Transcript More erosion landforms

Glaciers perform, in many ways, like an
excavator.
Although they can push weak material, like
gravel, like a bulldozer blade, they are far
more likely to lift material out of place, like a
backhoe, or scratch it in place, like a
ripper. And, like a bulldozer, glaciers are poor
at eroding rock unless it is already weakened.
Erosional landforms are influenced by a variety
of factors.
Pre-existing topography.
Bedrock types determine the relative
erodibility of the landscape being affected by
glaciation.
Morphology and flow of the ice itself. These
characteristics differ greatly between alpine
glaciers and continental ice sheets.
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Large
scale
Corrie
Cirque
Cwm
An armchair shaped hollow
surrounded by knife edged
ridges called arêtes.
Norwegian Alps
French Pyrenees
Steep headwall
and lateral flanks
Small rock lip
or moraine ridge
at the open end
Over-deepened basin
floor
Vary from a few 100m – over 15km wide
Walcott Cirque Antarctica – headwall 3km
Length to height ratio varies from (Blea
Water Corrie LD)2.8:1 to
3.2:1(Western Cwm Everest) from lip to
headwall top.
Cirques are heavily influenced by joint
pattern
Nivation or Snowpatch erosion
Formation
• Complex
• UK – usually develop on N to E facing slopes where
insolation lowest allowing rapid accumulation of snow
• In the mountains of the British Isles, the vast
majority of cirque basins are orientated between the
north west (315°) and south east (135°), this
preferred orientation occurs in response to two
environmental factors.
• North facing slopes receive less insolation than south
facing slopes, and therefore are colder, hence,
glaciers are more likely to form and survive for
longer.
• During the Ice Age the principal source of snowbearing winds were westerly airstreams, therefore,
basins on the more sheltered eastern side of
mountains acted as natural traps for snow .
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Pre glacial hollow
Site of orig snow accumulation
Hollow enlarged by freeze thaw
Further enlarged by nivation –
solifluction/transport/poss chem weathering
beneath and at the edges of snowpatch
• Process goes on until firn develops as basin
enlarges so more snow can accumulate
• At a critical depth and weight of ice, ice
begins to move out of the hollow by extrusive
flow in a rotational manner
• This rotational movement helps ice to erode
the hollow further by plucking and abrasion.
• Gradually the semi circular form of the
cirque is created.
• As ice rotates bergshrund created
Bergshrund
• A large crevasse near the upper limit of
the cirque formed as the ice pulls away
from the headwall.
• In the past this crack was thought to
assist freezethaw action
Randkluft
• Similar smaller crevasse – at backwall
where ice melts on contact with backwall
a randkluft may widen because the dark
rock next to it grows warm in the sunlight
and melts the ice nearby.
• Meltwater flowing down B and R helps in
continuing cirque growth. Meltwater
flowing down freeze thaw occurs – rocks
break down causing headwall to retreat
• Erosion of the headwall back called
headwall recession
• When ice disappears armchair shaped
hollow left. Melt water may remain
dammed by cirque lip left by rotational
erosion and the deposition of moraine
• Further development of headwall =
arêtes
• The formation of cirque basins begins with the development of a
topographic hollow beneath a permanent snow patch or firnbank.
This is progressively enlarged by nivation processes, which
involve frost shattering of bedrock at the base of the firnbank
and the removal of weathered debris by meltwater.
• Eventually, the hollow becomes deep enough to allow a
sufficient thickness of firn to compress the lower layers into
glacial ice, which then begins to flow downhill. Once movement
occurs erosion increases significantly by a combination of
headwall plucking and basin floor abrasion.
• Unlike other types of glaciers, cirque glaciers move by a process
of rotational sliding, this process is initiated by the steepness
of the headwall and causes over-deepening of the basin floor. At
the glacier snout where the thin ice limits glacial erosion, a rock
lip develops which usually exhibits a patchy cover of till or a
small moraine ridge . These features often act as a natural
barrier to surface drainage within the basin and dam a small
lake, called a tarn in the English Lake District. The removal
of large blocks of bedrock from the headwall by plucking
maintains its steepness and jagged appearance.
• The geological interest of Cwm Idwal lies in the
different rock types found in the area. Much of the
rocks are of hard, acidic rhyolite, which forms most
of the rock in the bowl of the cwm.
