Introduction to Agents of Gradation (3 pages long)
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AGENTS OF GRADATION.
Gradation is the process that changes the surface features of the earth through agents such as gravity, running water, waves, wind, and glaciers. Gradation includes erosion, transportation and deposition of earth materials. Gradational process can be divided into two general types:
a) Degradation, which refers to the wearing down of the land by erosion and transportation, and
b) Aggradation, which refers to the accumulation of eroded materials through deposition.
The combined effect of degradaton and aggradation is to smooth out the land surface by eroding places of high elevation and filling in areas of lower elevation to a point where erosional forces can no longer operate. This point is known as base level.
Rivers generally originate from the mountains or hills. Rivers are the most powerful agent of gradation. It has three basic functions, namely erosional, depositional and transportation. The river has three stages from its origin till it reaches the ocean. The upper course, the middle course and the lower course. The river has different amount of water in different seasons it depending upon its sources. Some rivers have water in them through out the year and some rivers do not water in them during the dry season. These rivers are called perennial and non-perennial respectably.
Glaciers are large mass of slow moving ice. They occur in two places 1) Polar Regions and 2) High altitudes above the permanent snowline. The permanent snow line decreases from the equator towards the poles. The glaciers are of two types continental glacier and the mountain glacier. The continental glacier occurs in the polar region. The mountain glacier occurs in the high altitudes above the permanent snowline.
Mountain Glaciers: -
Mountain glaciers are found in high altitudes such as the Himalayas and the Alps. In such regions ice and snow accumulate and due to its pressure glaciers are formed. These glaciers move down the valley causing erosion, transportation and deposition. The features made by a glacier are cirques, U-shaped valley and moraines.
Cirques: -
Cirque is an armchair-shaped hollow scooped out by ice. It is one of the most common feature of glaciated uplands. Cirques are hollows with steep sides and a relatively flat basin-shaped floor. The ice accumulates in such basins. Cirques generally enlarge owing to weathering of rocks along the edges. Cirques are observed in mountain region which were once covered with glaciers.
U-shaped Valley: -
U-shaped valley is a huge broad valley carved out by a glacier. Over thousands of years , glaciers carve out huge troughs which deepen, widen and straighten the existing river valley. Unlike V-shaped river valleys they form a distinctive wide U-shape.
Moraine: -
Moraines are piles of boulders, rocks, pebbles and clay. When a glacier melts away in the warmer lower slopes all the material carried by it gets deposited. Moraine are also formed when the glacier stops temporarily.
Wind is a stream of air moving from one place to another. Air on the surface of the ground always moves from areas of high pressure to area of low pressure. This movement carries the sand particles and the dust particles with it. These particles are carried hundreds of kilometers away and are deposited there. The main features made by wind are sand dune, mushroom rock and loess.
Sand Dune: -
Sand dune is a mound of wind-blown sand in the desert or a cost. In deserts such as Thar and Sahara there are vast areas of sand called Sand Sea. When the wind blows the loose dry sand is carried with it. When there is an obstruction or the speed of the wind decreases or when there is rain the sand particles get deposited. Resulting in the formation of Sand dunes. There are many kinds of Sand dunes. Dune type depend on the amount sand available, the variability of wind direction, and the amount of vegetation cover. Some types of dunes are star dunes, crescent dunes, sword dune and parabolic dune.
Mushroom rock: -
The Mushroom rock is also known as the Encrusted rock. The Mushroom rock resembles the shape of a mushroom. It has a narrow trunk and a broad top portion. The bottom portion is thinner because the wind carries more sand particles in the lower side than on the higher.
Loess: -
Loess are fine yellow sediments probably deposited by the wind. Wind action is important not only in deserts, but where ever light dry material is exposed such as in the coastal regions. Dust particles are carried are transported by winds over hundreds of kilometers and they get deposited in humid regions when rainfall occurs. These dust particles accumulate over an extensive area covering the whole surface. Loess deposits are found in China, central Europe and Siberia.
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