Introduction
The surface features of the earth are the result of the action of various geological agents. It is well known that the landforms are the distinclive configurations of the land surface mountains, hills, valleys, plains, and the like. In the evolution of landforms different exogenous as well as endogenous processes are mostly responsible. The configuration of the land surface are the outcome of the joint actions of some geological agents having both constructive as well as destructive effects on the existing surficial features.
The exogenous processes are those which derive their energy from external sources and ultimately from the sun. These processes are mainly caused by gelogical agents such as Blowing wind, Running water, Glacier, Sea-Waves etc. Since these agents originate upon the earth’s surface, they are known as Epigene—geological agents and their activities include processes like gradation, degradation, aggradation and weathering.
In a similar way, a process which originates within the earth’s crust is termed endogenous. The geological agents, which afe associated with these processes and have their origin underneath the surface of the earth are known as ‘Hypogene’ Geological agents. Earthquake, Volcanic eruptions as well as other earth movements are the results of the hypogene processes, Although - they originate within, they do affect the earth’s surface in a spectacular way. Mountains, plateaus and volcanoes are some of the striking features produced by hypogene agents.
Processes Associated With Epigene Geological Agents
Gradation
It is a three fold process; first the surface is decayed and eroded ; secondly the products of this decay and erosion are transported ; finally, they are deposited usually at lower levels. Thus ‘Gradation’ is the process by which the original irregularities of the earth’s surface are removed and a levelled surface is created. All gradation processes are directed by gravity. Thus it is clear that processes of gradation are divisible into two major categories :(i) Degradation.
(ii) Aggradation.
(i) Degradation
It constitutes those precesses by which material from a high relief feature is removed by exogeneous or external processes. It includes activities like weathering, mass wasting and erosion including removal and transportation.
Factors Affecting Degradation
(a) Properties of the materia on which the processes are operating,
(b) The energy available to the geomorphic agents, and
(c) The tools used by the agents.
(ii) Aggradation
It is the process of elevation of low-lying tracts, through deposition of materials, by various epigene geological agents like wind, river, glacier etc.The epigene geological agents are grouped under two headings.
1. On land. Running water, ground water, wind, glacier etc.
2. in oceans. Waves, currents, tides etc.
The activities of the epigene geological agents are divided into three major stages like, Erosion, Transportation and Deposition. These three activities contol the configuration of the tand surface of the earth.
Hypogene processes
The two main endogenous processes are : volcanism and diastrophism. The geomorphic features produced by them provide the setting for the operation of the exogenous processes.
(i) Orogenic and
(ii) Epeirogenic,
While Orogeny refers to mountain building with deformation of the earth’s crust ; epeirogepy refers to regional uplift without marked deformation.
The deformation of the crust is usually manifested in forms of folding, warping, faulting and with the occurrence of phenomena like continental drift, mountain building, volcanism, earthquakes isostasy etc.
Thus the hypogene and epigene-geological agents play their respective roles in shaping the surface features of the globe.
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