1
Q
Factors affecting glacial erosion
A
- the glacier size - determines ice thickness, and its thermal regime, also determine the importance and intensity of the erosional processes.
- when glacier ice is warm based (meltwater and abundant debris facilitate abrasion) and regelation (whereby water melts under pressure and freezes again when the pressure is reduced), one of the key elements of the plucking process, can also occur.
- the glacier’s velocity across the bed, the ice thickness and, hence, the power of the glacier to cause shattering.
- quantity and shape of the rock debris.
- Subaerial processes of freeze-thaw (congelifraction) combine with extensive mass movement from scree slopes to supply the tools for glacial erosion.
- The bedrock characteristics, such as density of jointing and hardness, are also significant in influencing compressional and extensional flow
- Essentially erosion rates are more intense when the glaciers are warm based, thick and fast moving and the bedrock relatively weak, often due to dense jointing.
- On the other hand, erosion rates are much slower where glaciers are cold based and the rock relatively resistant.
2
Q
Features and landforms of glacial erosion
A
- the landforms which are the output of the glacier landform system result from the interaction of the processes and characteristics of the landscape experiencing glaciation (bedrock type, structure and topography) operating through time.
- However, the processes do not operate at a constant rate during time, and the landforms have to continually adjust, especially after glacial retreat in the short paraglacial period and then post-glacially when the landforms shaped by glaciation are reshaped by water, weathering and mass movement.
- A further complication is that most present-day landscapes resulting from glacials are polycyclic/ polygenetic: the product of many successive advances of glacier ice because of the alternating ice-house-greenhouse conditions during the Quaternary period.
- As the last glacial period ended very recently in terms of geological time (the Loch Lomond Stadial, which ended around 11,500 years ago), the mountain areas of the UK provide clear examples of glacial erosional landforms.
- The Cairngorms and North West Highlands of Scotland, the Lake District and North Wales are all excellent areas for a glaciated highland case study.
- Many different features and landforms are produced by glacial erosion, and they can be classified in different ways based on scale/size range, relative altitude or even the dominant erosion process that formed them (plucking or abrasion).
- Table 6.1 provides a checklist of landforms associated with glaciated highland erosion, classified by scale.
3
Q
Key factors influencing abrasion and plucking rates
A
4
Q
Macro-scale features.
A
These are around 1 km or greater in size and form the major elements in a glaciated highland landscape.
They also contain many of the meso- and micro-scale erosional features, as well as depositional landforms.
Cirques (corries, cwms)
arete
pyramidal peak
5
Q
Cirques (corries, cwms)
A
- are armchair/bowl-shaped depressions usually found at relatively high altitudes.
- The initial stage of formation is for snow to accumulate in a sheltered mountain side location.
- In the northern hemisphere, cirques most commonly form on the northeastern side of mountains, in the lee from prevailing westerly winds, and in shadier sites protected from insolation.
- Once a sheltered area has accumulated snow, nivation or snow patch erosion begins, enlarging the hollow by a combination of freeze-thaw weathering to loosen the rock, and in summer melt water from melting snow transports the rock debris away, thus enlarging the hollow.
- Once a nivation hollow (a periglacial feature) is established, positive feedback occurs: the enlarged hollow traps additional snow and gradually enlarges to provide a site for glacial ice formation.
- the processes of plucking and abrasion combine to develop the cirque.
- The glacier ice may expand in area and move down valley during a glacial period.
- the cirque can be modified post-glacially with the formation of a small lake known as a tarn
- an example is Red Tarn on the northeast face of Helvellyn in the Lake District
6
Q
arete
A
- a steep, knife-like ridge produced from the intersection of two cirque headwalls
- formed when the two steep back walls meet
- Striding Edge (Helvellyn), The Lake
District
7
Q
pyramidal peak
A
if three or more cirques interact back to back around the flanks of a mountain
- forms a steep pointed peak called a horn because of its slope
- e.g Matterhorn, alps
8
Q
Glacial troughs
A
- When glacier ice bulldoses its way through mountain valleys, it straightens, widens and deepens them, changing the valley from V shaped to U shaped.
- More accurately, these glacial troughs are described as parabolic in shape.
- Glacial troughs can be of varied length, from Nant Ffrancon in Snowdonia, which is around 5 km long, to spectacular features such as Yosemite Valley in Yosemite National Park California.
