Erosion surfaces | Geomorphology | Principle Of Geography

 

Geomorphology

Principle Of Geography

Geography Complete Study Material

(Paper - I)

Erosion Surfaces

          The almost plain topographic surfaces having undulating ground surface and remnant low reliefs caused by dynamic wheels of denudational processes and cutting across geological formations and structures are generally called erosion or planation surfaces. Erosion surfaces form significant elements of landscape of given region and provide prominent clues for the reconstruction of denudation chronology (erosional and depositional history) of that region. R.J. Small has commented that 'it is perhaps unfortunate that by constant usage geomorphologists and have given the term 'erosion surface' a very limited and specialised meaning. It is, in fact, almost universally used to describe only flat or near - flat erosinal plains, formed very close to base-level and resulting form cycles of erosion that have reached well beyond the stage of youth and in many instances into the stage of old age. Thus, peneplains, panplains and plaines of marine erosion are all erosion surfaces in the accepted sense of the term. Besides, each plains, cryoplains (by periglacial processes) etc. are also major erosion surfaces. Besides, there are some minor erosion surfaces viz. valley side benches (terraces), river terraces, marine flats, marine terraces, raised beaches etc.

Pediments and Pediplains 

                       The concept of a peneplain (the word meaning ―almost a plain) emerged from W.M. Davis‘cyclic view of landscape evolution. As rivers and hillslopes reduced relief through the phases of youth, maturity, and old age, explained Davis, the eventual result was a plain of extremely low relief. This plain could only change very slowly since potential energy for fluvial action was greatly reduced. Valleys are thought to be V-shaped in youth, flat bottomed in maturity, after lateral erosion has become dominant, and to possess very shallow features of extensive plains in old age, after lateral erosion has removed all hills. Young landscapes are characterized by much flat topography of the original uplifted peneplain. Mature landscapes have deeper and wider V-shaped valleys that have consumed much of the interfluves bearing remnants of the original land surface. Old landscapes are characterized by a peneplain, in which the interfluves are reduced to minor undulations.

                     Landscape evolution in deserts is primarily concerned with the formation and extension of pediments. Gently inclined rocky floors close to the mountains at their foot with or without a thin cover of debris, are called pediments. Such rocky floors form through the erosion of mountain front through a combination of lateral erosion by streams and sheet flooding. Erosion starts along the steep margins of the landmass or the steep sides of the tectonically controlled steep incision features over the landmass. Once, pediments are formed with a steep wash slope followed by cliff or free face above it, the steep wash slope and free face retreat backwards. This method of erosion is termed as parallel retreat of slopes through backwasting. So, through parallel retreat of slopes, the pediments extend backwards at the expense of mountain front, and gradually, the mountain gets reduced leaving an inselberg which is a remnant of the mountain. That’s how the high relief in desert areas is reduced to low featureless plains called pediplains.