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Glacial Landforms

Corries    Arêtes    U shaped valleys    Ribbon Lakes    Hanging valleys    Glacial deposition  

Roche Moutonnées    Crag and Tail    Drumlins

How Corries Form

Corries form in hollows where snow can accumulate.  In the Northern hemisphere this tends to be on North west to south East facing slopes which because of their aspect are slightly protected from the sun, which allows snow to lie on the ground for longer and accumulate. It is in these locations that snow accumulation is highest and ablation is lowest. South westerly winds can also blow snow drifts from south westerly facing slopes into north east facing hollows, where it is protected from the sun’s strongest rays.  The snow compacts into ice and this accumulates over many years to compact into Névé. The hollow is deepened by Nivation (by the combined effects of repeated freezing and thawing and removal of material by melting snow) by the snow and Névé, and it grows into a corrie/cirque glacier. This moves down hill because of gravity, the mass of the ice, water at its base and the slope it is on.  It will move in a rotational movement because of the slope and the overlying pressure. The ice freezes to the back wall and as it does plucks rock out steepening the back wall.  Freeze Thaw and frost shatter above the hollow on exposed rocks shatters the rock and deliver shattered rock known as scree to the ice (both on top of the ice, within it and under it).

This material from plucking and frost shatter is then moved along under the ice abrading the hollow by scratching the surface rock. This is further aided under the ice by the fact that pressure melting point is often surpassed allowing melt water to exist at the base and allowing basal sliding to occur. This creates a steep back wall and a hollow known as a corrie or cirque.  In addition, water trickling down the Bergschrund encourages even more freeze thaw action encouraging the corrie to grow further. At the front edge of the corrie the ice thins out at is speeds up on its journey down valley, and this area is eroded less and crevasses form.  This leaves a lip of rock. When the ice melts a corrie lake can form. View an animation of this process.

In France and Switzerland Corries (the Scottish word) are known as Cirques (because of their near CIRcular shape), Coombe in England and in Wales it is Cwm. Grisedale Tarn in the Lake District is a great example of a Corrie lake.

Corries and aretes in Alaska

Grisedale Tarn in the Lake District

Arêtes and pyramidal peaks

Where 2 corries occur back to back, they can erode backwards through the processes outlined above.  As these corries erode backwards they steepen the back walls in both corries, which eventually leaves a steep knife edged ridge called an Arête.  Where 3 or more corries erode backwards towards one another, this can create a Pyramidal peak, a steep-sided pointed mountain like the Matterhorn. Striding edge in the Lake District is a fabulous example of an Arête.

U – Shaped Valleys or Glacial Troughs

As corrie glaciers leave their source regions and descend down old river valleys they can make huge changes to the landscape. One of the major changes they make is to the V-shaped valleys characteristic of the upper reaches of river valleys. The glaciers basically alter this V shape into a U, by creating a steep sided, wide valley in the shape of the letter U. They are formed by a valley glacier that moves down the valley because of gravity.

As the glacier moves down the valley it plucks the rock from beneath and those rocks then rub against the bed of the valley, eroding it further.  This deepens and widens the valley. At the front end of the glacier it acts like a bulldozer, shifting and removing soil, plucking rock from interlocking spurs and truncating them. This creates Truncated Spurs, which are interlocking spurs without the land that interlocks!  Originally, interlocking spurs are created as a river erodes the upper valley it cuts down into the rock and meanders in and out of the surrounding rock. During glaciations this rock is removed by descending ice sheets.

Lateral moraines and ground moraines also play an integral role, as this material is used as a tool to abrade the valley sides and floor further.

Melt water at the glacier base also plays a role, and where pressure melting point is exceeded the glacier can basally slide encouraging even more erosion. These processes are not even however, and extending and compressing flow and variable amounts of basal material can cause DIFFERENTIAL erosion to occur.  This means that some parts of the valley floor are over deepened creating Ribbon Lakes. Within U shaped valleys you can often find Ribbon lakes. These are long and thin lakes that collect from melt water and rain water after the glacier has melted.  During glaciations the glacier erodes some parts of the valley floor more than others.  This could be because of varying strengths of the bedrock or because there is thicker ice in one region of the glacier than another or because there is more moraine abrading the ground in one region than another. Often these are founds in areas where the ice has been subjected to COMPRESSING flow. When the glacier melts water fills the depressions (holes) where the valley floor was eroded most. These lakes can also form because melt water from receding glaciers is trapped behind Moraine, which is discussed below.  Where EXTENDING flow occurs more resistant less eroded rock steps can be left.

Corries and Arêtes near Mount Cook in New Zealand

U shaped valley

Where glacial troughs erode down to sea level or below and the either the ice retreats or the sea rises because of Eustatic or Isostatic processes a Fjord can be created, as shown below at Milford Sound in New Zealand

Milford Sound New Zealand - a Fjord

On the valley floor, Striations can be found carved into the bedrock.  These are long gauges or scratches in the rock where moraine has been dragged over the bedrock.  They run parallel to the direction of ice movement and therefore can be used to calculate the direction of ice movement once the ice has retreated.

