Glacial Troughs....and the rest !

Aim: To be able to understand what a glacial trough is and their major features.

Whilst glaciers flow down pre existing river valleys in upland areas, they widen, straighten and deepen these valleys through the processes of glacial erosion previously discussed. V shaped valleys are turned into U shaped valleys (see figure 1) due to the action of the ice, cpombined with large amounts of meltwater and sub-glacial debris, create large erosion rates than that of the river water. 

Figure 1: Image illustrating a U shaped river valley, shaped by glacier erosion. (Source: Reference)

Extending and compressing flows are acting upon the valley creating a variety of erosional rates. Compressing flows creates the valley floor to over deepen, which leads to the formation of rock basins.

Figure 2: Maybe not this type of rock basin !! See if you can find an image of a real glacier rock basin.

Major features of glacial troughs:

• Fairly straight with a wide base and steep sides – hence the U shape.
• Stepped long-profile with alternating steps and rock basins
• Glacial valleys can end abruptly at their heads in a steep wall. This is known as a trough end (Figure 3)
• Rock basins filled with ribbons lakes (Case study – Wastwater in the Lake District)
• Over deepening below the present sea level – this has led to formation of fjords when sea levels rose after the ice ages and submerges the lower parts of glacial valleys. (For example the coasts of Norway – where people going on a fjord cruise!) (Figure 4)
• Hanging Valleys are located on the side of the main glacial valley. These are either pre existing tributing river valleys which were never glaciated.
• Areas of land known as spurs, projecting from the river valley side have been removed by the glacier, producing truncated spurs. (Figure 5)
• Roches Moutonnees are left when small areas of rock on the valley floor are not always removed. The top is polished by the process of abrasion and the downstream side is made jagged by the process of plucking.
• Shallow lakes are created once ice has melted and filled glacial troughs. Their sides were modified by frost shattering and the development of scree which altered the glacial U shape valley.

Figure 3: Image illustrating the formation of a trough end(Source: Reference)

Figure 4: Image showing a fjord cruise (Source:Reference)

                            

Figure 5: Image showing an example of a truncated spur (Source:Reference)