The concept of cycle of erosion was formulated by William Morris Davis. According to him, in a cycle of erosion the surface features undergo changes as a result of the processes acting upon them. These changes are systematic and follow each other in a regular sequence. These sequences are termed stages. ‘Davis has recognized four stages.
However, most commonly, only youth, maturity and old-stages are used in the study of topographies produced by different geomorphic agents.
In the initial-stage the land-form is generally even and is raised high above the sea-level, where small and slow changes occur. Most of these features are erosional in origin.
However, vast changes occur by the time the landscape attains, maturity.
In the old stage again landscape evolution slows down.
Different geomorphic agents have their characteristic cycles of erosion. Thus we have fluvial cycle of erosion, glacial cycle of erosion, aeolian cycle, karst cycle and marine cycle of erosion.
The cycle of erosion, operates through the three stages until the initial relief is almost fully reduced and the surface is again levelled. Most of the cycles of erosion do not reach the final stage as some time during their operation either climatic or tectonic disturbances take place. An incomplete or partial cycle results from this. Topography returns to a youthful stage. This phenomenon is known as ‘rejuvenation’ whereby a mature topography becomes young.
The various land-forms and other topographic features associated with the different geomorphic agents owe their origin to the geological actions like erosion, transportation and deposition of the concerned geomorphic agents and are as follows:
1. Geological-action of river:
This phenomenon, which is associated with the geological action of river, is usually known as the fluvial cycle of erosion, or of the normal cycle of erosion.
The erosion caused by the running water is of two types:
(i) Mechanical erosion, and (ii) Chemical erosion.
(i) Mechanical erosion:
It is because of the physical forces associated with the running water and it takes place in four distinct manners like:
(a) Hydraulic action:
Forces inherent in the flow of running water can do a great deal of erosion of the bank and the bed-rock. It is mostly due to surface relief, i.e., gradient.
The materials which are being carried away by the running water acts as tools of destruction, and during their transportation, because of their rubbing against the surface of the bed-rock, they bring about a scraping of the surface. This process of erosion is also known as-‘Corrasion’.
Materials during their transit often collide among themselves and in turn get themselves teared and this is the process, through which big boulders are gradually reduced in size and finally reach the size-grade of sand and silt.
This is because of the presence of the air bubbles which create a whirling action at the time of penetration of water through the existing pores and fissures and the small sand particles along with the air bubbles play a loner role in widening the cavities
Factors Which Help Mechanical Erosion:
1. Hydraulic gradient.
2. Climate which determines precipitation and finally volume and velocity of water.
3. Nature of the bed-rock, whether it is hard or soft; whether the layering or jointing of the bed rock are parallel to the flow of the water or are perpendicular to the same ; whether the bed rocks ,-are igneous, sedimentary or metamorphic rocks.
4. Hardness of the transported materials.
(i) Chemical erosion. It is also known as solution or ‘Corrosion’, during which process the materials get dissolved in the water of the river and are transported in solution.
(1) Dissolving action of water due to the presence of carbon- dioxide.
(2) Solubility of the river bed.
There are two methods of transportation:
(a) Mechanical, and (b) Chemical.
(a) Mechanical transportation:
It takes place in three ways:
(i) Suspension (floating).
(ii) Traction (by creeping and rolling).
(iii) Saltation (through lifts and falls of materials).
These processes are aided with the following factors:
(a) Velocity of the river.
(b) Nature of the river-current.
(c) Density of the rock-material to be transported:
(b) Chemical transportation:
It is through the process of solution, usually in the form of carbonates, sulphates of calcium, sodium, potassium, magnesium etc.
It is the last geological action by the river, whereby materials transported get accumulated in an appropriate site where the following factors play major roles:
(i) Decrease in velocity of the transporting medium.
(ii) Decrease in slope.
(iii) Decrease in volume.
(iv) Change in channels.
(v) Chemical precipitation.
The main features of the ‘fluvial cycle of erosion’ are as follows:
The cycle begins on a recently uplifted landmass. It is initiated through the drainage system working on it.
1. During the ‘Initial Stage’ a river is formed and it involves- some of the characteristic processes like:
(i) Channel deepening due to bed-scouring,
(ii) Pot-hole drilling (by whirling current action).
(iii) Tributaries are fast developed.
(iv)Head ward erosion is maximum.
(v) Waterfalls, gorges and canyons are formed.