Striations

Glacial Trough Features: Hanging Valleys
Within glacial valleys there are main glaciers and smaller tributary glaciers (just like with rivers).  The main glacier can erode its valley to a much greater extent because they are wider, deeper; have more mass and more moraines to use as erosive tools.  The tributary valley glaciers are smaller, have less mass and moraine hence erode their valley less.  This means that the main valley is deeper, wider and steeper, and this becomes really evident post glaciation, when the tributary glacier is left hanging high above the main valley.  When rivers return, they often form waterfalls in these hanging valleys. This can all be seen in the images below. You can also see a fantastic animation of how hanging valleys are created here.

Glacial Trough Features: Roche Moutonnées

Roch Moutonnee

A Roche moutonnée is a mass of resistant rock which has a smooth, rounded up valley (STOSS) slope facing direction of ice flow and a down (LEE) slope formed by Plucking.

As the glacier encounters the obstacle pressure increases and allows melting to occur.  This allows the ice to melt and basal sliding to occur, rocks trapped in this ice abrade the bedrock. This abrasion on the up valley side of the glacier can leave striations as pieces of rock debris with ice were dragged across the surface under great pressure. On the lee side pressure falls and the water refreezes and as the ice moves downhill it pulls away masses of rock; plucking the rocks underneath.  This leaves a steeper sided more jagged Lee slope.

Glacial Trough Features: Crag and tail

A Crag and Tail consists of a large mass of resistant rock on the STOSS (upslope side) and a gently sloping tail (on the LEE side) of less resistant rock. This is a geological formation caused by the passage of a glacier over an area of hard rock and softer rock. A good example of crag and tail is the rock on which Edinburgh Castle is built. The Crag is composed of a hard volcanic pug of basalt whereas the Royal Mile runs down softer sedimentary rocks protected from erosion by the Crag. Crags are formed when a glacier or ice sheet passes over an area that contains a resilient rock.
Crag and tail
The force of the glacier erodes the surrounding softer material, leaving a rocky block protruding. It is a result from the abrasive base of a glacier sliding on the land surface. A tail is softer rock, Crags serves as a potential shelter from a glacier. However, usually the Tail has been removed by glacial erosion.
Glacial Deposition

Moraine – This is the material produced by glacial erosion.  The material tends to be unsorted (it contains really huge boulders and at the same time a fine powder called glacial flour).  It also tends to be very angular, as the processes that form the material involve freezing and shattering. 

Huge Lateral moraines next to Mount Cook

There are different types of moraine including:
Ground moraine - which is moraine spread all over the ground as a glacier retreats up valley in warmer times
Terminal moraine - which are rocks deposited in a ridge at the maximum advance of the ice
Lateral Moraine which are ridges of moraine that come from the valley sides and run parallel to those valley sides
Medial Moraine - these are a ridge of rocks running down the middle of a valley formed by 2 lateral moraines from 2 glaciers coming together. You can see an animation of moraine here (click on "watch glacier movie").

Recessional moraine – these often run parallel to terminal moraines and these ridges of material mark the retreat of a glacier.  Each recessional moraine marks a point where the ice has been static long enough in the glaciers retreat for material to build up.

Push moraines – these are mounds of material found where a drop in temperature or increase in precipitation allows glacial re-advance, and the glacier pushes previously deposited moraines forward into a new landform.  This can change the orientation of the stones found in the original landform.

Drumlins

There are 2 types of Drumlin, ROCK drumlins and till Drumlins. A Till Drumlin is a hill of compact, unstratified glacial drift or till, usually elongate or oval, with the larger axis parallel to the former local glacial motion. Drumlins (the crest of a hill) are a typical subglacial landform. A rock drumlin has a central core of rock!

The most identifying characteristic of drumlins are their shape, resembling an inverted spoon with its steep slope facing the direction from which the ice advances. Drumlins can be up to 7 km in length, 2 km in width and 30 m in height. There is no single composition typical of a drumlin, but most have a carapace of lodgement till. Some drumlins have a core of hard rock, or more resistant sediment, but some have no core at all.

Current theories of drumlin formation can be divided into two models: the deformational theory of drumlin formation, and the fluvial theory.

The fluvial theory, as proposed mainly by Shaw and Cox, attributes drumlin formation either to catastrophic flooding due to the release of melt water that is believed to have accumulated beneath melting ice sheets, or to floods caused by regional uplift due to tectonic movements.

However, the deformational theory for drumlin formation seems to be more widely accepted. Especially work by Boulton has shown that when a glacier moves over a potentially deformable bed there is a coupling between the glacier and its underlying sediment. This is called subglacial glaciotectonic deformation and takes place in the deforming layer beneath the glacier. Deformation is an intrinsic part of ice sheet flow, as well as of subglacial erosional and depositional processes.

Although the individual structures of drumlin cores are formed by different processes, the overall drumlin shape is thought to result from net subglacial deforming bed erosion. If more sediment enters a subglacial area than is removed (negative sediment flux) the sediment will build up and a subglacial till sequence – but no drumlins - will develop. In contrast, if more sediment is removed than enters in a subglacial area (positive sediment flux), then any resistant cores will be left behind as erosional remnants.

Drumlins Drumlins Cross section of a drumlin
  Think about it   

Research the Fundamentals of Physical Geography online textbook - an amazing resource

Compare the crag and tail with the Roche Moutonnee on this diagram

Brilliant website on all aspects of cold environments

Complete the Venn diagram at the base of the page

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