Periglacial Processes and Landforms (4)

Landforms due to Frost Action Weathering

Daily and seasonal cycles of heating and cooling in periglacial areas result in the regular alternation of freezing and thawing (frost action) which results in the dominance of mechinal weathering of heavily jointed and porous rocks, breaking them down and forming regolith (loose, angular, rock fragments). These freeze-thaw processes are common in periglacial environments and result in the formation of a number of features:

1. Blockfields (Felsenmeer)

Examples: Glyder Faw, North Wales; Cairngorm Mountains (Scotland)

These occur in fairly falt areas, commonly on mountain tops and plateaus which have been unaffected by glaciation but subject to periglacial conditions. These features are characteristed by an accumulation of large angular boulders on fairly flat surfaces. They are formed in situ by frost action (freeze thaw and frost heave) of jointed rock.

2. Scree Slopes

Examples: Wastwater Screes (shores of Wastwater Lake - Lake District); Cairngorm Mountains, Scotland

These features develop at the base of exposed cliffs / slopes. They form as mechanical weathering detaches angular rock fragments from the cliff face / upper slope. These weathered fragments move down slope as rockfalls under the influence of gravity. Due to the build up of a greater momentum, the largest boulders tend to be at the base of the scree slope as they have been able to travel the furthest. The lower part of the slope rests at an angle of between 25-30o whereas the upper slope which is made up of smaller angular material rests at angles of between 30-38o.

3. Tors

Example: Black Tor, Dartmoor, England

Tors, are resistant rock outcrops standing above the surface of the surroundings. There are a number of theories of how these features form. Whilst deep chemical weathering of rock, during early periods is thought to be partly responsible, having enlarged joints in rocks, it is believed that the formation may also be explained by frost-shattering of jointed rock in extreme freezing / thawing conditions, such as those found in periglacial areas. This frost shattering is believed to have been followed by the removal of the weathered material by solifluction processes, resulting in the more resistant rock being left behind, exposed as a rocky outcrop - i.e. a TOR.

Landforms due to Fluvial Action

Braided Streams

Braided Streams are stream channels which are subdivided into a number of continually shifting smaller channels separated by bar deposits. These form in periglacial areas due to the temporal patterns in discharge that result from the periglacial climate. For much of the year, streams may appear low or empty, due to the frozen conditions and lack of precipitations. However, when snowmelt occurs in the spring, for a few weeks there are very high discharges and in some cases flooding resulting from the underlying impermeable permafrost layer. During times of high discahrge, the streams are able to pick up large amounts of gravel and stones. When temperatures fall once again and freezing conditions and resumed, the large quantities of gravel and boulders transported at the time of high discharge are depoisted in the landscape, where they were being transported. These bars of material cause the subdivision of stream channels and re-distributed / shifted during subsequent periods of high discharge.

Another feature unique to periglacial stream are beaded channel which may form where streams flow over areas of ice wedges. The difference in thermal characteristics of the overlying water and the ice wedge, causes melting of the ice wedges, forming deep pools in the channel bed.

Dry Valleys

These are relict features of periglacial landscapes and provide evidence for the former existence of ground ice in an area and there are numerous examples to bou found in the UK, including the Watlowes Valley (near Malham Cove, Yorkshire) and valleys in the North Downs. These features are deeply cut valleys which are not occupied by a river. They are common on chalk and limestone. They form during periglacial times as the presence of permafrost, prevents downward percolation of water, rendering the underlying bedrock impermeable. This would have resulted in a relatively high drainage density as meltwater during summer thaws was forced to flow over the surface. This would enable the creation of valleys through the processes of gelifluction and meltwater erosion. When temperatures warmed at the beginning of the last ice age, the underlying permafrost melted which meant the bedrock began permeable once again and meltwater rivers disappeared as water was able to percolate into the rock, rather than passing over the surface. Thus leaving behind the dry valleys that are seen today.

Landforms due to Aeolian Action (Wind processes)

Aeolian processes are very effective in periglacial regions due to 3 main reasons:

1. There is a very good supply of sediments from glacial outwash plains - easily moved by saltation due to the strong winds common in periglacial environments.

2. Very cold climate - and therefore there is little vegetation cover so there are no roots to bind materaisl together and therefore the wind can be very effective - also the lack of vegetation cover means that there are no obstacles to break up the wind.

3. Periglacial environments tend to be dry environments (codl deserts) - this means that material is very loose and easily blown.

Winds in periglacial areas do have an erosion impact, as the high velocities of these unobstructed winds cause wind abrasion resulting in grooved and polished rock surfaces. They may also dislodge unconsolidated materials through deflation. The main landscape features associated with aeolian processes are however those created by the transport and deposition of materials by the strong winds.

1. Loess (also known as Limon in France)

This is fine silt (0.004 - 0.61mm diameter) which is carried through suspension in the air. The silt can be carried huge distances and large areas of what are often fertile deposits are found in North Europe and North America. These areas are often provide good farm land - for example East Anglia (Brick Earth deposits - rarely exceeding 2m in depth). Loess deposits are also found in the Paris Basin where they are known as Limon. The deepest loess deposits can be found in China, where depths of 300m+ may be found in some areas.

2. Coversands

These are coarser deposits with larger particles (e.g. sand - 0.061 - 1mm diameter). Due to the larger particle size, they are transported by wind through the process of saltation ('bouncing') and the formation of these deposits indicates the presence of storonger winds. Deposits form cover sands / sand dune features (examples include Netherlands and South Iceland)

Lesson Resources (Handouts and Extra Links)

Suggested Background Reading:

Anderson, D. (2004) Access to Geography: Glacial and Periglacial Environments, Hodder and Stoughton p.106-111

Blockfields in the Cairngorms (**very good)

Scree in the Cairngorms (**very good)

Formation of Tors in the Cairngorms

The Theory of Periglacial Weathering and Tor formation

Periglacial Weathering and Associated Landforms (BGRS) - includes Tor formation

Museum of Illionis - Loess

Loess: Face of the Earth

Periglacial Loess and Dune