Topic 2 EQ3 Flashcards
Processes of glacial erosion
Plucking, abrasion, crushing, fluvio-glacial erosion processes
What is the glaciers debris load derived from?
- weathering of the valley sides and floor which adds more debris to the glacier
- also from material falling onto the glacier from mass movements such as avalanches
What is glacial plucking?
- occurs where rocks are well jointed and weakened and meltwater is present
- the meltwater penetrates into joints and freezes onto the rock
- as the ice moves, it exerts an immense pulling force which will then be plucked from its position
What is Glacial abrasion?
- material plucked from the bedrock is frozen into the glacial ice and as the glacier moves downslope this material rubs against the valley sides.
- course rock debris will scrape the rock surface forming striations and fine material smoothies and polishes the rock
Where is the rate of abrasion highest?
Under thick fast moving ice with large amounts of coarse angular basal debris where the debris is hard rock and the bedrock is resistant
What may reduce rates of abrasion ?
High basal water pressure and a large amount of fine rock debris may reduce abrasion by protecting the bedrock
What is glacial crushing?
- Direct fracturing of weak bedrock by the weight of ice above it.
- the bedrock must first be weakened either by intense freeze thaw weathering or repeated glacier advance and retreat causing dilation
What is dilation?
Rock fracture due to the removal of overlying glacier weight
What does bedrock crushing produce?
Large , angular blocks of rock
Consequence of basal ice melting
Produce large volumes of meltwater which causes fluvial erosion processes such as abrasion , hydraulic action, attrition and corrosion.
What is attrition ?
Refers to the collision of rock fragments in the meltwater which breaks them into smaller more rounded fragments.
Why does subglacial meltwater travel fast under high pressure?
Because it is confined beneath the glacier . The force of the water may dislodge and remove rock debris through the process of hydraulic action.
Description of cirque
An amphitheatre -shaped depression in a mountainside with a steep back wall and a rock lip
Process of formation for a cirque
A large rounded hollow on a mountain side is eroded and deepened by plucking and abrasion due to the rotational ice movement of a cirque glacier
Description of an Arête
A narrow knife-edged ridge between two cirques
Process of formation for an Arête
Plucking and abrasion on the back wall of two cirques on a mountainside mean they erode backwards toward on another creating a narrow ridge
Description of a pyramidal peak
A pointed mountain peak with three or more cirques
Process of formation for pyramidal peak
Erosional processes within nearby cirques mean they erode backwards towards each other, creating a sharp, pointed mountain summit . Plucking is important
Description of a glacial trough
A u-shaped valley with steep sides and a wide, flat floor
Formation of a glacial trough
A v-shaped river valley is widened and deepened as a result of powerful plucking and abrasion by a valley glacier which goes through the landscape rather than round it
Description of a truncated spur
A steep rock valley side where spurs of a river valley used to interlock before glaciation
Formation of a truncated spur
Valley glaciers are less flexible than rivers and remove the ends of interlocking spurs by plucking and abrasion as they move down the river valley.
Description of a hanging valley
A small tributary V-shaped or small U-shaped valley high above the main glacial floor, often with a waterfall as the river flows over the edge
Formation of a hanging valley
Powerful thicker glacial ice in the main glacial trough eroded vertically downward more rapidly than thinner ice or rivers in tributary valleys . The floors of the tributary valleys are left high above the main valley floor.
What is a ribbon lake ?
A long, narrow lake along the floor of a glacial trough
Formation of a ribbon lake
Areas of increased plucking and abrasion by the valley glacier deepen part of the valley floor as a result of glaciers or weaker rocks.
Description of a Roche moutonee
A mass of bare rock on the valley floor with a smooth stoss (up - valley side) and a steep jagged lee (down valley side)
Formation of a Roche moutonée
A more resistant rock outcrop causes ice movement by creep and regelation around it. As the ice slides over the rock, it scours and smoothed the stoss while refreezing on the lee causing plucking
Description of a knock and lochan
A lowland area with alternating small rock hills (knock) and hollows often containing small lakes (lochan)
Formation of a knock and lochan
Scouring at the base of a glacier excavates areas of weaker rock forming hollows that fill with meltwater and precipitation following ice retreat
Description of a crag and tail
A very large mass of hard rock forms a steep stoss with a gently sloping tail of deposited material.
Formation of a crag and tail
A large mass of hard rock is resistant to ice scouring and creates a steep stoss. Reduced glacier velocity on the lee protects softer rock and allows deposition but the sheltering effect diminishes with distance creating a sloping tail
When do glaciers deposit their load?
When velocity is reduced or they become overloaded with debris or when ablation increases
What is till?
