Useful notes on the Evaluation of Penck’s Views

Penck was a German. He wrote in difficult phrase that reads awkwardly even in the original German. His main work has been translated into English which made his work obscure to most English speaking geomorphologists.

Penck’s views could not get wide acceptance, and were attacked by eminent American and British geomorphologists. Penck’s contention was that slope form and gradient are determined primarily by rates of river erosion.

Other factors like rock type, structure, vegetation, weathering and transportational processes were considered as subsidiary.

Penck’s concept of parallel retreat of slope and continued crustal movements was rejected by the American geomorphologists. However, his ideas about slope replacement and parallel slope retreat are now assessed as important and of much significance.

It is to be noted that there are some similarities between the work of Davis and Penck. Both presented models of slope and landscape evolution, although Penck made no use of the cycle concept of Davis.

Both of them followed the deductive approach, making assumptions and then inferring logically how landforms change with time. Penck’s Endrumpf surely is suggestive of Davis’s Peneplain.

Perhaps the contrasting views of these eminent geomorphologists were due to the fact that they hailed from different environments.

Penck came from recent fold mountains in Europe and worked in the Andes in South America, while Davis hailed from the heavily eroded old mountains of New England.

However, Davis and Penck both added new dimension to the subject of geomorphology. It cannot be denied that Penck presented an altogether new viewpoint and suggested a new line of investigation and research.

It is undoubtedly true that no single model or theory of landscape evolution is applicable to all environments.

Rejuvenation (Interruption in the Cycle) – According to modern views expressed by a number of geomorphologists, in reality true peneplains do not exist, since all erosional plains have more relief than the concept of the peneplains permits.

There are a large number of erosion surfaces that were once flat, but at later stage they are thoroughly dissected as a consequence of uplift.

When the cycle of erosion is interrupted at any stage due to uplift, the base level is lowered, and the rivers despite being graded, start vertical down-cutting with more vigour.

Then fresh waterfalls and rapids are created as a consequence of valley deepening. Under these conditions, the landscape and its rivers are said to be rejuvenated.

If further uplift occurs when the landscape and its rivers have reached the stage of maturity or old age with the consequent formation of large stream meanders, those meanders become entrenched due to the rapid incision of the rejuvenated streams.

At this stage the lateral erosion by the streams stops, and instead, the rejuvenated streams start vertical erosion. Where a previously mature valley is affected by rejuvenation, the incision of the rejuvenated streams produces a “valley-in-a-valley, which is very common in mountainous regions.

When uplift occurs in pulses separated by such periods, valleys may once again attain maturity. However, during the succeeding periods of uplift rivers may get rejuvenated further.

It is worthwhile to remember that during the Pleistocene there was lowering of the sea level which resulted in the rejuvenation of all rivers reaching the sea. Of course, it resulted from continental glaciations when the frozen water accumulated on the land.

At a later stage, during the warmer phase the subsequent deglaciation raised the sea level or base level so that the rejuve­nated valleys were filled with sediment.

Because of all these changes in the base level, broad, flat, aggradational flood plains were produced above buried former channels that were cut far below the sea level.

Thus, sea level changes, minor tectonic movements and climatic changes very often bring about a certain amount of rejuvenation in mature valleys with consequent deepening of such valleys. However, the old valley floor still exists in strips along the edges of the rejuvenated valley.

Old valley floors are called stream terraces or river terrace. It may be noted that incised meanders are also produced as a result of the rejuvenation of mature or old age landscape as well as streams. Sometimes paired terraces are produced, if the new valley is formed in the middle of the old valley.

Spurs and benches are also the product of rejuvenation. These landforms are produced due to the work of tributaries rather than that of the main stream.

These features are commonly found in mountainous and plateau regions. Hanging valleys as well as waterfalls and rapids are most common feature in recently uplifted mountains. The above mentioned topographic features are good evidence of rejuvenation.