GENERAL GEOLOGY

Geological work of fluvial or arid zone.

geological work of fluvial

The term stream includes the channelized flow of any size from the smallest brook to a very large river like Ganga, although the term river and stream are used synonymously.

The geological work of stream is to “erode” the valley “transport” the material thus eroded and “deposit” the same in the lower reaches at favorable sites.

  (a)Stream erosion

  • Chemical action
  • Hydraulic action
  • Abrasion
  • Attrition

 (b)Stream transportation

  • Dissolved load
  • Suspended load
  • Bed load

 (c)Stream deposition

Features of stream erosion:-  the main types erosional features develop geological work of stream are

  • Pot holes:-pot holes are the circular and deep holes cut into solid rocks by sand grains and pebbles’, swirling in fast eddies. They are commonly found on the channel floor.                                                             
  • Waterfalls:-the falling of stream water from a height is called a waterfall. Waterfalls occur at place where the stream profile makes a vertical drop. Such a situation is usually found where gently inclined erosion resistant beds overlie the nonresistant beds. The softer rock is eroded fast while the harder one offers resistance and forms a ledge at a height, from which the stream water falls down deep into the gorge.                                                         
  • Gorges or canyons:- the erosion is confined to down-cutting of its channels only it gives rise to a deep-cut narrow valley with steep or vertical walls known as gorges or canyons.
  • Escarpments:- these are erosion land forms produced by rives in regions composed of alternating beds of hard and soft rocks. The differential erosion of rock give rise to a steep slope called escarpment.
  • Hogbacks:- these are sharp crested often saw tooth ridge formed of the upturned edge of a resistant rock layer of sandstone, limestone or lava . in the cases the beds dip at a high angle roughly in excess of 45o so that the dip slope becomes almost as steep as the escarpment.
  • Cuestas :-these erosional landform are developed on resistant strata having low to moderate dip. This has the shape of an asymmetric low-ridge or hill belt with a steep scarp on one side a gentle slope on the other.
    • Mesa and Butte:-in regions of horizontal strata in which isolated portion of land is capped by a hard erosion-resistant bed the erosional landform produced will have an isolated tableland area with steep sides commonly known as Mesa.

    Butte with continued erosion of the sides a mesa is reduced to a smaller flat-topped hill known as butte.

    • Monadnock:-sometime some mounds or small hillocks of hard rock persists on the pen plains and are known as monad nocks.

     

    Features found in arid regions:-most common feature is found arid zones are:-

    • Alluvial fans and cones:- on descending to the plains from the hills the velocity of a river and the carry capacity are reduced. At this point the river-sheds a large amount of load which assume a fan or conical shape.

    In the lower parts many fans join laterally to produce a bajada or piedmont plain.

    • Pediment:- it is a plain of eroded bed rock in an arid region developed between mountain and basin areas. Pediments converge to form pediplains.
    • Playas:-in deserts that consist of basins enclosed by mountain ranges the drainage is toward the centre of the basin from all margins when there is sufficient water this plain is coved by a broad shallow lake called playa.
    • Wadies:- these are channels formed during rains in desert or arid regions.
    • Inselbergs:- these are isolated mound rising above the general level of a pediment. These are equivalents to monadnocks humid regions.

     

    Features of stream depositional:- the main types depositional features develop geological work of stream are

    • Alluvial fans:- the alluvial material which flows down from mountains accumulates at foot hills where the stream enters a plain. The deposition occurs due to abrupt change in the gradient of river valley. Such deposits spread out in the shape of flat fans and are called alluvial fans.
    • Flood plains:-during floods a river overflows its bank and submerges the adjacent low lying areas where deposition of alluvial material takes place. A wide belt of alluvial plain formed in this way on either side of a stream is called flood plain
    • Natural levees:- natural levees are the low ridges which are formed on both sides of river channel by the accumulation of sediments. in this way successive floods build up ridges on both sides of a river channel which are called natural levees.
    • Meander, point bars, oxbow lake:-the symmetrical S- shaped loops found in the course of a river are called meander. This develops in mature rivers. Mature rivers are those which have cut down to an approximately graded profile. In such rivers side cutting becomes very prominent which results in the development menders.
      • Point bar:- in meandering rivers sediment deposits occur as point bars. The point bars are the crescent shaped deposits which occur at inside bends of a river channel.
      • Oxbow lake:-thus a meander loop gets abandoned. This phenomenon is called a cut off. The abandoned channel thus constitutes a loop-shaped lake known as an oxbow lake.
      • Deltas:-deltas are deposits built at the mouth of stream. The deltas are usually triangular in shape with their apex pointed upstream. When a stream enters an ocean or lake the currents of the flowing water dissipate quickly. This result in the deposition of the series of sedimentary layers which make up the delta .the material of most deltas is well sorted and many deltas are uniformly graded. The structure of a delta deposit. it consist of three set of beds.

                 (1) top-set beds    (2) fore set beds   (3) bottom set bed

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