Glacial processes: erosional features and landforms

Glacial processes: erosional features and landforms

1. Freeze thaw- also called frost-shattering as it occurs in cold climates when temperatures are often around freezing point and where exposed rocks contain many cracks. Water enters the cracks during the warmer day and freezes during the colder night. As the water turns into ice it expands and exerts pressure on the surrounding rock, causing pieces to break off.

   • Weathering- the break-down or decomposition of rock by biological, physical or chemical processes.
     • Nivation – hollows form under the emerging glacier as a result of the freeze-thaw cycle and mass wasting. Over time these may enlarge and start to form corries
     • Frost action – an umbrella term for freeze-thaw processes where meltwater percolates into cracks and freezes causing fissures to expand under pressure of ice and, with repeated cycles, shatter the surrounding rock.
   • Glacial troughs (or U-shaped valleys) These are valleys formed by the glacial processes of weathering, erosion and transportation. They have steep sides and flat floors. Unlike V-shaped river valleys, glacial troughs are straight as they have truncated any interlocking spurs which existed prior to glacial advance.

2. Abrasion – rock fragments and debris carried by the glacier scrape the rock below/adjacent to the ice flow in the valley with which it has contact leaving striations (scratches) in the bedrock and incorporating the eroded rock into the ice mass, from where it can cause erosion subsequently.

3. Plucking – meltwater flowing along the base of the glacier freezes to the bedrock, fragments of which are ripped out by the moving glacier.

4. Rotational scouring – as layers of rock debris accumulates on the surface of a glacier from valley sides, over successive winters it becomes embedded within the compressed ice. Bands of slowly rotating frozen rock are then scraped over the bedrock as the glacier moves downhill.

5. Pressure release/dilation – where the variable weight of glacier ice on top of bedrock can cause fractures to open up, expand and extend deeper.

6. Basal sliding: occurs only in warm glaciers where temperatures are such that significant meltwater lubricates the contact between the base layers of glacier ice and the bedrock. The meltwater occurs either as a result of internal tunnels transferring it via surface moulins (vertical tunnels), or increased pressure as the basal ice encounters resistant bedrock, which may reach the pressure melting point that causes basal ice to melt.

7. Internal deformation: occurs predominantly in cold glaciers where gravity and the pressure of ice in the accumulation zone causes ice crystals to slide over each other in a series of parallel planes in a ‘crumpling’ deformation. This can result in deep crevasses at the surface.

8. Extensional flow: where the gradient becomes steeper the ice moves faster ‘stretching’ the ice mass and becoming thinner through a series of fractures which form crevasses at right angles to the direction of flow.

9. Compressional flow: where the gradient becomes less steep or the ice encounters a major obstacle the ice mass slows, backs up, crevasses close and there are thrust fractures as the ice mass compresses. The increased thickness of ice exerts greater pressure on bedrock and can result in more extensive pressure erosion.

10. Cirque/ corrie- These are amphitheatre-shaped mountain-side hollows found where the build-up of snow was greatest (in the northern hemisphere this is typically on north or east facing mountain slopes). They are formed when lying snow accumulates over a number of years and compresses lower layers into ice (firn) in a hollow. Once it has developed a sufficient volume, gravity causes it to move in a rotational manner out of the hollow and down the mountain side, eroding the floor by plucking and abrasion. Corries are the sources of Alpine glaciers.

11. Arêtes Where two corries occur back to back and headward recession (cutting back) takes place over time forming a narrow, steep, rocky ridge between the two corries.

12. Truncated spurs These are where interlocking spurs (the result of meandering river erosion) which predate the glacier are eroded and amputated by the straight-flowing glacier as it drives a path down a valley.

13. Hanging valleys These are formed by tributary glaciers which enter a glacial trough. Unlike in river landscapes they do not cut down to the level of the main valley as the glaciers they carried were considerably smaller, but instead lie suspended above in the valley side at the end of the ice advance. They are often represented by a waterfall emerging from the higher tributary valley in the post-glacial landscape.

14. Roches moutonnées These are asymmetrical rocky outcrops in the trough floor. The up-valley side (stoss) gently slopes and is smoothed by abrasion of the glacier. The down-valley side (lee) is steep in gradient, and subject to plucking as ice accelerates past it. They represent significantly more resistant rock on the glacial trough floor that the glacier found more difficult to erode than surrounding rocks.

15. A Crag and Tail consists of a large mass of resistant rock on the STOSS (upslope side) and a gently sloping tail (on the LEE side) of less resistant rock. This is a geological formation caused by the passage of a glacier over an area of hard rock and softer rock. A good example of crag and tail is the rock on which Edinburgh Castle is built. The Crag is composed of a hard volcanic pug of basalt whereas the Royal Mile runs down softer sedimentary rocks protected from erosion by the Crag. Crags are formed when a glacier or ice sheet passes over an area that contains a resilient rock.

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