and
summer meltwater activity enlarge these hollows allowing small corrie glaciers
to form as glaciation progresses.
Rotational flow of the ice and active abrasion within
the hollow tend to over-deepen the base leaving a
raised rock lip at the edge of the hollow. Meltwater
which lubricates and aids glacier flow may get under
the ice through crevasses, particularly the large
bergshrund crevasse along the backwall.
Photo source: U.S. Geological Survey (public domain)
Where a series of corries form around a mountain peak,
they create other unique landforms. Two corries eroding
into the mountain eventually leave a narrow, knife-edged
ridge or arete between them. Striding Edge in the Lake
District provides a classic example.
Photo source: http://trekking-hiking-outdoors.co.uk/
Where three or more corries erode backwards around a mountain, they create a
characteristic triangular pyramid peak or horn. One of the most spectacular
examples is the Matterhorn.
Photo source:
http://www.skizermatt.com/mattnet/pics/batch
/11/photo3.htm
Image source: http://oz.plymouth.edu/~sci_ed/Turski/Courses/Earth_Science/Images/8.glaciatedtopo.jpg
In mountain environments, valley glaciers severely modify former river valleys
to produce very deep, steep-sided, flat-floored U-shaped valleys or glacial
troughs.
Variations in rock resistance
or locations where glaciers
merge give rise to overdeepening of the valley floor
and the formation of long,
narrow ribbon lakes. Where
over-deepening occurs along
the coasts, deep sea fjords
may form as sea-levels rise
and flood the former glaciated
valley.
Along the sides of the glacial troughs are truncated spurs, rocky outcrops
which form the ends of former interlocking spurs that have been eroded by the
valley glacier. Tributary river valleys contain only small valley glaciers and due
to the small amount of erosive power that they have, these valleys remain at a
higher level and form hanging valleys, often with dramatic waterfalls where
tributary streams rejoin the main valley. In the glacial troughs post-glaciation,
small misfit streams occupy the now enlarged valleys.
Other erosion features include striations, roche moutonnees and crag-andtail landforms.
Striations or scratches are found everyone on
bare rock surfaces and are useful to indicate
direction of glacier movement.
Picture source: http://www.gov.ns.ca/natr/
Roche moutonnees are large rock
obstructions that have been smoothed
by abrasion on the upstream side
(stoss) but have irregular, jagged
surfaces on the downstream side (lee)
where plucking has occurred.
As glaciers move across the landscape, they come across large rock obstructions
such as volcanic plugs or particularly resistant rocks. These outstanding crags
remain after glaciation and may protect a tail of softer material which slopes gently
away from the crag on the leeward side. Edinburgh Castle stands on one of these
crag-and-tail landforms.
Material eroded and subsequently transported by glacial ice may be deposited
as unsorted till material as the ice melts or it may be further transported by
glacial meltwater and then deposited as sorted fluvioglacial material.
Till deposits, sometimes referred to as boulder clay, are a mixture of
unsorted sand, clay and rock particles. The rock fragments are sub-angular in
shape. The majority of this material has been transported as supraglacial
debris and is dropped in situ at the glacier snout or more generally at the ends
of ice ages when glaciers disappear. Some of the till deposits form distinctive
landforms but much of it is simply deposited as a layer which masks the
former pre-glacial landscape.
Till fabric analysis can be
used to analyse the rocks
embedded in the sand/clay
matrix in order to
determine the direction of
glacier or ice sheet flow.
The long axes of the larger
rocks align themselves in
the direction of flow.
Picture source: http://www.gov.ns.ca/natr/
Sometimes glaciers pick up and transport rocks
with distinctive geological characteristics. Once
deposited, these erratics can be used to trace
back the route followed by the glacier. The
photo shows a sandstone Norber Erratic in
Yorkshire lying on top of limestone which has
been chemically eroded by acidic rainwater in
the 13,000 years since the boulder was
deposited.
When glacial debris is deposited, five main types of landform may be created:
Lateral moraine, medial moraine, terminal moraine, recessional moraine
and push moraine.
Lateral moraine forms a ridge along valley sides. They are formed from debris
originating from freeze-thaw activity on valley sides. The material falls onto the
glacier and is carried mainly as supraglacial material. The material remains
angular in nature. Medial moraines, found in the middle of valleys, are usually
formed when two glaciers coalesce. They are rare in post-glacial landscapes as
they are easily destroyed by rivers flowing along the glacial troughs.
Terminal or end-moraines are located at the snout of glaciers or the edges of
ice sheets and mark the furthest point of advance. They are created from
supraglacial, englacial and subglacial material when ablation is active. They form
crescent-shaped ridges which can be several hundred metres high. Terminal
moraines may act as dams at the edges of corrie basins or in glacial troughs.
If there is a major re-advance of the glacier or ice sheet, the end moraines are
bulldozed forward to create push moraines. When there are long pauses in the
deglaciation process, a series of recessional moraines, often smaller than
terminal moraines, may form to mark the various stages of glacial retreat.
Sometimes, moraine material is shaped into low, egg-shaped hills called
drumlins. These tend to occur in large swarms on valley floors or in lowland
areas. They are blunt and steep at the stoss end, narrow and gently-sloping at
the lee end. Valley drumlins are around 5-10 metres in height but lowland
examples can reach 50 metres in height.
Photo source:
http://oz.plymouth.edu/~sci_ed/Turski/Courses/E
arth_Science/Images/
Summary of key points:
• glaciers are capable of transporting their load as supraglacial material (on the surface),
englacial material (within the ice) and subglacial (below the ice)
• where freeze-thaw weathering is highly active, it can weaken and break up rocks to make
glacial erosion easier
• glacial erosion takes place by abrasion (sandpapering action using rocks embedded in the
ice) and by plucking (ice freezes on to loose rocks and pulls them free)
• key landform in upland areas is the corrie, a large armchair-shaped hollow with a steep
rocky, backwall, often filled with a glacial lake or tarn. Around the corries, steep, knife-edged
aretes may form and some peaks are shaped into pyramidal peaks or horns
• former v-shaped river valleys are transformed into deep u-shaped glacial troughs with
truncated spurs and hanging valleys
• glaciers deposit their loads as unsorted glacial till or sorted fluvioglacial material
• when glacial debris is deposited, five main types of landform may be created: lateral
moraine, medial moraine, terminal moraine, recessional moraine and push moraine