P2 Flashcards
Glacial Weathering and Erosion:
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Physical weathering processes dominate e.g. freeze-thaw.
Líttle biological or chemical weathering is evident in glacial environments as both work best at high temperatures.
Processes of Glacial Erosion:
Abrasion:
Plucking:
Rotational Movement:
Abrasion:
- angular material is embedded in the glacier as it rubs against the valley sides and floor, gradually wearing it away.
- The scratching and scraping action may leave striations (elongated grooves) as well as generally smooth, gently sloping landforms.
Plucking:
- occurs where the ice freezes onto rock outcrops, after which ice movement pulls away masses of rock.
- The pressure of overlying ice generated by frictional contact may cause partial melting of ice on the upstream side of obstructions, and then the removal of pressure on the downside causes regelation (refreezing) helping attach material.
- It is generally previously loosened material that is removed.
- Jagged-features landscapes are formed.
Rotational Movement:
is the downhill movement of ice pivoting around a central point of rotation.
Corries are created by this rotational scouring of depressions and this process is most effective where temperatures fluctuate around OC (as with plucking) to allow freeze thaw to operate, particularly in areas of jointed rocks where weaknesses may be exploited.
Corries:
Description:
- Armchair Shaped Hollow
- Found in glaciated upland areas
- Steep back wall - can lead to Arête or Pyramidal peak formation
- Over deepened basin with rock lip made of moraine
- Often contains a small lake called a Tarn
- Usually faces between North and East in the Northern hemisphere
- Evidence of frost shattering on back wall in the form of scree.
- May be striations too
- Examples are Easedale tarn at Easedale or Red Tarn at Hellvellyn
Corries:
Formation:
Pre-glacially:
- Climate worsens and becomes subarctic, below OC with constant frost and heavy snow.
- Snow collects in hollows of NE facing slopes. The depth of snow increases over winter.
- In summer it melts and meltwater seeps into ‘nooks and crannies’ leading to freeze thaw action. The rubble created is removed by solifluction or meltwater streams.
- This process is repeated many times and the hollow soon deepens. (process is nivation)
Corries
- Corries form when snow continues to build up in a depression or nivation hollow, eventually compacting to form a glacier.
- The glacier becomes trapped within the hollow, meaning the only way it can move is through rotational slip.
- The back wall is eroded through plucking and frost shattering, and the hollow is deepened through rotational abrasion.
- Water can fill corries to make tarns (lakes).
- As corries are eroded rocks, they last a long time and are minimally affected by erosion.
- This is why corries have lasted thousands of years.
Corries: formations Postglacially:
Ice melts leaving a hollow containing a small lake.
• There is still evidence of winter freeze-thaw from the scree on the back wall.
There may be fluvial erosion due to the meltwater streams flowing from the tarn.
Arêtes:
A knife-edged ridge formed between two corries (when the two steep back walls meet). If three meet, they create a point called a pyramidal peak.
Similar to corries, arêtes last a long time.
* E.G. Striding edge above Red Tarn in the Lake District (850m)
* E.G. the Minarets - Sierra Nevada, California (3735m high, 338m prominence)
Pyramidal Peaks:
• Where three or more corries erode back toward each other or 3-4 arêtes radiating from a central point.
• A very steep, sharp mountain peak.
It often has near-vertical sides.
• E.G the Matterhorn in Zermatt, Italy/Switzerland. (4478m)
• E.G. Mont Blanc in Chamonix, Eastern France (4810m)
Glacial Troughs:
Description:
- A u-shaped valley formed by a glacier bulldozing and eroding through a river (v-shaped) valley.
- The glacier has enough force to erode away a river’s interlocking spurs.
- This leaves smooth but steep truncated spurs on the valley sides and a wide, flat valley floor.
- The river that originally flowed through the valley will continue to flow, giving it the name misfit stream due to its small size in comparison to the surroundings.
- U shaped valleys last for a long amount of time.
- e.g nant ffrancon
Examples of Glacial Troughs:
Nant Ffrancon in Wales
In long profile, there is a basin and step formation;
The basin is made up of less resistant rock
• The step is more resistant rock
where Ice came from:of Glacial Troughs:
- A highland field - tongues of ice spilled out down valleys
- Corries - ice flowed over the lip down steep sides into river valleys
- Ice accumulated at the head of the valley itself and steepened the back wall forming a trough end
- Ice action steepened, widened and straightened pre-existing river valleys changing long and cross profiles.
Cross profile of glacial troughs
- the valley is overdeepened by the sheer mass of ice and erosion on base.
- It is straightened by removal of interlocking spurs.
