Plains of Fluvial Erosion. The plains formed by river erosion have a lot of variation because of two factors -
(i) The stage of erosional development
(ii) The initial slope and the structure of basal rocks.
(a) The Dissected Plains of the Youth - The broad water-divides, large valleys distantly situated from one another, etc., are the main characteristics of this type of plains. The tributary valleys undergo limited development but there is a crowding of such valleys close to the main valley. It gives a dissected look to the plain. The drainage is dendritic in nature.
The divides have a great economic value. Agriculture, colonies, transport and urban development are encouraged by the peculiarities of landforms. Big river valleys encourage agriculture but the floors of smaller river valleys are not suitable for it. The Colorado, Kansas, Nebraska, etc., east of the Rockies, belong to this category of plains.
(b) The Dissected Plains of Maturity. These are the plains where the development of tributary valleys has dissected the high lands. Areas of gentle slope are very much limited in these plains. Plain areas are available more in the valleys. Gradually the plain area of the divides goes on decreasing. Finally, the divides are reduced to small ridges. Fields, buildings, roads, villages, etc., develop in the broad areas of valleys. The plains of this stage are found in North Missouri, Southern Iowa, Eastern Nebraska, etc.
(c) Plains of Old Age - Peneplains and Panplains are usually mentioned in this type of plains.
(i) Peneplains - The slope of the plains is so gentle that it can barely transport the debris. The tributary streams have comparatively steep slopes. In fact peneplain is a low land of faint relief. Its water divides are broad and a bit convex. They separate the wide valleys from one another. Peneplain should take 60 to 70 million years for evolution.
(ii) Panplains - Some problems about peneplains have been discussed. Peneplanation does not solve the problems. In order to solve them, Krickmay used a new concept of Panplain in 1933.
In order to solve these problems Crickmay described another process. He observes that the river valleys stop the work of deepening their valleys, in the last stage of maturity in the fluvial cycle of erosion and start lateral erosion. This process is known as lateral planation. The flood plains broaden and coalesce into one another as a result of the erosion of divides. The plains thus formed are called by Crickmay as panplains. The existence of erosional river terraces confirms the views of Crickmay. Meanders also bring out the importance of the effect of lateral erosion. This process steepens the slope of the divides. The remaining divides appear as monadnocks.
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