GLACIAL ENVIRONMENTS 2 Glacial processes and landforms: glacial erosion processes corries and associated landforms glacial troughs and associated landforms glacial deposition landforms

Ice is capable of transporting huge quantities of rock Ice is capable of transporting huge quantities of rock. Some rocks fall on to the surface of the ice from the valley sides and are transported as supraglacial debris. Some material finds its way into the ice via crevasses to be transported as englacial debris. Where there is basal sliding, debris may also be picked up below the ice and be transported as subglacial debris. Glaciers that move relatively quickly and that transport large amounts of debris at the base, are capable of powerful physical erosion which can drastically alter the pre-glacial landscape. Chemical erosion, because of the low temperatures is relatively ineffectual. Weathering, in the form of frost-shattering (freeze-thaw) aids the erosion processes by providing a ready supply of broken rock debris. If this debris is incorporated into the sides and base of the ice, abrasion becomes active, sandpapering the rock surfaces to produce smooth, gently sloping landforms. Striations or grooves may show the direction of ice flow. Image copyright: http://tvl1.geo.uc.edu/ice/other/use.html

Plucking is a process that is now regarded as only a minor erosion process as only a small quantity of already fractured rock is capable of being removed by ice which freezes to the rock surface and then moves forward, pulling out the loose blocks. Plucking produces jagged slopes to landforms. Rates of erosion will vary considerable but are greatest where: temperatures fluctuate around freezing point where rocks are more jointed and faulted providing weaknesses where slopes are slightly steeper leading to more rapid glacier movement (very steep slopes can lead to extended flow, a thinning of the ice and reduced erosive power two or more glaciers meet and combine to give an increased depth of ice ice moves by rotational flow in corrie glaciers leading to over-deepening of the hollow

Cirques (France), corries (Scotland) or cwms (Wales) are glacial hollows with a very steep backwall and a basin that may contain a lake or tarn when the glacier retreats and melts. Snow collects in pre-glacial hollows, particularly on sheltered, north-facing slopes and a combination of freeze-thaw weathering, solifluction flow and summer meltwater activity enlarge these hollows allowing small corrie glaciers to form as glaciation progresses. Rotational flow of the ice and active abrasion within the hollow tend to over-deepen the base leaving a raised rock lip at the edge of the hollow. Meltwater which lubricates and aids glacier flow may get under the ice through crevasses, particularly the large bergshrund crevasse along the backwall. Photo source: U.S. Geological Survey (public domain)

Where a series of corries form around a mountain peak, they create other unique landforms. Two corries eroding into the mountain eventually leave a narrow, knife-edged ridge or arete between them. Striding Edge in the Lake District provides a classic example. Photo source: http://trekking-hiking-outdoors.co.uk/ Where three or more corries erode backwards around a mountain, they create a characteristic triangular pyramid peak or horn. One of the most spectacular examples is the Matterhorn. Photo source: http://www.ski-zermatt.com/mattnet/pics/batch/11/photo3.htm Image source: http://oz.plymouth.edu/~sci_ed/Turski/Courses/Earth_Science/Images/8.glaciatedtopo.jpg

In mountain environments, valley glaciers severely modify former river valleys to produce very deep, steep-sided, flat-floored U-shaped valleys or glacial troughs. Variations in rock resistance or locations where glaciers merge give rise to over-deepening of the valley floor and the formation of long, narrow ribbon lakes. Where over-deepening occurs along the coasts, deep sea fjords may form as sea-levels rise and flood the former glaciated valley. Along the sides of the glacial troughs are truncated spurs, rocky outcrops which form the ends of former interlocking spurs that have been eroded by the valley glacier. Tributary river valleys contain only small valley glaciers and due to the small amount of erosive power that they have, these valleys remain at a higher level and form hanging valleys, often with dramatic waterfalls where tributary streams rejoin the main valley. In the glacial troughs post-glaciation, small misfit streams occupy the now enlarged valleys.

