Basic Concepts of Geomorphology and Typical-Landforms

Basic Concepts of Geomorphology and Typical-Landforms | Types, Cycle, Factors

Geomorphology: Geomorphology is the systematic study of land-forms and the interpretation of them as records of past-history.

Certain agents function to bring about gradation and forms. These may be called geomorphic agents, such as river, wind, result changes in land glacier and sea-waves. These geomorphic agents basically serves three functions :-

1. Erosion 
2. Transportation, and
3. Deposition. 

(1) Erosion: The breaking-down or disintegration of rocks due to the physical forces associated with the natural agencies, followed by removal of the dislodged rock-fragments and particles, and the sum total of the process is defined, as erosion .

(2) Transportation: It is the process by which the weathered materials are removed from the site of their formation by various geomorphic agents.

(3) Deposition: It is the process, whereby accumulation of the transported materials, which results from the loss of the trans porting capacity of the geomorphic, agents. This happens when the velocity is reduced or the material being transported exceeds the capacity for transporting. It may also happen because of chemical precipitation of the materials transported in solution.

Each geomorphic agent functions in a way peculiar to it and as a result produces erosional and depositional land-forms which are characteristic of that agent. It is therefore possible to recognize the origin of land-forms although the processes by which they were formed may have ceased to operate; it is because 'geomorphic processes leave their imprint upon land-forms'.

The structure of the crust acts as a control in the formation of land-forms. Folds, faults, unconformities, differences in the hardness and susceptibility of rock units to decomposition and disintegration

Provide a frame-work within which the geomorphic agents function. structure of the underlying rock. Thus the combination of a parti. The resulting physiographic features are always influenced by the cular structure being eroded primarily by one geomorphic agent will result in a predictable land-form. 

Cycle of erosion:  

The concept of cycle of erosion was for 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'.

1. Initial stage
2. Youth stage
3. Stage of maturity
4. Old-stage.

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 cycle 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. associated with the geological action of river is usually known as This phenomenon, which is the fluvial cycle of erosion, or of the normal cycle of erosion.

Erosion. The erosion caused by the running water is of two Types : 

1. Mechanical erosion,
2. Chemical erosion.

1. Mechanical erosion. It is because of the physical forces associated with the running water and it takes place in four distinct manners like :
1. 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.
2. Abrasion. 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'
3. Attrition. 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.
4. Cavitation. 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 sard particles along with the air bubbles play a major 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's 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.

ii. 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.

 Factors 

1. Dissolving action of water due to the presence of carbon dioxide.
2. Solubility of the river bed.

 Transportation. There are two methods of transportation :  

(a) Mechanical,
(b) Chemical. 

(a) Mechanical transportation. It takes place in three ways :

(i) Suspension (floating).

(ii) Traction (by creeping and rolling). 

(ii) 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.

Deposition. 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) Headward erosion is maximum.

 (v) Waterfalls, gorges and canyons are formed.

 2. Youth-stage. River system is to some extent established. River capture or river piracy takes place, in which case, one river reaches the course of another one and if the course of the second river is diverted because of the greater gradient of the earlier river, this phenomenon will be known as river-piracy. The point where the course of the second river is diverted is known as elbow of capture'. The captured river is called the 'misfit-river' and the abandoned part of the channel, through which no water flows is the 'wind-gap'.

 In this stage, there is conspicuous formation of V-shaped valleys.

3. Mature-stage. through lateral-cutting. valleys develop strikingly. In this stage there is maximum erosion The landmass is fully dissected; ridges and As a result of heavy erosion and deposition this stage is. comprised of a large number of landforms such as hog-backs, cuestas, plateau tables, meanders, ox-bow lakes, terraces, alluvial. fans etc.

 4. Old-stage. Down-cutting stops at the base level of erosion, which is the mean sea level produced inland. Frequently the river is over-flooded and builds up flood-plains on both sides. The initial irregular surface has become practically flat at this stage. The river is mostly engaged in depositing and does little of erosion and transportation. The topography is characterised by a set of distributional features which comprise delta, distributaries etc.

 Features and typical-landforms associated with an ideal fluvial cycle in a humid region :

 (i) Strath. When the width of the valley is greater than the width of the river, the valley is called a strath.

 (ii) Knick-point. During the process of regrading, there is a more or less marked change of slope at the point of intersection of newly graded profile with the older one, and it is known as the knick-point.