• Higher up on the edge of the cwm are some areas of
slates. Less acidic rocks enter the cwm from Twll Du
(Devil's Kitchen), a dark cleft in the centre of the
cliffs surrounding the cwm; the cliffs of Twll Du, and
boulders and rocks on the scree below the cliffs are
formed of basalt - a lime-rich rock.
• You can see folded rock layers in these cliffs. There
are also areas of glacial moraines deposited in places
around the cwm which tend to be fairly acidic.
Arêtes and more….
• Overtime, progressive headwall erosion allows cirque
glaciers to cut backwards into an upland plateau.
• Cirque growth may eventually progress to the point
where two adjacent headwalls are only separated by a
narrow ridge called an arête, such as Striding Edge
and Swirral Edge on Helvellyn in the Lake District .
• Where three or four cirques converge, a steep-sided
pyramidal peak or horn develops . Classic examples of
pyramidal peaks, such as K2 in the Karakoram
Mountains are restricted to high altitude mountain
ranges.
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Glacial Valley
These are the most spectacular and important landforms in glaciated regions. They consist
of steep-sided troughs between mountains and are formed by the erosive action of valley
glaciers
Their dimensions vary greatly from the modest features, several hundred meters deep, in
upland parts of Britain and Ireland to the massive troughs of the Himalayas/Karakoram,
which are several thousand metres deep.
The formation of glacial valleys commences as ice from an ice cap or cirque glacier spills into
a pre-existing river valley. Glacial erosion then widens and deepens the initial V-shaped
valley into the characteristic U-shape of glacial valleys .
The valley is also straightened by the removal of inter-locking spurs to form truncated
spurs. This occurs because glaciers are rigid bodies and are less able to negotiate river
bends in the same manner of flowing water. Furthermore, lowering of the valley floor leads
to the truncation of tributary rivers and the formation of hanging valleys .
The floor of a glacial valley often exhibits irregularities consisting of rock bars and rock
basins. These features are generally considered to reflect changing rates of erosion. For
instance, rock bars usually comprise rock that is more resistant to erosion and able to form
upstanding landforms. In contrast, rock basins form in response to localised increased
erosion, which can occur for the following reasons.
bedrock is less resistant and has already been weakened by frost weathering prior to the
onset of glaciation
transition zone from an extensional to compressive flow regime.
tributary glaciers converge to locally increase ice thickness and glacial pressure
constriction of valley walls increases the rate of ice flow
Valley Changes
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The course will be straightened
The floor widened and flattened
Sides steepened and spurs truncated
Rock steps and basins will be created when weaker
rocks are literally scooped out
• During interglacial phases weathering will continue
• After glaciation form is further affected by
processes that have operated since the last
glaciation. Screes from frost shattering litter the
sides of the valleys.
• Steep wall – trough
end. Above this you
will find corries
that supply the
valley with its ice.
• Many troughs are
parabolic – due to
deposited moraine.
TROUGH
END
YOSEMITE VALLEY
• Troughs are the outcome of a number of processes and
sequences of changing conditions.
• Before the onset of glaciations and ahead of the ice
accumulation active freeze thaw weathering takes place under
periglacial conditions which will weaken the floor and sides of
the pre existing valley, so preparing it for rapid erosion. Once
the valley occupied by moving ice it will be
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Its course will be straightened
It’s floor will be flattened
It’s sides will be steepened and spurs truncated
Steps and basins will be created where weaker rocks are
literally scooped out.
• During interglacial periods with periglacial conditions
there will be further weathering of the rock surfaces
previously stripped of their weathered material by
the passing ice.
• The present shape of the trough depends on the
nature of the geomorphologic activity – weathering
mass movement etc that has taken place since the
last glaciation. Frost shattering will have produces
screes that now cover the sides of the trough.
Moraines will lie irregularly on the floor here and
there damming back melt water to form ribbon lakes.
Rivers also reestablish them selves.
Hanging Valley
• Hanging Valley
Hanging Valley
Roche Mountonne
• Roche Moutonnee are outcrops of resistant bed rock
with a gentle abraded slope on what would have been
the upstream side of the ice (stoss slope) and a steep
rougher slope on the downstream side (lee slope). The
name is French and translates into English as 'sheep
rocks', a good description of them when seen from a
distance. The smooth upstream slope is probably
caused by abrasion as the ice advances over the rock,
and the rough 'tail' is due to the action of plucking
where ice has attached to the rock and literally pulled
rock fragments away. Plucking could occur because as
the ice moved up the stoss slope there was a
reduction in pressure, allowing liquid water to refreeze and attach the ice to the underlying rocks.
