NCERT Notes: Geography- Erosional landforms

Subject: Geography
Category: The Fundamentals of Physical Geography 
Topic: Erosional landforms 

NCERT notes on important topics for the UPSC civil services exam. These notes will also be useful for other competitive exams like banking PO, SSC, state civil services exams and so on. This article talks about the Erosional landforms for IAS exam.

 

Valleys
  • Valleys start as small and narrow rills. These rills will progressively develop into long and wide gullies.
  • The gullies will again deepen, widen and lengthen to give rise to valleys.
  • The valley types depend upon the type and structure of rocks in which they form.
  • Depending upon sizes and shapes, several types of valleys like V-shaped valley, gorge, canyon, etc. can be recognized.
  • A gorge is a deep valley with very steep to straight sides.
  • It is almost equal in width at its top as well as its bottom.
  • A canyon is characterized by steep step-like side slopes and might be as deep as a gorge.
  • It is a variant of the gorge.
  • A canyon is wider at its top than at its bottom.
  • It is commonly formed in horizontal bedded sedimentary rocks and gorges form in hard rocks.

  

Potholes
  • Potholes are cylindrical holes drilled into the bed of a river that varies in depth and diameter from a few centimetres to several metres.
  • They are found in the upper course of a river where it has enough potential energy to erode vertically and its flow is turbulent.

 

Plunge Pools
  • A sequence of such depressions ultimately joins and the stream valley gets deepened.
  • At the foot of waterfalls also, large potholes, quite deep and wide, form because of the absolute influence of water and rotation of boulders.
  • These large and deep holes at the base of waterfalls are called plunge pools.
  • These pools also help in the deepening of valleys.

  

Incised or Entrenched Meanders
  • Entrenched meanders are symmetrical and form when the river down cuts quickly. 
  • The speed of the river downcutting gives less opportunity for lateral Thus giving them symmetrical slopes.
  • These are very deep and wide meanders can also be found cut in hard rocks.
  • It is common to find meandering courses over floodplains and delta plains where stream gradients are very gentle.

 

River Terraces
  • River terraces are surfaces marking old valley floor or floodplain levels.
  • They may be bedrock surfaces without any alluvial cover or alluvial terraces consisting of stream deposits.
    • Paired terraces: The river terraces may occur at the similar elevation on either side of the rivers.
    • Unpaired terraces: When a terrace is present only on one side of the stream and with none on the other side or one at quite a different elevation on the other side.
  • The terraces may result due to
    • Change in hydrological regime due to climatic changes.
    • Sea level changes in case of rivers closer to the sea.
    • Receding water after a peak flow.
    • Tectonic uplift of land.

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