- The ice thickness and velocity are greatest over the central part of the valley floor so erosion is greatest and deepest in the center.
Long profile: of glacial troughs
- A distinct trough end due to accumulation of ice at head of valley
- Irregular long profile due to extending and compressing flow
- Basins on the valley floor associated with greater erosion caused by compressing flow where a tributary glacier provides additional ice so the enlarged glacier can achieve greater downcutting (or an area deeply weathered prior to glaciations, or a band or less resistant rock or due to constriction of valley walls)
Often find a ribbon lake in the basin - glacial flow is compressional, the ice gains a rotational movement causing enhanced abrasion and deepening of the rock basin.
- Steps on the valley floor marked by the position of a more resistant band are the zone of extending flow - the ice is stretched, thins and so erodes less.
erosional processes of Glacial Troughs:
Plucking -
- ice loosens, picks up and removes masses of rock varying in size.
- Most effective in areas with well jointed rocks or permeable rocks where water produced by pressure melting percolates into cracks in the bedrock then freezes and shatters the rock.
Abrasion -
- using the material entrained in the ice from plucking, the glacier will smooth the truncated spurs and will overdeepen the valley floor.
Pressure Release -
- when a certain thickness of the bedrock is removed it is replaced by ice which is 1/3 the density of rock and so causes the uppermost layers of the rock to separate along the sheet joints.
- This weakening in the upper bedrock allows other erosive processes to operate rapidly.
Roche Moutonnees:
- A mound of rock shaped by a glacier flowing over it and eroding it.
- The glacier would be moving right (stoss side) to left (lee side) in the picture.
- The glacier hits an obstacle that is too large and hard to pluck, it must move over it.
- The glacier hitting the obstacle increases friction and pressure, therefore increasing
melting as the lower ice can reach the pressure melting point. - This meltwater allows the glacier to slide over the rock, and smaller rocks will abrade the stoss side
- When the glacier reaches the top of the obstacle, friction and pressure drop ●
- Meltwater refreezes
- Frozen rocks are plucked from the lee side
- These landforms last a long time as they are made of rock.
- E.g. Cairngorms in Scotland
Crag and Tail:
- Consists of a larger mass of resistant rock or crag and gently sloping tail of less resistant rock and/or sediment on one side.
- E.G. in Edinburgh. Castle sits on hard basaltic rock, the Royal Mile runs down softer sedimentary rocks.
Striations:
When glaciers move across exposures of rock, angular debris embedded within the ice may leave parallel scratches or grooves called striations.
Glacial Deposition:
Subdivided into:
- Till - all material deposited directly by the ice, largely unsorted in nature.
- Fluvioglacial material - sediments deposited by meltwater streams. These usually have been sorted with coarse material nearer the original glacier snout and finer particles carried further away by meltwaters.
Till (boulder clay):
• Unsorted mixture of rocks, clays and sands.
• Once carried as supraglacial debris and later deposited to form moraines, it was deposited during ice movement or glacial retreat.
• There is little rounding of debris and it tends to remain subangular in form.
Erratics:
- Fragments of glacial debris which range in size from pebbles to large boulders.
- They have been carried by glacier ice before being deposited.
- E.G. Big Rock in Alberta, Canada (16500 tonnes).
- They are said to be ex situ.
- They are usually distinguishable by their lithology - they are likely to be of a different rock type from the underlying rock and by their attitude, they do not lie in the same manner as the local strata.
Lateral Moraine:
Description:
- With the eventual melting or retreat of glacier, such accumulations of moraine appear as hummocky, linear embankments running along the valley sides parallel to the ice movement.
- They are unsorted, unstratified, angular material in clay
- Examples: Cwm Idwal in Wales
Lateral Moraine:
Formation
- Derived from loose weathered rock that moves down valley sides and is gradually fed onto the glacier below.
- The combination of this load supply and the movement of the glacier creates lines of debris that gradually become part of the moving body of ice.
- When the ice has melted/retreated material falls onto the valley sides and floor. It may slump or erode after this.
Medial Moraine:
Description:
- Often made up of only one metre or so of coarse stony debris.
- The material is largely supraglacial and central and so rarely gives rise to landforms in post glacial times.
- They are unsorted, unstratified, angular material in clay
- They mark the confluence of two glaciers vallies and lie parallel to the direction of flow.
- They are less defined towards the snout.
- Generally between one and 30m high and one and 20km long.
- Between 50-100m wide.
- Examples: Kaskawesh in Yukon, USA. 1km wide moraine, narrowing to 60m.
Medial Moraine: Formation:
• Formed when two glaciers meet.
• The two lateral moraines that converge subsequently flow as one in the middle of the enlarged glacier (explain lateral moraine formation).