Material deposited directly underneath the ice
What are erratics?
Till transported in areas with different geology
Why are a erratics valuable indicators of the direction of past ice movement ?
The rock types can be traced back to their source
Example of erratics used as an indicator for past ice movement
Micro-granite erratics from the Scottish Island of Ailisa Craig are found in Pembrokeshire, south wales - transported 400km
What is Morraine?
The collective term for rock carried and deposited by a valley a glacier
Description of lateral moraine
A ridge of till deposited along the valley sides
Formation of lateral moraine
Debris from freeze-thaw weathering of valley sides falls onto a glacier and is then transported and deposited at the edge of the glacier when it melts
Description of medial moraine
A ridge of till deposited in the middle of the valley - parallel to the valley sides
What is the formation of medial moraine?
The confluence of two tributary glaciers, the lateral moraines join to form a medial moraine
Description of terminal moraine
A high ridge of till extending across a valley at right angles to the valley sides
Formation of terminal moraine
Debris is deposited at the maximum extent of a glacier
Description of recessional moraine
A lower ridge of till across the valley, parallel to the terminal moraine
Formation of recessional moraine
Debris is deposited during interruptions in the retreat of glacier ice, when the glacier remains stationary long enough for a ridge of material to build up.
Description of a drumlin
Smooth, elongated mounds of till, with a long axis parallel to the direction of ice movement and with a steep stoss and gentle lee
What are a large number of drumlins in an area called?
Drumlin swarms and form a basket of egg topography
What is the formation of drumlin ?
Controversy over the origins. A popular view is that they are formed by deposition when glacier ice becomes overloaded with debris when exiting an upland area. The deposits are streamlined and shaped by the moving ice.
Description of till plains
A large, relatively flat plain or undulating landscape of till in a lowland area
Formation of till plains
When a sheet of ice retreats, large amounts of material are deposited over a sizeable area due to melting.
Description of lodgement till
Deposits of angular rock fragments in a fine matrix -unstratified (not layered) , unsorted (mixture of clast sizes) and contain erratics
Formation of lodgement till
Till is deposited by actively moving ice forming landforms such as drumlins. It is lodged or pressed into the valley floor beneath the glacier.
Description of ablation till
Deposits of angular rock fragments in a fine matrix - unstratified , unsorted and containing erratics
Formation of ablation till
Till is deposited by melting ice from stationary or retreating glaciers, forming landforms such as terminal and recessional moraines.
How can corries be used to reconstruct former ice movement ?
In Britain, Corries are mostly oriented between North-West through to the North-East. Corrie orientation indicates the direction of the ice movement .
How can till fabric analysis be used to identify the direction of ice movement?
As a glacier moves, it turns the larger till rock fragments to point in the direction of the ice movement
How can glacial troughs be used to show ice extent ?
On some u-shaped valley sides, it is possible identify a trim line . Below this evidence of abrasion such as striations and polished rock surfaces . The trim line indicates the extent and height to which the trough was filled with ice.
How can Roche Mountonee be used to reconstruct former ice movement ?
- Stoss always points upslope
- Lee always points downslope
- Striations are parallel to ice movement
How can crag and tails be used to reconstruct past ice movement ?
The tail of a crag and tail is the lee side which always points downslope
How can ground moraine be used to reconstruct former ice extent?
Ice extent can be determined by studying the distribution of ground moraine and mapping the position. The greater the area of ground moraine , the greater the glacier.
How can terminal Moraine be used to reconstruct former ice extent?
Shows the maximum extent to where the till was at.
How can drumlins be used to reconstruct former ice movement ?
By identifying the stoss and lee, it is possible identify ice direction . Ice moves from stoss to lee
How can erratics be used to reconstruct former ice movement ?
They give clues to establish the direction of previous ice movement . By working out the source of the erratic , the origin of the ice can be established.
Issue of using till fabric analysis to reconstruct ice movement
Requires samples to be dug from the lip of a Corrie
Issue of using striations to reconstruct ice movement
Striations can be mistaken with chatter marks which are perpendicular to ice flow
Issue of using erosional and depositional landforms to identify ice movement
Can be difficult to identify these landforms in the field because they can be modified and confused with other glaciated landscapes . For example later glaciations such as the Devensian glacier which happened 11,000-12,000 years agi
Issue of using drumlins to reconstruct former ice movement
Many drumlins are altered or destroyed by human activity through farming, construction and urbanisation which reduces the value of using drumlins. Eg Glasgow is built on a drumlin field and glacial deposits on the valley floor has been disturbed by the development of transport routes
Factors that create lowland depositional landscapes
- ice retreat
- moving ice
- melting ice
