4. Glacial Erosion - Glacial troughs and associated landforms

Question 1

Explain what a Glacial Trough is and describe its features.

Answer 1

​Glacial troughs (also called U-shaped valleys) are steep-sided valleys with flat bottoms. They’re formed by the erosion of V-shaped river valleys by glaciers. As the glacier erodes through the V-shaped valley, it makes them deeper and wider.

The major features of glacial troughs are:

• usually fairly straight with a wide base and steep sides – a U-shape
• stepped long profile with alternating steps and rock basins
• some glacial valleys end abruptly at their heads in a steep wall, known as a trough end, above which lie a number of corries
• rock basins filled with **ribbon lakes **e.g. Wast Water in the Lake District
• over-deepening below the present sea level - this led to the formation of **fjords **when sea levels rose after the ice ages and submerged the lower parts of glaical valleys, for example on the coasts of Norway and southwest New Zealand (Milford Sound)
• **hanging valleys **on the side of the main valley (e.g. the valley of Church Beck which flows down into Coniston Water in the Lake District). These are either pre-existing tributary river valleys which were not glaciated, or tributary glacial valleys. In tributary glacial valleys there would have been less ice and therefore less erosion than in the main valley. The tributary valley floor was therefore left higher than that of the main valley when the ice retreated.
• areas of land projecting from the river-valley side (spurs) have been removed by the glacier, producing truncated spurs.
• small areas of rock on the valley floor are not always completely removed and this leaves roches moutonnées. These have an upstream (stoss) side polished by abrasion and a downstream (lee) side made jagged by plucking.
• after ice retreat, many glacial troughs were filled with shallow lakes which were later infilled, and their sides were modified by frost shattering/ freeze-thawing and the development of screes which alterted the glacial U-shape (e.g. Great Langdale, Lake District).

Question 2

Explain the formation of a Glacial Trough.

Answer 2

Glaciers flow down pre-existing river valleys as they move from upland areas. They straighten, widen and deepen these valleys, changing the original V-shaped river feature into the U-shape typical of glacial erosion. The action of ice, combined with huge amounts of meltwater and subglacial debris, has a far greater erosive power than that of water.

As both extensional and compressional flow are present, the amount of erosion varies down the valley. Where compressional flow is present, the glacier will over-deepen parts of the valley floor, leading to the formation of rock basins. It is also suggested that over-deepening is caused by increased erosion at the confluence of glaciers, areas of weaker rocks or zones of well-jointed rocks.

An example is Nant Francon Valley, Snowdonia, North Wales.

Question 3

Draw a diagram to show the formation of a glacial trough.

Answer 3

Question 4

Draw a diagram to show the formation of a hanging valley.

Answer 4

Question 5

Draw a diagram to show the formation of a roche moutonnée.

Answer 5