Landforms of Glacial Erosion
Prior to glaciation, a mountain area has narrow V-shaped river valleys with interlocking spurs. River erosion is prominent.
When temperatures are cool, snow accumulates in hollows on north-facing slopes and compacts to ice under its own weight. The glacier flows downhill, following the course of the river valley. The processes of plucking and abrasion causes erosion of the rock below and to sides of the glacier. Plucking is when the glacier freezes onto the rock and as it moves pulls pieces of rock with it. Abrasion occurs when the glacier scours away at the rock in a sandpaper-like action. Frost shattering (freeze-thaw weathering) is likely to occur on the slopes above the glacier, producing scree. The pieces of rock (moraine) are transported by the glacier and later deposited.
When the ice melts, the result is a U-shaped valley with steep, straight sides and a flat floor. Truncated spurs occur where the ends of interlocking spurs are 'cut off' by the glacier. A misfit stream may flow in the bottom of the valley and appears to be out of place as it would not have carved the valley as deep and as wide as the glacier had done.
A hanging valley is another landform common in a U-shaped valley. It is formed when a smaller tributary valley filled with a glacier erodes at a slower speed than the main valley (also filled wit a glacier). When the ice melts it is left suspended above the main valley. A waterfall is common at a hanging valley.
Ribbon lakes are commonly found in a U-shaped valley. They are long and thin bodies of water. There are two ways in which a ribbon lake can form. The first is when the glacier hits a band of softer rock and can erode this area much deeper, leaving a hollow in the valley floor. The second is when water is dammed in between terminal and recessional moraine (see Revision:Glacial Deposition) also creating a ribbon lake.
When temperatures drop, snow accumulates in a north-facing hollow. It compacts to ice under its own weight and flows downhill out of the corrie, feeding the valley glacier. The ice within the corrie moves with a rotational movement, deepening the corrie and steepening the back and side walls. Plucking occurs mainly on the backwall and abrasion occurs mainly in the hollow. Frost shattered rocks fall onto the glacier and into bergshrunds and will be used in the erosive process.
After the ice melts, a deep armchair-like bowl shape is left. The corrie has steep back and side walls and has an over-deepened bottom. A rock lip, where erosion was less, holds back a corrie lochan or tarn.
Arête and Pyramidal Peak
An arête is formed when two hollows that exist side-by-side are filled with snow. Corries are formed. As the ice flows downhill in a rotational movement it deepens the corries (by abrasion) and erodes into the backwalls are steepened (by plucking). The intervening high ground between the two corries is eroded and narrowed. Continued erosion reduces this ground to a narrow, knife-edge ridge.
A pyramidal peak occurs in between three or more corries. It is formed in a similar way to an arête. The ice eroding away at the sidewalls of the corries forms several arêtes. These arêtes lead up to a jagged peak. This is known as a pyramidal peak or glacial horn. A good example of a pyramidal peak is the Matterhorn in Switzerland.