• Lembert dome
Crag and Tail
• A crag and tail is a larger rock mass than a roche
moutonnee. Like a roche moutonnee, it is formed from
a section of rock that was more resistant than its
surroundings. On the lee side of the resistant rock,
the bed rock was protected from the erosional power
of the glacier.
• The volcanic rocks on which Edinburgh Castle
(Scotland) is built provided protection against the
erosion from an advancing glacier and Royal Mile, the
main street in Edinburgh, extends along the gently
downward sloping lee slope, or tail.
• Over-deepened rock basins in formerly glaciated valleys are
usually in-filled by lakes. Elongated examples are often referred
to as ribbon lakes. Fiords are glaciated valleys that have
subsequently been flooded by rising sea levels. The lower flanks
of glacial valleys are usually covered by sheets of till (debris
deposited by melting glaciers) or display rock surfaces that have
been smoothed and polished by abrasion. In contrast, the upper
slopes that were exposed above the surface of the glacier
exhibit rock surfaces frost shattered by severe climate cold.
• The boundary between these two distinct landscapes is called
the trimline and its position can be used to reconstruct the
thickness of former glaciers.
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Roche moutonees, whalebacks, rock drumlins and crag and tail
(Mesoscale)
Roche moutonees are bedrock outcrops that are asymmetric in profile
and streamlined in the direction of glacier flow. (Photo) They vary
greatly in size, but are generally 0.5 to 100m long and 0.1 to 10m high.
They exhibit a smooth abraded stoss slope (i.e. facing the direction of
ice movement), a steeper craggy lee slope formed by plucking or frost
weathering and meltwater erosion if a lee side cavity develops. (Fig)
Whalebacks are smooth streamlined hills that range in length from 1 to
1000m. Rock drumlins are similar, but larger features and may be
several km’s long. Crag and tail consist of large rock outcrops and a lee
side rampart of till. They form by erosion on the up glacier side of the
outcrop and deposition of debris in the low-pressure lee area. (Fig) The
most well known example of crag and tail occurs in the centre of
Edinburgh. The high crag, which is composed of an ancient volcanic plug,
is the site of Edinburgh castle while the
• Striations, chattermarks and cresentic fractures (Microscale)
• Striations are scratches formed by the abrasive action of rock
fragments frozen into the base of sliding glaciers. (Photo) They
are 1 to 50 mm wide and 0.5 to 10 mm deep, usually occur in
clusters parallel scratches and are often well developed on the
stoss side of roche moutonees. Larger variants, up to 20 cm
deep, are called grooves and form along zones of weak rock.
Chattermarks and cresentic fractures are small cresent shaped
surface scars, 5 to 50 cm wide, formed by glacial crushing.
Chattermarks usually occurs as lines of cresentic scars that are
concave down ice, whereas, cresentic fractures are concave up
ice.
Paternoster Lakes
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Paternoster lakes (named for their imagined
resemblance to rosary beads) are a series of lakes that
form in the low spots of a u-shaped valley. They are
linked by a stream that flows through the valley. The
presence of such lakes is diagnostic of recent glaciation,
as rivers cannot cut basins, but rather, attempt to fill
them from upstream and drain them from downstream.
The number of lakes in a trough can vary as a function
of the weakness, jointing, and lithology of the underlying
bedrock. Glaciers can also vary in erosive power along
their length based on bed temperature, valley
steepness, and extending or compressive flow. The
lakes shown occupy the Grinnell Creek valley below the
Grinnell Glacier (behind the camera), Glacier National
Park, Montana. Note the difference in color between
Grinnell lake (closest to the camera) in which silt from
Grinnell Glacier outwash is deposited, and Josephine lake
(second from the camera), which is much clearer. The
"bathtub ring" around Sherburne lake (farthest from
the camera) results from management of water level by
a dam.
• http://geology.wlu.edu/intro3d/ maps
3d
• http://www3.interscience.wiley.com:810
0/legacy/college/strahler/0471238007
/animations/ch20_animations/animation
2.html Hanging valley
• What is a corrie, what are it’s
distinctive features?
• Comment on the statement that
Glaciated uplands contain ‘distinctive’
erosional landform features.
• http://www.bgrg.org/pages/education/a
level/coldenvirons/