Terminal Moraine (End Moraine): Description:
- Mark the maximum advance of a glacier and the boundary between glacial and proglacial landscapes.
- From a plan view, they are typically arc-shaped.
- Consist of a ridge of material (or several mounds/hummocky hills) stretching across a valley.
- Elongated at right angles to the direction of ice advance
- Often steep-sided, particularly the ice contact side (20-30 degrees, distal slope is 10-20 degrees), and reach heights of 50-60 meters.
- There is only one per glacier.
- They are typically 30-60 meters high.
- They are perpendicular to the valley floor and glacier direction.
- Sometimes it creates a dam creating a proglacial ribbon lake.
- They are often crescent shaped, moulded to the form of the snout.
- They are formed from unsorted, unstratified, angular material in clay.
- Examples: Cape Cod - North East USA.
Cromer Ridge, Norfolk - 8km wide, 9 meters high. - The Franz Loset glacier in NZ. Highest recorded at 430 meters.
Terminal Moraine (End Moraine): formation
- When ice melts and the material it has been carrying is deposited. This is why they contain a range of unsorted material, from clay to large boulders.
- Occurs when the glacier has a positive mass balance causing it to advance - boulder clay is pushed along the glacier snout and forms a pile.
- The glacier retreats due to a now negative mass balance, leaving the pile of unsorted, unstratified and angular debris as a ridge.
- The height is determined by how long the ice remains stationary at the maximum point.
- It may be weathered by freeze thaw action or slump in post glacial times.
Recessional Moraine:
Description:
- Also forms parallel to the glacier snout (right angles to ice flow)
- Proglacial areas may have more than one recessional moraine
- They will be positioned between the snout and terminal moraine
- Sediment it consists of tends to be unsorted, unstratified, angular and in clay
- Smaller and less steep than terminal moraine as snout is less steep when retreating
- Can be from 0.5-100km long, 20-500m wide and 3-50m high
Recessional Moraine: Formation:
• As the glacier retreats, it is possible for a series of moraines to be formed along the valley, marking points where the retreat halted for some time - this is recessional moraine.
• The halt is usually due to a climate change
• (+formation exactly as terminal moraine above)
Push Moraine:
Description:
- Can be recognised by the orientation of individual pieces of rock which have been pushed upwards from their original horizontal position.
- Also form parallel to the ice margin (right angles to ice flow).
- Small hummocky ridges due to short-lived winter re-advance.
- They often include glacio-fluvial sediment of more rounded clasts in small ridges.
- Consist of unsorted, unstratified, angular material in clay
- Examples:
- Athabasca Glacier, Canada - 0.7-2m high (seasonal shift)
- In front of the Axel Heiberg Island glacier
In the Canadian high arctic
Push Moraine: Formation:
• If the climate cools for some time, leading to a glacial advance, previously deposited moraine may be shunted up into a mound forming push moraine.
• It is not uncommon for them to be destroyed from one winter to the next due to summer melting.
• They can also disappear due to an increase in snow input into the system, causing the snout to advance.
What does terminal moraine tell us about ice movement?
• Where the glacier was stationary and for how long
• The maximum extent of ice
• Where the ice came from through material
What does recessional moraine tell us about ice movement?
• That the glacier was retreating but stationary for some periods - climatic indications
• The time it was still for - from height
• Where the ice came from through material
Drumlins:
Description:
- Smooth oval shaped hills
- Can reach 50m in height, more than 1km in length and up to 0.5km in width
- Average is approx 400 by 40
- Have a steep stoss end and a tapered lee end
- Elongated in the direction of ice advance - stoss is the upstream end
- Made up of glacial debris/till - unstratified, unsorted, angular and in clay
- Often found in ‘swarms’ and so have a ‘basket of eggs’ topography
- Found in lowland plains such as central lowlands of Scotland - lower end of glacial vallies
- Degree of elongation is usually between 2.5:1 and 4:1
- Examples: Hellifield, The Ribble Valley - the largest is Rise Bigg Hill
- New York state has over 10,000
- Much of Northern Ireland
Drumlin Formation:
- The most wide-spread theory is that Drumlins are formed when ice is melting and overloaded in a lowland area.
- The glacier does not need much to encourage deposition.
- When an obstacle such as a rock outcrop is passed material is deposited on the upstream end of the obstacle and the movement of ice and erosive power of it helps to create a smoothened lee end.
- The fluvial theory, as proposed mainly by Shaw and Cox, attributes drumlin formation either to catastrophic flooding due to the release of meltwater that is believed to have accumulated beneath melting ice sheets, or to floods caused by regional uplift due to tectonic movements.
