Question.
Discuss the causes of rejuvenation of the fluvial cycle of erosion, and describe the characteristics of the resultant polycyclic landforms with examples. (UPPSC 2011)
Classify the nature of interruptions in the cycle of erosion. Also, describe their geomorphic expressions. ( UPPSC 2003)
Write explanatory notes on Polycyclic landforms. ( UPPSC 2002)
Answer.
The fluvial erosion cycle is led by the running water geomorphic agents. When the river's erosion rate suddenly increases, we call it rejuvenation of the fluvial erosion cycle. Apart of increase the rate of erosion, it increases the length of the erosion cycle as well as the erosion cycle returns from the mature stage or old age again to the young stage.
Following are the main reasons for the rejuvenation of the fluvial cycle of erosion:
- Vertical elevation of land due to endogenous forces. For example, the formation of Peatlands in Ranchi has rejuvenated the river erosion cycle in the Damodar valley.
- The land also rises vertically due to the release of weight. For example, in a high latitude region such as Alaska, global warming is causing glaciers to melt, causing the land to rise vertically as pressure is released.
- The fall of the base level of the outlet or sea level also leads to the rejuvenation of the river cycle of erosion. The fall in sea level is caused by either global cooling or tectonic depression.
The rejuvenation of the river cycle of erosion increases the time of the erosion cycle and forms polycyclic landforms.
Polycyclic landforms are those landforms that are formed by multiple erosion cycles. For example, the valleys of the Uttarakhand rivers have been formed by three successive uplift processes, so they are examples of polycyclic landforms.
Characteristics of polycyclic landforms:
- There are multi-storeyed valleys or valleys on top of valleys that are formed due to multicyclic erosion. The Uttarakhand river valley is an example of a multi-story valley. As we know, a narrow V-shaped valley is a feature of the young phase of the river cycle and a wide valley is a feature of the old stage of the river erosion cycle. If the valley passes through a multicyclic process of erosion, then a narrow and deep valley is seen below the broad valley.
- presence of peneplain on high ground is also an example of multicyclic erosion. In fact, we know that the peneplain is formed almost at the base level at the end of the erosion cycle. But due to the uplift of the land in the polyclinic erosion cycle, the same peneplain is found on highlands. For example, the Patland of Ranchi is an example of an uplifted plain.
- The presence of older fluvial landforms on top of younger fluvial landforms is also an example of multicyclic erosion. For example, narrow and meandering river channels are an example of a polycyclic landform.
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