- Along their lengths (long profile) many glacial troughs have a stepped profile, reflecting differential erosion as a result of both irregularities in the underlying bedrock and variations in intensity of erosion.
- For example, where several cirque glaciers meet at the head of a valley the enlarged glacier erodes very deeply to form a trough end to the valley.
- After deglaciation, successive rock basins down a glacial trough are separated by riegels or rock steps.
- Longer and deeper basins may contain linear lakes termed ribbon or finger lakes.
- Post-glacial weathering and mass movement has led to infill of glacial troughs, which are now commonly occupied by misfit streams.
- With relative sea level rises at the end of the last glacial period, many coastal glacial troughs were flooded by the sea to form sea lochs (Scotland) or fjords (Norway).
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Q
Hanging valleys
A
- occur where a small side tributary glacier meets a larger main valley glacier.
- During the glacial phase the surface ice elevation of the tributary and main valley glaciers is the same but, because the rate of erosion beneath the main valley glacier is much greater, once the glaciers have retreated the tributary valley can be left hanging hundreds of metres above, often with a waterfall plunging from the hanging valley to the main valley below (for example Pistyll Rhaeadr in the Berwyn Mountains).
- e.g Yosemite Valley, California
10
Q
truncated spurs
A
- Many glacial troughs show truncated spurs, marked by very steep, almost vertical, side walls where original interlocking spurs have been cut away or truncated by glacial erosion, because of the inflexibility of glaciers moving down the valley (for example Lauterbrunnen Valley in Switzerland or Yosemite in California).
11
Q
Diagram of Macro features of a glaciated upland area
A
12
Q
Meso-scale features
A
- Meso-scale features are largely found within macro features, for example the whalebacks and roches moutonnées found on the floor of the Yosemite glacial trough.
- These intermediate-scale landforms can range from around 10 m to 1 km in length.
- Streamlined bedrock features such as whalebacks are the most common, where a glacier moves over a resistant rock knoll, so abrading it.
13
Q
Roches moutonnées
A
are stoss and lee features; abrasion smooths the up-glacier, stoss-side of a bedrock knoll, while glacial plucking makes the down-glacier, lee-side rugged and rough, thus producing an asymmetric landform.
Figure 6.4 shows how they are formed beneath the ice.
Average-size examples, such as those found in the Cairngorms in Scotland, are around 300 m long and about 30 m in height.
14
Q
Micro-scale features
A
- Micro features of glacial erosion are those that are a few metres in size or less.
- They include striations, Chatter marks, crescentic gouge
- Look out for these micro features on abraded surfaces.
- Micro features are not only useful for helping glaciologists understand which direction the ice came from (its provenance) but also for determining the maximum altitude of glacial erosion, where there are no micro features (that is, beyond the trim line on the valley side).
- In the Glyders you can see how block fields, screes and tors, clearly indicative of periglacial activity, supersede abraded, ice-scratched rocks.
15
Q
Chatter marks and crescentic gouges
A
Chatter marks are irregular chips and fractures in the rock, whereas crescentic gouges have a more regular pattern and are usually concave up-glacier. Look out for these micro features on abraded surfaces.
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Q
striations
A
- which are scratches on hard bedrock caused by debris being dragged across the surface during abrasion, almost like chisel marks.
- They tend to be parallel to the direction of ice movement, with the deepest part of the scratch at the initial point of impact, and are therefore useful for tracking the direction of past glacier movement.
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Q
Ice-eroded landscapes formed by glacial scouring p1
A
- When ice sheets and glaciers expand out beyond constrained mountain valleys, they erode large areas of lower relief by the process of areal scouring.
- the landscape consists of extensive tracts of subglacially eroded bedrock, composed of many whalebacks, roches moutonnées and over-deepened rock basins.
- This type of landscape is associated with extensive coverage by warm-based ice, quite slow moving, which differentially eroded the hard bedrock.
- The structure of the underlying rock therefore has a major impact on the orientation and scale of the erosional landforms.
- In North West Scotland (Sutherland and the Isle of Lewis) the landscape is called knock and lochan topography because the higher areas of resistant rock (knocks) are interspersed with numerous small lakes in the rock basin (lochans).
- A chaotic drainage pattern has resulted, often where patches of residual moraine interfere with the drainage.
