2A.7 processes of erosion and erosional landforms Flashcards
Abrasion
The sandpapering effect of ice as it grinds over and scours the landscape.
Quarrying / plucking
Basal meltwater freezes around part of the bedrock at the base of the glacier. Any loosened rock fragments are plucked away as the glacier moves forward.
Crushing
The pressure of the weight of the glacier on the underlying bedrock.
Basal melting
Melting at the base of the glacier due to a reduction in the pressure melting point.
Subaerial freeze thaw
When water freezes in the cracks and joints of rock, it expands. This exerts stresses within the rock, enlarging cracks and pores. Eventually, large chunks of rock break away. Caused by the weather in the atmosphere.
Mass movement
The movement of material downslope as a result of gravity
Micro-scale landforms
Small scale landforms up to 1m long
Meso-scale landforms
Medium-scale landform (ranging from 10m to 1km in length) that are found within macro features (e.g. glacial troughs). Have been created mostly by erosion but also can be created by meltwater erosion
Macro-scale landforms
Large-scale landforms (around 1km or greater in size) that form the major elements of a glaciated landscape. Have been created by a combination of glacial processes over time
Subglacial environments
Below a glacier or ice sheet
Marginal environments
At the sides or end of a glacier or ice sheet.
Proglacial environments
In front of, at, or immediately beyond the margin of a glacier or ice sheet.
Periglacial environments
At the edges of ice sheets and glaciers.
Areas where sediment / other depositional material may lie:
- supraglacial
- englacial
- subglacial
movement of sediment
- aeolian
- fluvial
- glacial
Striations - Inc. scale, environment and definition
ENVIRONMENT: subglacial
SCALE: micro
DEFINITION: The remaining trapped debris and larger rocks, create long grooves, called glacial striations, as they flow over the bedrock
These striations indicate the direction of ice flow
Chatter Marks - Inc. scale, environment and definition
ENVIRONMENT: subglacial
SCALE: micro
DEFINITION: They occur mainly on hard, brittle rocks such as granite and are formed under a glacier by the pressure and impact of boulders moved along by irregular rolling or sliding
Corrie/cirque/cwm - Inc. scale, environment and definition
ENVIRONMENT: subglacial
SCALE: macro
DEFINITION: Corrie, cwm and cirques are all the same feature and are deep, pre-glacial hollows of accumulated snow and ice
In Wales corries are called cwms and in France they are called cirques and in scotland corrie
Found at the apex of a glacial valley, on the coldest aspect of the mountain, with the greatest accumulation of snow and ice
As the accumulated ice begins to flow; basal/rotational sliding along with plucking and abrasion, hollows the mountain into a bowl-shapen (amptheatre / armchair)
Debris is pushed to the edges of the corrie, which acts as a dam (corrie or rock lip) to the accumulating snow
As the ice thickens within the hollow, it flows over the corrie lip and downhill as a glacier
Plucking, abrasion and freeze-thaw weathering, steepen the back wall of the corrie, into the familiar armchair shaped landform
Examples include Helvellyn Corrie in the Lake District and Cwm Idwal in Eryri National Park (Snowdonia)
constrained by topology
Corrie, tarn or cirque lakes form when the ice within the corrie melts
Because of the corrie lip at the bottom end, the meltwater is held in place and a circular body of water is formed
Examples include Red Tarn, Helvellyn in the Lake District and Cadair Idris in Eryri National Park (Snowdonia)
Aretes / Pyrimidal peaks - Inc. scale, environment and definition
ENVIRONMENT: subglacial
SCALE: macro
DEFINITION:Arêtes are knife-edge, steep-sided ridges
Formed when two corries cut back into the mountainside
As each corrie glacier erode either side of the ridge, the edges become steeper and the ridge narrower
This gives the arête it’s a jagged profile
Examples include Crib Goch in Eryri National Park, and Striding Edge in Lake District England
a prymidal peak is a three-sided, pointed mountain peak
Formed when three or more back-to-back corrie glaciers carve away at the top of a mountain
This creates a sharply pointed mountain summit
Examples include Yr Wyddfa (Snowdon) in Wales and Buachaille Etive Mòr, Glencoe, Scotland
Glacial through / U-shaped valley/ ribbon lakes - Inc. scale, environment and definition
ENVIRONMENT: subglacial
SCALE: macro
DEFINITION: Glacial troughs are steep-sided valleys with a flat floor
They start as V-shaped river valleys but due to the size and weight of the glacial ice it changes to a U shape as the glacier erodes the sides and bottom making the valley deeper and wider
Examples are found all over the UK, but Nant Ffrancon and Nant Gwynant in Eryri National Park are good examples
At the end of the last Ice Age, many coastal troughs (glacial) flooded to form fjords (Norway) or sea lochs (Scotland)
Ribbon lakes - As a glacier flows it travels over hard and softer rock
