Cycle of erosion

The geographic cycle or cycle of erosion is an idealized model that explains the development of relief in landscapes. The model starts with the erosion that follows uplift of land above a base level and ends eventually in the formation of a peneplain. Landscapes that bear evidence of more than one cycle of erosion are termed "polycyclical". The cycle of erosion and some of its associated concepts have, despite popularity, been a subject of much criticism.

William Morris Davis, the originator of the model, divided it into stages whose transition is gradual. The model begins with an uplifted or to-be-uplifted landscape. Then Davis defined a youthful stage where river incision is the dominant process shaping the landscape. During the youthful stage height differences between uplands and valley bottoms increase rapidly. The youthful stage is followed by a mature stage where height differences between valley bottoms and uplands are greatest. Beginning in the mature stage slope decrease becomes a more important phenomenon and uplands lose height more rapidly than rivers incise, effectively diminishing relief. In the very latest stage erosion has acted so long that the landscape – despite original height – is reduced into a rolling lowland. This landscape of low relief is called a peneplain and may contain residual heights standing out from the general level. The peneplain can be uplifted starting a second erosion cycle.

Davis acknowledged that a full cycle was a special case and that initial uplift was not necessarily rapid nor followed by a prolonged period of quiescence. However, as Walther Penck pointed it out, Davis and his followers did usually –if not almost always– employed a rapid uplift and quiescence approach to explain landscapes. This means that the model, as understood by most, assumes rapid and episodic tectonic uplift. Another characteristic of the model is that slopes evolve by decline, meaning that initially steep slopes are worn out by erosion forming successively gentler slopes. Weaknesses of the model are that it is mostly theoretical and deductive in nature; further it does not take into account the complexity of tectonic movements or climate change. The nature of surface processes is also poorly represented by the model. The model in its original form is intended to explain relief development in temperate landscapes where erosion by running water is assumed to be of prime importance. Nevertheless, the cycle of erosion has been extended, with modifications, into arid, glacial, coastal, karst and periglacial areas.

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