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Q
Ice-eroded landscapes formed by glacial scouring p2
A
- Other areas where landscapes of areal scour occur include Central Finland Lake Belt and the Canadian Shield, both areas of ancient resistant igneous and metamorphic rocks where differential erosion is controlled by the density of jointing
- These areas may also contain examples of crag and tail (meso-scale landforms)
19
Q
Micro- and meso-scale features are again useful for
A
researching the provenance of the ice.
- Work in Antarctica confirms that ice sheets did not create the overall landforms of the great shield areas, which had acquired their almost-level surfaces by denudation before the Ice Age.
- What these ice sheets did do, however, was to considerably modify the underlying surface over which they passed.
- This is confirmed as there is generally a low amplitude of relief (less than 100 m) with many meso- and micro-scale features.
20
Q
crag and tail
A
- (meso-scale landforms) where the glacier ice is forced around a large and resistant rock obstacle, such as a volcanic plug, which protects the less-resistant material on the leeside causing the feature to taper into a tail in the down-glacier direction.
- Edinburgh Castle is a famous example of a crag and tail, with a steep up-glacier stoss end and a long, gently sloping tail on the leeside that runs for 1.4 km down to Holyrood Palace and the Scottish Parliament.
21
Q
Glacial debris
A
- Once rock has entered the glacial system and is being transported, it is classified into three kinds of debris: supraglacial, englacial and subglacial.
- Supraglacial refers to material being transported along the surface of the ice.
- If rock debris from surrounding slopes falls on to the glacier in the accumulation zone it will become buried by new snow and become englacial.
- If it falls on the ablation zone area, it will most likely stay on the surface till it reaches the snout, which is often very dirty from accumulated debris.
- Supraglacial debris also can become englacial by falling into deep tranverse crevasses opened up by extensional flow, or it can be carried downwards by meltwater in warm (wet) based glaciers.
- Subglacial debris is transported beneath the ice all the way to the glacier snout, but it can be thrust upwards during compressional flow.
- The transportation and eventual deposition of debris by glaciers is just as significant as glacial erosion in modifying the pre-glacial landscape, in this case by covering it over.
22
Q
glacial erratics
A
- The presence of large boulders known as glacial erratics, so called because they are of a different rock type to the bedrock they ‘sit’ on, testifies to the sheer scale of the ability of glaciers (especially the vast ice sheets) to transport enormous quantities and weight of rock debris over great distances.
- Huge erratic - boulders weighing up to 16,000 tonnes - were carried over 300 km from the Canadian Rockies to the plains of Alberta by the Cordilleran Ice Sheet.
- Some are actually dumped as perched boulders, for example the Bowder Stone in Borrowdale in the Lake District.
- If the erratic is made of a rock source of a distinctive geology from a restricted location - for example Ailsa Craig granite from West Scotland - you can precisely map the direction of movement of the glaciers
- Erratics from Scandinavia have been found in the boulder clay of the Northumberland, Durham and Yorkshire coasts of North East England, confirming the presence of the continental ice sheets from Scandinavia.
23
Q
The 4 processes of glacial deposition
A
The main processes by which glaciers deposit material are:
* Lodgement
Ablation:
Deformation
Flow
All of these processes produce till or boulder clay of different compositions, enabling scientists to analyse the types of depositional process. Lodgement till has relatively rounded clasts because of the grinding that occurs at the ice bed interface, not within a matrix of clay or silt-size particles (rock flour).
Ablation till consists of more angular clasts as they are not ground down, and also the matrix is of larger-sized material and less compact.
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Q
Lodgement:
A
- this process occurs beneath the ice mass when subglacial debris that was being transported becomes ‘lodged’ or stuck on the glacier bed.
- Lodgement occurs when the friction between the subglacial debris and the bed becomes greater than the drag of the ice moving over it.
- It is commonly associated with glaciers carrying huge loads of debris and where the glacier is very slow moving, if not static.
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Ablation:
this process refers to debris being dumped as the glacier melts and thaws. It can include supraglacial and englacial material, as well as subglacial material.
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Deformation
is a less-common process associated with weak underlying bedrock, whereby these sediments are defined by the movement of the glacier.
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Flow
occurs if high meltwater content causes the glacial debris to creep/slide or flow during deposition.
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moraine
refer to an accumulation of glacial debris, whether it is dumped by an active glacier or left behind as a deposit after glacial retreat. There are two broad categories:
• moraines formed beneath the glacier (subglacial)
• moraines formed along the margins of a glacier (ice-marginal).
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Subglacially formed moraines
- These moraines are composed primarily of lodgement till as they are formed from glacial debris beneath the glacier.
