While planation surfaces refer mostly to erosional surfaces, the present writer is inclined to include depositional surfaces or plains also under the term. This can be done if we concentrate our attention on the processes, which produce level surfaces or plains. No doubt depositional plains are absolutely level and devoid of rocky projections “but rocky ground or projection may be present on the margins of depositional plains as on the IsoiShern margin of the Indo-Gangetic Plain.
These marginal rocky features “have been under erosion but the plantation there has largely been effected by Alluvial filling. In mountain areas, the basins of deposition may be relatively narrow but alluvial filling causes the formation of level plain surfaces. In such areas as the Dehra Dun or Kangra Valley or Kathmandu basin planation or plain formation is mainly alluvial plantation but it is also erosional plantation because through the course of geological time the surrounding hills are being eroded and while receding, they cause the extension of the plain.
Now alluvial planation is the cause of the formation of the most extensive plains of the world like the Indo-Gangetic Plain, the Irrawaddy plain, the plains of Chinese, Indochinese, Siberian and Russian rivers, the plains of Danube, that Mesopotamia, the Nile Valley, the Amazon basin, The Mississippi plain, etc. All these are the examples of alluvial planation. Some of these are tectonic depressions by alluvium. Some are shallow depressions on rigid shields.