(iii) Bad-land. Due to pronounced erosion by running water the areas are intricately traversed by gullies, which mostly develop on argillaceous rocks.

 (iv) Escarpment. It is a steep slope resulted from differential. weathering of rocks. 

(v) Cuesta. In a region of sub-horizontal beds, a gentle slope is developed along the gentle dips of strata, such a landscape is known as cuesta. 

(vi) Mesa. An isolated table-land area with steep sides. 

(vii) Butte. With continued erosion of the sides a mesa is reduced to a smaller flat-topped hill, known as 'butte'. 

(viii) Hog-back. It is a cuesta, in which the dip slope and scrap slope are both approximately 45°.

 (ix) Braided-river. In this case distributaries or branches develop in large number in a region of flatness. They are commonly formed where the amount of load is excessive and the stream is incapable of transporting all of it. The coarser fractions of the load tend to form islands in the centre of the stream, which breaks up paths around them. Thus a braided river is formed.

 (x) Peneplains. The peneplains are formed in the old stages of rivers and are the plain lands produced by the river.

(xi) Monadnock. Sometimes some mounds or small hillocks of hard rocks persist on the peneplains and   are known as monadnocks.

 (xii) Natural levee. On the flood-plains, long depositional ridges extending parallel to the river are found, which are known  as Natural-levees. 

Features Found in Arid Regions

 (i) Alluvial fans and cones. On descending to the plains from the hills the velocity of a river   and the carrying capacity are reduced. At this point the river-sheds a large amount of lead which assumes a fan or conical shape. from the hills the velocity of a river and the carrying capacity which assumes a fan or conical shape.In the lower parts, many fans join laterally to produce a 'bajada or piedmont plain.'..

 (ii) Pediment. It is a plain of eroded bed rock in an arid region developed between mountain and basin areas. Pediments converge to form pediplains.

 (iii) Bornhardts. If pediments occur on both sides of a mountain range, they gradually converge. Eventually the range is reduced to a very broad dome with the slope equal to that of the pediments on either side of the crest that consists of a narrow ridge of small scattered domes. These residual hills are known as Bornhardts.

(iv) Playas. In deserts that consists of basins enclosed by mountain ranges the drainage is towards the centre of the basin from all margins When there is sufficient water, this plain is covered by a broad shallow lake called playa.

 (v) Wadies. These are channels formed during rains in desert or arid regions.

 (vi) Inselbergs. These are isolated mounds rising above the general level of a pediment. These are equivalents to monadnocks in humid regions. 

Depositional Features 

(vii) Deltas. These are submerged equivalents of alluvial Fars.

 (viii) Sloughs. Depressions on the flood-plains of meandering rivers, which are excavated during floods due to the tendency of the overflowing water to follow a short-course. Important Terminologies Associated with the Actions of this Geomorphic Agent :

 (a) Effluent and influent river. In case of effluent rivers, the regional water-table lies near the earth's surface, as a result the river or stream is fed; whereas in case of influent rivers, the water-table is located at a great depth, a part of runoff is scheduled to percolate downwards.

 (b) River pattern : 

(i) Antecedant. Rivers existing before the surface relief was impressed upon the area.

 (ii) Consequent. The flow of the river occurs as a consequence to the existing surface relief. 

(iii) Subsequent. The river which joins the consequent river arising later as erosion proceeds.

 (iv) Insequent. It displays no reason for its particular course, such as that upon homogeneous terrain.

 (v) Obsequent. Here the river drains in the opposite direction to the original consequent-river. 

(vi) Resequent. It drains in the same direction as the original consequent, but at a lower topographical level. 

 (vii) Super-imposed. At some places. old rocks may be covered under a sheet of new deposits. Any river developed on such an area will follow the surface relief of the overlying cover and will not have any relation with the older rocks lying below. Gradual erosion removes the overlying cover and the river flows on the older rocks below. Here, the river is said to be super-imposed on the older rocks below.

 (c) Drainage pattern. The joining of the tributaries with the master stream produces a pattern termed drainage pattern. The common drainage patterns are :

 (i) Dendritic. Which is characterised irregular branching of tributary streams in a similar pattern 'as that of a tree's branches.

 (ii) Parallel pattern. Develops on steep slopes where the tributaries and the master stream flow parallel to each other.

 (iii) Trellis pattern. It develops in a topography created on a folded structure of synclines and anticlines, faults or joints etc.

(iv) Radial pattern. It consists of drainage lines radiating from a central part as on a dome.