- Till plains of ground moraine are extensive flat areas that cover pre-existing topography, often to depths of 50 m.
- In some places beneath active glaciers, lodgement till is moulded into streamlined mounds called drumlins that have their long axis orientated parallel to the direction of ice movement.
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drumlins
- Drumlins vary widely in size, usually ranging from 10 to 50 m high and between 200 and 2000 m long.
- The steeper, blunt end of the drumlin (stoss end) is the up-glacier side, whereas the gently sloping, tapered end occurs down-glacier.
- Drumlins usually occur in ‘swarms’ forming what is often called a ‘basket of eggs’ topography.
- They often occur regularly spaced, with a length-to-width ratio never more than 50, and are typically found in lowland areas in relative close proximity to upland centres of ice dispersal.
- Excellent examples of drumlin swarms occur in Northern Ireland, the Ribble Valley (Lancashire), the Cheshire Plain, North Shropshire and the Eden Valley, Cumbria
Drumlins are an example of equifinality, in that a number of mechanisms have been proposed for their formation.
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Not all drumlins are necessarily formed in the same way; some drumlins have a rock core, which also needs an explanation, whereas others do not.
• The Boulton-Menzies theory suggests that a drumlin is formed by deposition in the lee of a slowly moving obstacle in the deforming layer. The obstacle of bedrock, or thermally frozen material, forms the core of the drumlin and ground moraine is plastered round it.
• The Shaw theory suggests that all drumlins, even rock core drumlins, were formed by subglacial meltwater in flood causing irregularities to form in the river bed which were subsequently moulded into drumlins and streamlined by the advancing ice.
It is only recently that time-lapse geophysical surveys have been carried out subglacially. These actually show a drumlin forming from deforming sediments beneath the Rutford Ice Stream in West Antarctica, which helps to reveal more about their formation.
In some areas lodgement till remoulded into streamlined flutes, with a length-to-width ratio in excess of 30.
These long, narrow features are usually less than 3 m in height and less than 100 m long.
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dimensions that can be measured into order to analyse drumlin morphometry.
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Lateral moraine (linear)
A ridge of moraine along the edge of the valley floor
Exposed rock on the valley side is weathered and fragments fall down on to the edge of the glacier. This is then carried along the valley and deposited when the ice melts. Parallel to ice flow.
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Medial moraine (linear)
A ridge of moraine down the middle of the valley floor
When two valley glaciers converge, two lateral moraines combine to form a medial moraine. Material is carried and deposited when melting occurs. Parallel to ice flow.
35
Terminal or end moraine (linear)
A ridge of moraine extending across the valley at the furthest point the glacier reached
Advancing ice carries moraine forward and deposits it at the point of maximum advance when it retreats. The up-valley (ice contact) side is generally steeper than the other side as the advancing ice rose over the debris. Transverse to ice flow.
36
Recessional moraine (linear)
A series of ridges running across the valley behind the terminal moraine
Each recessional moraine, and there may be many, represents a still-stand during ice retreat. They are good indicators of the cycle of advance and retreat that many glaciers experience. Transverse to ice flow.
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Push moraine (linear)
A ridge of moraine with stones tilted upwards
Any morainic material at the glacier snout will be pushed forward during advance. The faster the velocity of advance, the steeper the angle of tilt or stones. Transverse to ice flow.
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Hummocky or disintegration moraine (non-linear)
Chaotic jumble of till mounds
Originally considered a product of ice stagnation and dropped from a debris-rich glacier, now associated with active glacial retreat. Limited orientation.
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Ice-marginal moraines
- These glacial depositional landforms are complex to interpret as they are frequently interspersed with features of fluvio-glacial deposition.
- Moreover, as glaciers expand and retreat, or still-stand, they often rework older glacial deposits into new forms, adding to the complexity of depositional landforms.
- As well as creating distinctive landscapes in both lowland areas such as eastern Denmark or in the floors of glaciated valleys, glacial depositional landforms are particularly useful in helping glaciologists to understand not only the extent of ice cover but also which direction the ice came from (its provenance).
- In some cases it is the orientation of the feature and in others it is the contrasting up-glacier and down-glacier shape or the actual debris that yields the clues.
- The overall geography of the assemblage of features is also very important, in particular behind and in front of any terminal morainic ridges, as these mark glacial snouts or ice sheet edges.
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A quick guide to provenance
