Many physical and chemical processes operate because the earth's surface undergoes alterations. These are known as geomorphic processes. As Penck designated endogenic processes such as epeirogenic (continental building) diastrophic (mountain-building) and volcanism originates from forces within the earth's crust, responsible to build up or restore areas, which have been worn down by the external forces. According to Thornbury these geomorphic processes leave their distinctive imprint upon landforms and each geomorphic process develops its own characteristic assemblage of land forms.
Here we will study exogenic forces, which are operating on earth surface in forming landforms and labeling them to sea level or base land. Over the time of these operation, various processes underplay to give rise to various kind of land forms ranging from simple, compound to multicyclic landscapes.
Geomorphic Evolution. Geomorphic evolution is more complex. It is a rare thing to find landscape assemblages attributed to one geomorphic process. The landforms may be simple, compound, monocyclic or exhumed or resurrected landscapes. Multicyclic landscapes are generally more common than monocyclic and they are either simple or compound. Newly created land surfaces are restricted to uplifted portion of the ocean floor, volcanic cone, lava plain or plateau and beneath the Pleistocene glacial deposits. The older forms are limited to upland remnants or by benches along valley sides above present valley floors. Thus mature or old age topography is likely to have superimposed upon its youthful features as a result of rejuvenation. Rejuvenation is due to interruptions in a cycle and may be caused by epeirogenic uplift, eustatic changes of sea level and by topographic unconformity. Valley in valley, knick points, incised meanders, river and marine terraces and accelerated river piracy are the result of rejuvenation.
Davis considered the interruptions in the fluvial cycle at the penultimate stage on account of the existence of peneplains at various levels on the surface of the earth. Peneplain is a low and gently undulating plain developed at the penultimate stage of a geomorphic cycle. Some of the erosional surfaces are described as peneplain. Davis attributed this to sub aerial denudation while Penck described it as sub aerial planation. There are a few surfaces confused with peneplain such as structural plain. The terms like pediplain and panplain are applied to describe the land forms identical to peneplain of penultimate stage by King and Crickmay.
The geologic and climatic changes during the Pleistocene are all responsible for the complex evolution of land forms. Most of the geomorphic forms are older than Tertiary but not older than Pleistocene. The geomorphic evolution is due to the influence of more than one set of climatic conditions and geomorphic processes. Therefore, polyclimatic landscapes and polycyclic. They are relict land forms, buried exhumed land forms Relict land forms are those, which formed landscape. The remnants of former erosion surfaces in Africa and Australia, relict drainage systems of both positive and negative manifestations (ridges, cementation of river bed deposits and abandoned valleys), and disrupted drainage systems are a few examples of relict land forms. Buried land forms are those buried beneath some type of covermass, commonly marine or terrestrial sediments and have been preserved essentially intact. Buried erosional surfaces, buried valleys, buried submarine canyons, buried paleokarst plain, buried pediplain or peneplain and buried sand bodies are good examples. Exhumed landforms are those, which are formed as surface topographic expressions and then were buried beneath a covermass of some sort and still later were resurrected. The covermass must have been removed to the extent that the formerly buried feature now constitutes a part or whole of present day landscape. Sherman peneplain of Wyoming, Precambrian peneplain of Canada, Prekaroo landscape in S. Africa, Peros depression in New Mexico, Oligocene ridges in Paris basin, are a few examples of exhumed land forms. Most exhumed topographic forms are likely to be composed of the distant bedrock material, which has escaped destruction in the process of exhumation.
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