Rejuvenation and Polycyclic Landforms

Sunday, July 19, 2020

Rejuvenation is defined as the sudden acceleration in the cycle of erosion caused by varied factors like rise in land or downfall in outlet.
It leads to modification in the land form of valley coarse. Rejuvenation can be divided into 3 parts :-

1. Dynamic Rejuvenation 

The rejuvenation caused by dynamic aspects:
I . Upliftment of landmass. 
II. Downfall in outlet. 
III. Tilting of land.

2. Eustatic Rejuvenation 

This is caused due to changes in sea level due to:
I Glaciation lead to decrease in sea level. 
II. Diastrophic Factors (subsidence of sea floor or rise in coastal lands) 

3. Static Rejuvenation

This is caused without any vertical change as follows:
I.  Increase in river volume due to High rainfall or high melt water.
II. Increase in river volume due to use main river capture. 
III. Decrease in river load.

Polycyclic Landforms

These are the topographical expressions of rejuvenation. These are of mainly to four types: 

1. Topographical Discordance 

This can be defined as decrease in uniformity of valley of rivers Basic cally this denotes creation older topographic forms above and younger topographic forms below the older one. 

Example of Polycylic valleys and paired river terraces
  • Topographical discordance leads to formation of Valley in Valley topography' or paired terraces.
  • Basically this happens due to rise in the land of coarse of river. Which lead increase in down cutting of the river valley leading to creation of a paired terraces on both sides due to fall in river.
  • Repeated upliftment of land form cause creation of new paired terraces after each uplift of land.
Example 1 - Damodar River near Rajaroppa in Hazaribagh district, Jharkhand . 
Example 2 - Almost all Uttarakhand rivers have 3 paired terraces reflecting 3 phases of upliftment.

2. Uplifted Peneplain

This type of landform develops due to upliftment in regional areas. Not in the source region of the river from where it originates. These are basically formed due to continuous cycle of erosion caused due to continuous rejuvenation leads to formation of Patlands Mesas. 
Example:-  Patlands of Ranchi Plateau. 
Patlands of Ranchi Plateau

3. Incised Meanders 

As the name suggest these are special kind of Polycylic landform formed due to vertical incision of hill valley by rivers due t fall in the sea level which increases the carrying capacity of rivers leading to vertical incision of valley floor of river in form of meanders. 
Incised Meanders

4. Knick Point 

This is one of the most important feature of Polycyclic land-forms. It represents breaks in slope in the longitudinal profile of river .
This is formed due to rejuvenation which lead to fall in the longitudinal of river hence, this is also called as 'head of rejuvenation' which registers gradual recession. 
Graphical presentation of Nick Points
Example:-  Hundru Falls (76.67m) (Subarnarekha River) 
Nick Point leads ot formation of waterfalls which is also known as 'Nick Point falls' or 'Nick falls'.