Exogenetic or Exogenous or External forces are those that work on the earth’s surface from outside and result in the formation of new landforms. These forces include the elements of the earth’s atmosphere, other agents of denudation, and the formation of the force of gravity. They are essentially processes of destruction.
Agents of exogenetic forces
- running water
- ground water
- sea waves and shore currents
- periglacial processes
- work of wind
- weathering and mass movements
- Mass wasting
- Gradation Process: Gradational processes are continuously engaged in removing the vertical irregularities created by endogenous processes through denudational (weathering and erosion) and depositional activities.
- Agents of Gradational Process: erosional, transportation, and depositional works of running water or rivers, glacier, wind, groundwater, and works of ocean waves and shore currents.
- Gradation = Erosion + Transportation + Deposition.
- Associated landforms
- Flood plains
- Formation of delta
- Moraine – A moraine is a material left behind by a moving glacier.
- Stalagmite – Stalagmites are formed by the slow dropping of water containing the mineral lime.
Weathering is the process that breaks down the rocks into smaller fragments by static agents of weather.
- Physical Weathering: Physical weathering is the process that breaks rocks apart without changing their chemical composition caused by weathering changes the temperature of rocks.
- Chemical Weathering: When chemicals in rain and moving water react with rocks and minerals to change or weaken them in some way and break down the rock into parts is known as chemical weathering.
Mass wasting is the process of movement of soil or rock mass from one region to another mainly due to gravity. It occurs slowly or gradually (eg. Soil creep, Mudflow, rock slides, etc.)
Works on River
Definition of River
Definition-A River is an important part of the Hydrological cycle, in which water gets transported from one point to another over the earth’s surface along a channel.
According to Jackie Smith, ‘A river is a large stream of fresh water flowing downhill within a channel to enter another river, lake or sea’.
A river is a natural flowing freshwater watercourse, flowing toward an ocean, sea, lake, or another river.
The river performs three works
- River water is the main agent of erosion, in this activity rivers wear away the river bed (land), sediments, rocks, and other materials, and its banks are known as erosion.
- In other words, Bank erosion is the wearing away of the banks of a stream or river.
- Four major actions take place in this process.
- Hydraulic action
- Corrasion or Abrasion
- Hydraulic Action – Quarrying and removing of the loose bedrocks by the force of the running water are known as hydraulic action.
- Corrasion or Abrasion: When erosion takes place by a collision between the flowing rock, fragments, and the static rocks of the wall of the channel is known as Corrasion.
- Two types of Corrasion – Vertical Corrasion, and Lateral Corrasion.
- Attrition – When the flowing rock fragments strike against one another and wear down, it is known as attrition.
- Solution – It is the action in which the river dissolves the soluble rocks and minerals.
- Transportation: Streams transport eroded materials, from one place to another. This activity is known as transportation.
- This process is done in four ways
- Traction: When the heavier and large rock fragments are forced by the flow of water to roll on the floor of the river channel. These fragments can roll, slip and bump. This is called traction.
- Saltation: The fragments of the rocks move downstream by jumping continuously. This process is known as saltation.
- Suspension: When the finest particles are carried by the stream, their weight is reduced by the buoyancy of the water. The reduced weight of the fragments keeps them in suspension.
- Solution: Soluble particles are carried in solution.
- Deposition: In this process, the stream of the river accumulates the transported material is called deposition. Materials deposited by rivers in their channels or over surrounding flood plains are called alluvium.
|Work of River||Upper Course||Middle Course||Lower Course|
Terms used in the River system
- Source: The beginning of a river is called its source.
- Channel: The bed of the river through which the river flows.
- Course: The path of a river from its source to its mouth is known as the course of the river.
- Tributaries: Many smaller rivers having their sources elsewhere, and flowing into the main river are known as Tributaries.
- Distributaries: Near its mouth, a river often divides up into smaller streams. Such streams which go out of the main river are called Distributaries.
- Mouth: The end of the river is its mouth. The river may enter into a lake, a sea, or an ocean.
- Confluence: Confluence is the place where two or more two rivers meet.
- River Basin: The area drained by the main river along with all its tributaries and distributes is known as the River Basin.
- Watershed – On the highland or mountains region multiple distributaries separate from the main river which marks the boundary between areas drained by the two streams is known as a watershed.
Sixth power law of river
If the velocity of river is doubled, the transportation capacity of that river increases by 64 times or 26 = 64 times. This is known as sixth power law of river.
Courses of the river
- From its source to its mouth a river is usually divided into three important sections based on the slope of the land.
- upper course or mountain course
- middle course or plainland course
- lower course or deltaic course.
Works of Glaciers
Definition of Glacier
- The term ‘glacier’ is derived from the French word Glace, meaning ice.
- Glaciers are rivers of Ice.
- According to Richard Flint “A Glacier is a mass of ICE, lying entirely or largely on land which was formed chiefly by compaction and recrystallization of snow and which flows or some time has float”
- A glacier is a moving mass of ice at speeds averaging between 30 to 40 cm and 15 to 18 meters per day.
Bergschrund and Crevasses
Bergschrund is a German word ‘Berg’ means mountain and ‘schrund’ means crack.
On high mountains, cracks are sometimes formed on the glacier due to unequal movement of different parts of solid ice mass. The deep vertical crack which opens up at the head of a glacier, when the ice breaks away from the rock wall behind, is called Bergschrund.
Bergschrund is also called ‘Rimaye’ (French Word).
If there is not ice apron on the rock wall, the gap is called a Randkluft.
A series of cracks that develop on the surface of a glacier is known as Crevasse.
Delta: The velocity of a river is greatly reduced where it meets the sea or the lake. So, the heaviest deposition occurs here. The river channel often gets blocked and the river waters have to find their way to the sea through a number of channels and distributaries that form a network. The Ganga Brahmaputra Delta is the largest delta in the world.
There are three types of deltas
- (i) Bird’s foot Delta – where delta formation is river-dominated and less subject to tides and waves. E.g. The Mississippi Delta,
- (ii) Arcuate Delta – where the delta has a rounded convex outer margin. Eg. The Nile Delta.
- (iii) Cuspate delta – where the material brought down by the river is evenly spread on either side of the channel. It is shaped like a cup. Eg.-Tiber delta in Italy.
Favorable Conditions for delta formation
- (i) The river should have a long course.
- (ii) The river should have many tributaries.
- (iii) The sea at the mouth should be shallow.
- (iv) The Ocean where the river meets should be calm.