Erosion

Erosion essentially involves the removal of material which has often already been weathered. The key agents of erosion are rivers, moving ice (glaciation) and the sea.

Rivers

A river’s ability to erode depends mainly upon:

• The speed of the river (increases with steepness of slope)
• The volume of water in the river (increased water means increased ability to carry material)

A river transports its material :

• In suspension (carried in the water)
• In solution (dissolved in the water)
• As bedload (rolled along the bed of the river)

A river deposits its material :

• When it slows down (usually when it comes to flatter land)
• When its volume decreases (e.g. in times of drought)
• When it reaches the sea.

The three stages of a rivers course are:

1. Upper Course

2. Middle Course
3. Lower Course

The Upper Course
Here, the river is mainly concerned with erosion. The river valley has a V-shape.

The Middle Course
Here the river is concerned with both erosion and deposition. It is eroding its sides and now the valley has a broader flatter profile. The river twists and turns (meanders) with its broader valley.

The Lower Course
At this stage, the river is flowing on a very gentle slope, therefore it is flowing more slowly. This lack of speed means the river is unable to carry all its load.

Rivers and mankind

Advantages of rivers include:

• Can provide hydro-electric power
• Can be used for transport
• Can be used for recreational purposes
• Can be used for irrigation purposes.

Disadvantages include:

• Can cause flooding which may lead to loss of life, damage to crops and damage to property.

Moving Ice (Glaciation)

• Key features of glacial erosion
• Key features of glacial deposition
• Glaciers and Mankind.

How glaciers form

Glaciers form is cold conditions when winter snows do not melt in the summer. Over time this snow accumulates and, due to the overlying weight, it turns to ice. In time this ice becomes a glacier and moves downslope due to gravity.

Glacial erosion

Glaciers erode by

(a) plucking — here the glacier pauses and melts a little. When it re-freezes and moves on it plucks material from the floor and sides of the valley.
(b) abrasion — in this case, the glacier scrapes the bed and sides of the valley with the boulders it has embedded in it.

Key features of erosion are:

• cirque (corrie) — the birthplace of a glacier
• glaciated (U-shaped) valley
• ribbon lake (found on the glacial valley floor)
  In general, erosion takes place in the upland and deposition on the lowlands.

Glacial deposition
Glaciers normally deposit material when they reach lower levels and begin to melt. This material is usually unsorted and remains as it was when first picked up by the glacier. Typical examples of this are:

• Boulder clay (glacial till) which gets spread over large areas.
• Drumlins, which are oval-shaped hills formed by glacial deposition.

However, sometimes the features deposited on lowlands are associated with melt-water flowing from the front of the glacier. These features differ in that the material is sorted and rounded as it has been affected by running water. An esker (a long winding ridge of sand and gravel laid down on the bed of a stream flowing with the melting glacier) would be the most significant of these features.

Glaciers and mankind

Advantages of glaciations include:

• Many glaciated eskers and valleys provide excellent routeways
• Glacial lakes can sometimes be used as reservoirs or to help in hydro-electric power production
• Glaciated areas often provide very attractive scenery
• Sands and gravel from glacial deposits can be used in the construction industry.

Disadvantages include:

• Some of the soil laid down by glaciers can be of very poor quality and badly drained
• Glaciers sometimes remove the soil cover from an area and leave just bare rock.

The Sea (Coastal Landforms)

As with rivers and ice, the sea erodes, transports and deposits material, thereby changing the shape of coast lines.

Sea Erosion is caused by waves. The strength of the waves depends on the strength of the wind and the fetch (the distance the wave travels). Waves reaching the West of Ireland will have a greater fetch than those on the east coast.

Waves erode mainly by:

• Hydraulic action (power of the water hitting the shore)
• Abrasion (stones etc. bashed against the shore by the waves)
• Solution (when certain rocks e.g. chalk are dissolved by the water)

Key features of erosion would include bays, cliffs and caves.

Destructive waves is the term used for waves which erode the coastline.
Constructive waves are those which lead to the formation of new features along the coast. This usually happens when the coastline is shallow. Here, waves are forced to slow down and they then deposit material which may have been removed elsewhere.

Sea deposition

A beach is formed when constructive waves deposit material along the shore. This is most likely to happen in a bay.
Most other features of deposition are associated with longshore drift. This is where material (sand etc) is carried along the shore by currents. When this material meets an obstruction such as a headland, it gets deposited.
Key features of longshore drift include:

• spit (a ridge of sand projecting out from the shore)
• tombolo (where a spit links an island to the mainland)
• lagoon (a lake formed when a spit claces off a bay)

Coasts and mankind

The sea is of huge benefit to man for food, transport etc.
Many cities are situated on the coast and mankind has often tried to control how the sea shapes the coastline.
Mankind tries to control coasts in order to prevent flooding ; reclaim land from the sea; stop the erosion of amenities such as beaches.