Macro erosional landforms | Parabolic Valleys | Hanging Valleys | Fjords | Aretes
What are macroscale erosional landforms?
Macroscale erosional landforms are larger than 1 km in dimension and often contain smaller landforms (micro- and mesoscale) 1.
Macroscale erosional landforms, such as Parabolic Valleys, Arêtes and Pyramidal Peaks (see Figure 1) are without a doubt some of the most impressive geomorphological features found in mountain regions and provide evidence of past glaciations 2.
Types of erosional landforms
Parabolic Valleys
The cross-section of a glaciated landscapes is generally described as being ‘U-shaped’. Parabolic valleys have smoothed, polished, and steep walls and flat floors (see Figure 2).
Formerly known as ‘V-shaped’ fluvial valleys 1, parabolic valleys are formed by subglacial erosion as the glacier flows from the cirque down the valley, using a combination of glacial abrasion (erosion by rocks carried within the glacial ice) and plucking or quarrying (removal of chunks of rock by the glacial ice) 3.
As the glacier erodes the valley, the landscape is transformed, with many fluvial landforms altered or removed. For instance, fluvial spurs are bulldozed and leave behind what are known as truncated spurs (as can be seen in Figure 1).
You can find spectacular examples of parabolic valleys across the world, with sweeping views from the valley floor to the mountain peaks (see the image of Glen Coe as an illustration Figure 3).
Hanging valleys
The erosional power of a glacier is generally controlled by a variety of factors, including velocity, slope, discharge, and thermal regime. Therefore, large main valley glaciers have different erosional powers to smaller tributary glaciers.
As a result, where a tributary glacier flows into a main glacier (known as a confluence) due to the difference in erosional power a step develops between the two valley floors 2,3 leaving the tributary glacier perched at a higher level than the main glacier 1.
Following deglaciation, landforms known as hanging valleys form where the higher valley floor from the tributary glacier flows into the main glacier (Figure 1).
Many spectacular waterfalls now occupy hanging valleys, for example Bridalveil Falls in Yosemite.
Fjords
A glacial valley that has been submerged by sea water is known as a Fjord. Fjord landscapes are common in the arctic and Antarctic, as well as along maritime fringes in southern south America, northwest Europe and Norway, Canada, Alaska, and New Zealand, home to one of the most famous Fjords – Milford Sounds 1.
Arêtes and pyramidal peaks
Glacial cirques (known as corries or coires (Scotland) and cwms (Wales)) are common to many mountainous regions (see this post) 4 and often serve as evidence of past glaciation 2.
In areas where multiple cirques develop in adjacent valley’s, steep knife edge ridges known as arêtes form (here at least 2 cirques must be present). There are numerous examples of arêtes across the UK, perhaps the most well-known being Striding Edge in the lake district (Figure 5a).
Where 3 or more cirques form, sharp horn-like Pyramidal Peaks establish. You can find examples across mountainous regions (Figure 5b), with the Matterhorn in the Dolomites, one of the most famous examples.
Over time, adjacent cirques can erode back and remove the bedrock or arete between them. At this point a sharp pass known as a col forms (Figure 1).
