Formation of features of glacial erosion

Main features of a corrie:

• A corrie is a armchair shaped hollow. 
• Steep back wall, over-deepened basin and a ridge called a lip at the front.
• Corries vary greatly in shape and size, some can be a few hundred metres across but some can be 15km wide.

Main features that shape corries:

• Nivation- responsible for the the initial enlargement of hillside hollow and development of corries. Nivation is a combination of freeze-thaw action, solifluction, transport by running water and (possibly) chemical weathering.
• Erosion
• Abrasion
• Plucking

• Begins with nivation of a small hollow on a hillside (usually in mountainous regions which are not facing the sun.)
• Snow collects and accumulates in the hollow year on year. Over time the snow accumulation increases and lots of snow is compressed into Neve ice.
• Erosion, nivation and weathering all work togetherand continue to erode the hollow which continues to get deeper.
• At a critical depth, the ice starts to rotate under its own weight, this causes the hollow to enlarge further.
• The rotation also causes plucking of the back wall which makes it very steep. This action can also cause bergschrund crevasses to form.
• The debris from this as well as from weathering above the hollow then falls into the bergschrund crevasse.
• Once the hollow is deepened further, the thinner ice at the front is unable to erode as rapidly so a lip is left. The lip may also consist of  glacial till which has been deposited as the ice moves out of the corrie.
• Post glaciation- the corrie may become filled with water, forming a circular lake or tarn.

A bergschrund crevasse is an irregular crevasse usually running across an ice slope in the accumulation area, where active glacier ice pulls away from ice stuck to the steep mountain side.

A Tarn is a small lake occupying a hollow eroded out by ice or dammed by a moraine, especially common in corries.

Aretes (Meaning edge or ridge in French)

Main features of a Arete

• An arete is a narrow, steep sided ridge found between two corries.
• Often very narrow so is described as knife -edged
• Aretes are often very pronounced and are sharpened by frost action.

Arete formation

Aretes form from glacial erosion, with the steepening of slopes and the retreat of corries that are back to back or alongside each other.

When three or more corries erode backwards towards one another, this can create a pyramidal peak.

Weathering of this can sharpen its shape as with aretes.

An example of a pyramidal peak is the Matterhorn which is located in the swiss alps, it is over 1200m high

These are also called glacial troughs. They are the characteristic shape that a valley leaves behind.

They are formed when a valley glacier travels down a slopw and erodes the sides of a valley through the process of abrasion. An example is Great Langdale in the Lake District.

1. A Corrie Glacier starts to move down into an old river valley.

2. 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 through abrasion.

3. This deepens the original V shape of the valley into a deep U shape. Which is the glacial trough.

4. The truncated spurs form very steep sides which forms the U shape.

What is found in the glacial trough?

In the trough of a U shaped valley, there are different features left behind by the glacial meltwater.

• Misfit streams are rivers which would have originally filled the V shaped valley, but is now too small for the U shaped valley so it meanders and looks almost lost.
• Ribbon lakes are long lakes which form from rain and meltwater which collects in areas where the glacier eroded deeper into the rocks than in other areas.

Plucking

Plucking is a form of erosion where ice at the base and sides of the glacier freeze onto the rock.

As the glacier moves forward down the valley the pieces of rock are pulled away.

Abrasion

Where debris in the ice of a glacier scrapes against the base and sides of the valley, wearing it away.

Finer material smooths the rock

Coarser, harder material will scrape, scratch and smooth the rock to wear it down.

 A hanging valley is a valley which has been cut off by a deeper valley at a right angle.

A tributary glacier may flow into the main glacier leaving behind another U shaped valley.

When the tributary glacier melts, it leaves behind another ribbon lake which then may flow down into the valley below as a waterfall.

Truncated spurs

Are formed by both the processes of plucking and abrasion as the glacier moves down the valley "cutting" through the spurs in the previously V-shaped valley.

Because the spurs are truncated it leaves behind the shape of the U-shaped valley because it leaves behind very steep vertical sides.

Fjords are found in locations where current or past glaciation extended below current sea level.

A fjord is formed when a glacier retreats, after carving its typical U-shaped valley, and the sea fills the resulting valley floor.

This is becauseall the glacial melt water causes the sea level to rise and fill the valley forming a fjord.

The fjord is then connected to the sea by the mouth of the fjord.

It is very cold and very deep-> no swimming

A roche moutonne is an isolated lump of resistant rock, found at the bottom of a valley glcaier or trough. The surrounding rock gets eroded away and allows the resistant rock to stick out.

As a glacier advances over the rock pressure is built up and causes the ice to melt around the rock, known as internal deformation.

As the ice passes over the rock small bits of sediment shapes and smoothens the rock forming a ramp shape in the direction of ice flow.

Occasionally larger shards of rock that are being transported by the glacial ice pass over the Roche moutonne causing scrapes in the rock striations. When ice has passed over the rock, it re-freezes and attaches onto the rock at the back of the Roche moutonne, this is known as plucking.

The name of the scrapes left on the rock due to rock shards in the ice passing over it are called striations.

Ellipsoidal basins are erosional landforms created by ice sheets.

For example, The Laurentide ice sheet covered much of North America between about 95,000 and 12,000 years ago. At its maximum extent, it spread as far south as latitude 37*N and covered more than 13 million Km squared. Its thickness in some areas reached 3000m or more.

Erosion by the Laurentide ice sheets produced a series of ellipsoidal basins in north america.

The master basin holds Hudson Bay with smaller basins containing the great lakes.

As well as the significant erosional impact of the ice, the weight of the ice sheet also led to isostic lowering of the surface landscape.