Softer rock is less resistant to erosion, so a glacier will carve a deeper trough over this type of rock
When the glacier has melted, water collects in these deeper areas
This creates a long, thin lake called a ribbon lake
Examples include Lake Windermere in the Lake District and Llyn Ogwen in Eryri National Park
The areas of harder rock left behind are called rock steps
Hanging valley / Truncated spurs - Inc. scale, environment and definition
ENVIRONMENT: subglacial
SCALE: meso
DEFINITION:Truncated spurs are past interlocking spur edges of past river action that have been cut-off forming cliff-like edges on the valley side
Found between hanging valleys and are an inverted ‘V’ shape
Formed when past ridges/spurs are cut off by the lower valley glacier as it moves past
An example is Nant Ffrancon Valley in Eryri National Park
Hanging valleys are small tributary glaciers found ‘hanging’ above the main valley floor
When melting occurs, there are waterfalls onto the valley floor
An example is Cwm Dyli in Eryri National Park
Scouring
Scouring is the ability of the ice mass to erode large areas of pre-existing landscape
Occurs when the glacier overspills its containing valley
Ice sheet scouring produces the following landscapes:
Crag and tail - Inc. scale, environment and definition
ENVIRONMENT: subglacial
SCALE: macro
DEFINITION: tapered ridge of glacial deposits, extended to one side, found in both upland and lowland areas
Formed when a ‘crag’, a large section of hard, resistant bedrock e.g. volcanic rock, protects a section of softer, unconsolidated material, the ‘tail’
The advancing ice sheet is forced to go around the upstream or **stoss **section of resistant rock (crag), leading to melting and refreezing around loose rocks and boulders, which are plucked as the ice mass moves forward
The **leeward **or downstream side of less resistant material is protected directly behind the crag
Velocity and pressure is reduced and deposition occurs
As the ice mass continues, abrasion of the softer rock occurs and the tail is drawn and smoothed by abrasion with the plucked crag debris
Example is Castle Rock (crag) and Royal Mile in Edinburgh (tail)
Roche moutonees - Inc. scale, environment and definition
ENVIRONMENT: subglacial
SCALE: meso
DEFINITION: (sheep’s back)
resistant, bare mass of rock, on the valley floor, that has been sculpted by flowing ice
Reversed crag and tail, with plucking occurring on the leeward side and smoothing on the upstream, stoss side
The upstream or stoss side of the outcrop, is smoothed due to abrasion by the glacier
The moving ice leads to localised pressure melting
This eases basal sliding and increases erosion over the rock, creating striations across the top of the rock
On the leeward or downstream side, the pressure reduction refreezes the meltwater
This bonds the base of the glacier to the outcrop
As the glacier continues to flow, loose rocks/boulders are plucked out and a jagged, steep surface is left behind
Knock and Lochan - Inc. scale, environment and definition
ENVIRONMENT: subglacial
SCALE: macro
DEFINITION: scoured rugged, lowland features
Consist of ‘knocks’, small rounded hills with no vegetation, and ‘lochs’ of small lakes
Created by the intense erosion of the surface by an ice sheet
The knocks are resistant bedrock hills with the lochs being formed by over deepening hollows
Examples are found in the Outer Hebrides, Scotland
Differential geology
This is the differences in geology that contribute to the landforms that are generated by processes in glaciated landscapes
It is the orientation, structure, and pattern of the bedrock that has a major impact on the scale and type of erosional landforms
For example:
It is the pattern and orientation (direction) of resistant to less resistant rock that creates ‘knock and lochans’, rather than physical ice processes
With a crag a tail formation, it is the sheer size of resistant rock that forms the steep, upstream stoss which protects the soft, leeward rock by reducing velocity and pressure of the ice mass as it passes over
A sloping tail forms as the protection deteriorates with distance
Rock flour
Ice sliding across the bedrock, grind the debris into a fine, but gritty powder called rock flour
Rock flour polishes the surface of the bedrock to a smooth finish called glacial polish
Difference between youthful, mature and aged glacial environments
Youthful
This marks the beginning of erosional landforms
The shaping and hollowing of a corrie by ice
The beginnings of aretes and horns
Mature
Corries are well-formed and begin to meet
The glacial valley takes on its ribbon-shaped with a regular, stepped graded contour
Hanging valleys are visible
The valley floor begins to deepen and takes on the shape of a trough
Aged
‘U’-shaped valley is clearly defined
Development of the outwash plain, including features of drumlins, eskers, kettle holes, etc.
Corries converge, mountain summit heights decrease and their peaks become rounded
Scouring
The motion of glacier ice, thanks to its “arming” by moraine boulders, has long been known to carry out large-scale erosion of the preexisting landscape