Other erosion features include striations, roche moutonnees and crag-and-tail landforms. Striations or scratches are found everyone on bare rock surfaces and are useful to indicate direction of glacier movement. Picture source: http://www.gov.ns.ca/natr/ Roche moutonnees are large rock obstructions that have been smoothed by abrasion on the upstream side (stoss) but have irregular, jagged surfaces on the downstream side (lee) where plucking has occurred. As glaciers move across the landscape, they come across large rock obstructions such as volcanic plugs or particularly resistant rocks. These outstanding crags remain after glaciation and may protect a tail of softer material which slopes gently away from the crag on the leeward side. Edinburgh Castle stands on one of these crag-and-tail landforms.

Material eroded and subsequently transported by glacial ice may be deposited as unsorted till material as the ice melts or it may be further transported by glacial meltwater and then deposited as sorted fluvioglacial material. Till deposits, sometimes referred to as boulder clay, are a mixture of unsorted sand, clay and rock particles. The rock fragments are sub-angular in shape. The majority of this material has been transported as supraglacial debris and is dropped in situ at the glacier snout or more generally at the ends of ice ages when glaciers disappear. Some of the till deposits form distinctive landforms but much of it is simply deposited as a layer which masks the former pre-glacial landscape. Till fabric analysis can be used to analyse the rocks embedded in the sand/clay matrix in order to determine the direction of glacier or ice sheet flow. The long axes of the larger rocks align themselves in the direction of flow. Picture source: http://www.gov.ns.ca/natr/

Sometimes glaciers pick up and transport rocks with distinctive geological characteristics. Once deposited, these erratics can be used to trace back the route followed by the glacier. The photo shows a sandstone Norber Erratic in Yorkshire lying on top of limestone which has been chemically eroded by acidic rainwater in the 13,000 years since the boulder was deposited. When glacial debris is deposited, five main types of landform may be created: Lateral moraine, medial moraine, terminal moraine, recessional moraine and push moraine. Lateral moraine forms a ridge along valley sides. They are formed from debris originating from freeze-thaw activity on valley sides. The material falls onto the glacier and is carried mainly as supraglacial material. The material remains angular in nature. Medial moraines, found in the middle of valleys, are usually formed when two glaciers coalesce. They are rare in post-glacial landscapes as they are easily destroyed by rivers flowing along the glacial troughs.

Terminal or end-moraines are located at the snout of glaciers or the edges of ice sheets and mark the furthest point of advance. They are created from supraglacial, englacial and subglacial material when ablation is active. They form crescent-shaped ridges which can be several hundred metres high. Terminal moraines may act as dams at the edges of corrie basins or in glacial troughs. If there is a major re-advance of the glacier or ice sheet, the end moraines are bulldozed forward to create push moraines. When there are long pauses in the deglaciation process, a series of recessional moraines, often smaller than terminal moraines, may form to mark the various stages of glacial retreat. Sometimes, moraine material is shaped into low, egg-shaped hills called drumlins. These tend to occur in large swarms on valley floors or in lowland areas. They are blunt and steep at the stoss end, narrow and gently-sloping at the lee end. Valley drumlins are around 5-10 metres in height but lowland examples can reach 50 metres in height. Photo source: http://oz.plymouth.edu/~sci_ed/Turski/Courses/Earth_Science/Images/

Summary of key points: glaciers are capable of transporting their load as supraglacial material (on the surface), englacial material (within the ice) and subglacial (below the ice) where freeze-thaw weathering is highly active, it can weaken and break up rocks to make glacial erosion easier glacial erosion takes place by abrasion (sandpapering action using rocks embedded in the ice) and by plucking (ice freezes on to loose rocks and pulls them free) key landform in upland areas is the corrie, a large armchair-shaped hollow with a steep rocky, backwall, often filled with a glacial lake or tarn. Around the corries, steep, knife-edged aretes may form and some peaks are shaped into pyramidal peaks or horns former v-shaped river valleys are transformed into deep u-shaped glacial troughs with truncated spurs and hanging valleys glaciers deposit their loads as unsorted glacial till or sorted fluvioglacial material when glacial debris is deposited, five main types of landform may be created: lateral moraine, medial moraine, terminal moraine, recessional moraine and push moraine