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 —if conditions allows— 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.[A] 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.
|Arid||Davis, 1905||At the beginning of the cycle of erosion in arid climate there are numerous small basins to where material is washed during the scarce rainfall events. In the next stage (youthful stage) valleys are developed and highlands dissected by these. Gentle slopes and basins accumulated material derived from the highlands. In the mature stage drainage basins coalesce. At the end a stage is reached where the terrain has lost much of its relief and deflation hollows interfere with the drainage systems, breaking it up into local systems. During all stages sand and dust might be exported by wind to other landscapes.|
|Coastal||Johnson, 1919||Alternate models are proposed for shore profiles: one for shore profiles of emergence and another for shore profiles of submergence. A complementary model can be applied to shore lines where different parts might undergone submergence and emergence.|
|Glacial||Davis, 1900||The glacial cycle of erosion deals with mountainous regions and lack any mature stage since Davis argues that nothing more developed than mature glacial landscapes exists at present. A glacial cycles of erosion is begins in a pre-glacial landscape. Over time valley glaciers erode the underlying rock at different paces creating valleys and glaciers that are more entrenched than other. Over time the less-entrenched glaciers reach the same levels of entrenchment than the more entrenched ones since the deeper a glacier erodes erosive power diminish. In a mature stage valleys form smooth-sided troughs.|
|Karst||Cvijić, 1918||The cycle of erosion in karst region has tree phases. At first the upper parts of fractures are dissolved, enlarged and filled with water. Normal fluvial valleys develop on surface, small poljes might exist. Subsequently, re-routing of water by the growth of a karst system disorganizes the fluvial drainage pattern with valley bottoms developing large dolines and uvalas. Ridges between uvalas gradually disappear as these landforms coalesce. If the bedrock is underlain by unsolube rock normal fluvial valleys will slowly re-appear once the underground river systems reaches the unsoluble rock. In the end soluble rock appear only as isolated hills. Contrary to Davis' original cycle of erosion the karst one does not end in the formation of a peneplain.|
|Periglacial||Peltier, 1950||The periglacial cycle of erosion begins with a non-periglaciated landscape. Once-periglaciated mass wasting of regolith exposes bedrock in the upper slopes. These outcrops are then subject to frost weathering that makes slopes retreat forming extensive blockfields at the base of the bedrock areas. At a later stage solifluction wears down summits and fills in topographic lows.[B]|
Early acclaim and criticism
After the model was first developed by William Morris Davis around 1900 it received wide acclaim, but was never universally accepted. The initial enthusiasm and strength of the cycle of erosion model has been attributed to various causes. First, the model provided a framework to study areas and epochs in Earth history where erosion is the dominant process. Second, the model fitted well into the grand evolutionary thought that had emerged in the 19th century with Darwin's evolution theory.[C] Lastly, some popularity was indebted to Davis's lucid writing style. The model achieved its greatest popularity in the 1900–39 period when numerous studies on denudation chronology based on the model were published. In these studies usually two to five erosion cycles were identified. The approach of doing denudation chronology with the cycle of erosion model lost popularity from the 1930s onward. Arguably this was so because the approach did not provide any unforeseeable insights. An increasing number of geomorphologists had begun to study processes happening in the present and not in the past as done with the cycle of erosion model. These process geomorphologists soon realized some of their observations were at odds with Davis's model. Other geomorphologists turned away from the cycle of erosion to work instead on climatic or tectonic geomorphology.
While the model was at first widely accepted among Anglo-Saxon scholars, in Continental Europe it met some resistance with German scholars Albrecht and Walther Penck, Siegfried Passarge and Alfred Hettner standing out as early opponents to the model. Walther Penck and Lester Charles King proposed alternative cycle theories in the 1920s and 1960s respectively.
Intensified criticism (1960s)
While King's ideas were an attempt at refuting Davis's cycle of erosion they were themselves of cyclical nature and contributed to ushering in a wave of criticism in 1960s against both his and Davis's models. This criticism has been called "Davis bashing" by Cliff Ollier and constitutes to Ollier the ridicule of cyclical theories in geomorphology without any alternative model being proposed. Since Davis' ideas were being discredited other ideas, like that of climatic geomorphology, were attacked by their perceived association to it.
The notions of time, uplift, slope and drainage density evolution in the erosion cycle have been criticized. Further the validity of some whole concepts associated with the cycle of erosion have been questioned including stream grade, base level and most of all that of peneplains.
Writing in 1971 geomorphologist Ronald Flemal summarized the situation as follows:
- Currently geomorphologists are divided into three camps: those who still adhere to Davisian concepts, either in the original or a modified form; those who desire to replace Davisian ideas by a different cyclic erosional model; and those who reject cyclic erosion completely.
Despite considerable criticism the cycle of erosion model has remained part of the science of geomorphology. The model or theory has never been proved wrong, but neither has it been proven. The inherent difficulties of the model have instead made geomorphological research to advance along other lines. In contrast to its disputed status in geomorphology, the cycle of erosion model is a common approach used to establish denudation chronologies, and is thus an important concept in the science of historical geology. While acknowledging its shortcomings modern geomorphologists Andrew Goudie and Karna Lidmar-Bergström have praised it for its elegance and pedagogical value respectively.
- Alternative models of slope evolution are parallel slope and scarp retreat, championed by Lester Charles King, and slope replacement first proposed by Walther Penck. King considered scarp retreat a dominant process across the globe and claimed that slope decline was a special case of slope development seen only in very weak rocks that could not maintain a scarp. The modern understanding is that the evolution of slopes is much more complex than the classical models of decline, replacement and retreat imply.
- An earlier cyclical model on the formation of cryoplanation terraces was proposed in 1943 by Soviet scientists S.G. Boch and I.I. Krasnov.
- However, the model was more influenced by Neo-Lamarckian thought that was currency in late 19th-century United States. Darwin's ideas were a lesser influence. As such the model cycle of erosion employs concept of "evolution" rather than "change" implying a predictable direction of landscape and landform change. It is thought that Davis received some Neo-Lamarckian influence from his tutor Nathaniel Shaler. Other influences included the biological theories of orthogenesis and recapitulation both of which are linked to Neo-Lamarckianism.
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