Thegeographic cycle, orcycle of erosion, is an idealizedmodel that explains the development ofrelief inlandscapes.[1] The model starts with theerosion that follows uplift of land above abase level and ends, if conditions allow, in the formation of apeneplain.[1] Landscapes that show evidence of more than one cycle of erosion are termed "polycyclical".[1] The cycle of erosion and some of its associated concepts have, despite their popularity, been a subject of much criticism.
William Morris Davis, the originator of the model, divided it into stages whose transition is gradual similar to the human life i.e. Youth Stage, Mature Stage and Old Stage.[2] The model begins with an uplifted sate of landscape.[3] Then Davis defined ayouthful stage whereriver incision or vertical erosion is the dominant process shaping the landscape. During theyouthful stage height, differences between uplands and valley bottoms increase rapidly. Theyouthful stage is followed by amature stage in which height differences between valley bottoms and uplands are at their greatest. In themature stage,slope decline becomes a more important phenomenon as the lateral erosion dominates,[4] 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 its original height, is reduced into arolling lowland. That landscape of low relief is called apeneplain and may contain residual heights standing out from the general level. The peneplain can be uplifted, which starts a second erosion cycle.[5]
Davis acknowledged that a full cycle was a special case and that initial uplift was not necessarily rapid or followed by a prolonged period of quiescence. However, asWalther Penck pointed out, Davis and his followers usually used a rapid uplift and quiescence approach to explain landscapes.[6] This means that the model, as understood by most, assumes rapid and episodic tectonic uplift.[7]Another characteristic of the model is thatslopes evolve by decline, with initially-steep slopes worn out by erosion forming successively-gentler slopes.[7][A] Weaknesses of the model are that it is mostly theoretical and deductive in nature and it does not take into account the complexity of tectonic movements orclimate change. The nature of surface processes is also poorly represented by the model.[7] The model in its original form is intended to explain relief development intemperate landscapes in which erosion by running water is assumed to be of prime importance.[5][7] Nevertheless, the cycle of erosion has been extended, with modifications, intoarid,semi-arid,savanah,selva,glacial,coastal,karst andperiglacial areas.[7][10] Writing in 1950 Louis C. Peltier claimed the cycle of erosion inmaritime andboreal climates were the only one that had not been described in detail.[10]
| Environment | Proposed by | Details |
|---|---|---|
| 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 themature stage drainage basins coalesce. At the end, a stage is reached in which the terrain has lost much of its relief anddeflation hollows interfere with thedrainage systems, breaking it up into local systems. During all stagessand anddust might beexported by wind to other landscapes.[11] |
| Selva | Cotton, 1942[10] | Rainforest hinders the erosion of hillsopes.[10] Hillslopes have thick soil.[10] |
| Semi-arid | Cotton, 1942[10] | Forms together with the savannah cycle intermediate cycle between the "normal" and arid variants.[10] |
| Savanna | Cotton, 1942[10] | Associated withinselbergs andpediplains.[10][12] Rivers are notincized enough to erode fresh rock, eroding instead "red soils" andweatheredregolith.[12] Flattish surfaces in savanna landscapes are eroded by river's lateral migration and by regularly flooding that leads thewashing away of fines.[12] The over-all effect a gradual loweing of this surfaces. Large inselbergs are polycyclic, but smallertor are more recent features.[12] |
| Coastal | Johnson, 1919 | Alternate models are proposed forshore profiles: one for shore profiles of emergence and another for shore profiles of submergence. A complementary model can be applied toshorelines where different parts might have undergone submergence and emergence.[13] |
| Glacial | Davis, 1900 | The glacial cycle of erosion deals with mountainous regions and lacks an old stage since Davis argues that nothing more developed than mature glacial landscapes exists at present. A glacial cycle of erosion begins with a pre-glacial landscape. Over time valley glaciers erode the underlying rock at different rates, creating valleys and glaciers that are more entrenched than others. As time passes the less-entrenched glaciers reach the same levels of entrenchment as the more entrenched ones, since the deeper a glacier erodes, its erosive power diminishes. In a mature stage, valleys form smooth-sided troughs.[14] |
| Karst | Cvijić, 1918 | The cycle of erosion in karst regions has three phases. At first, the upper parts offractures are dissolved, enlarged and filled with water. Normal fluvial valleys develop on the surface, smallpoljes might exist. Subsequently, re-routing of water by the growth of a karst system disorganized the fluvial drainage pattern, with valley bottoms developing largedolines anduvalas. Ridges between uvalas gradually disappear as those landforms coalesce. If the bedrock is underlain by insoluble rock, normal fluvial valleys will slowly re-appear once the underground river systems reach the insoluble rock. In the end, soluble rocks appear only as isolated hills. Contrary to Davis's original cycle of erosion, the karst cycle does not end in the formation of apeneplain.[15] |
| Periglacial | Troll, 1948[16][10] Peltier, 1950 | The periglacial cycle of erosion begins with a non-periglaciated landscape. Once-periglaciated mass wasting ofregolith exposes bedrock in the upper slopes. The outcrops are then subject tofrost weathering that makesslopes retreat forming extensiveblockfields at the base of the bedrock areas. At a later stage,solifluction wears down summits and fills in topographic lows.[17][B] |
There had been some ideas on cyclical erosion in theGraeco-Roman world and then in theIslamic world and Europe during theMiddle Ages. However the immediate influences ofWilliam Morris Davis, the creator of the cycle of erosion model, were 19th century American explorers. The end of theAmerican Civil War (1861–1865) led to a resumption of the exploration of the western United States. Three explorers,John Wesley Powell,Clarence Edward Dutton andGrove Karl Gilbert, wrote about the geomorphology and geology in the landscapes they encountered. It was from these works that Davis borrowed many concepts used to construct the model.[18][C]
It has been argued that Davis was also influenced by ideas from the field ofbiology, especially theNeo-Lamarckian thought that was current in the late 19th-century United States. It is thought that Davis received some of this Neo-Lamarckian influence from his tutor,Nathaniel Shaler. Other biological theories that may have shaped the cycle of erosion are those oforthogenesis andrecapitulation both of which are linked to Neo-Lamarckianism.Darwin'sevolution theory was a lesser influence relative to Neo-Lamarckism. The impact of these ideas can be hinted in the models' employment of the concept of "evolution" rather than "change", implying a predictable direction of landscape and landform change.[19] It has been argued that "Davis consciously applied Darwinism to landscape".[20]
Davis conceived the cycle of erosion model in the 1880s while studying theAppalachians inPennsylvania andVirginia.[18][20] Davis wrote down the Cycle of erosion theory in detail for the first time in 1889.[20] More fully developed by 1900 it received wide acclaim, but was never universally accepted.[1][7] 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 whereerosion is the dominant process. Second, the model fitted well into the grand evolutionary thought that had emerged in the 19th century withDarwin'sevolution theory.[21] Lastly, some popularity can be attributed to Davis's lucid writing style. The model achieved its greatest popularity in the 1900–39 period when numerous studies ondenudation 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.[21] 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. Theseprocess 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 onclimatic ortectonic geomorphology.[21]
The model spread fast. In 1901Hans Reusch was using it to explain theundulating plateau of southern Norway.[22][23] Very much influenced by DavisWalter Wråk moved to study the relief of the northernScandinavian Mountains, describing among other things theBorsu surface.[24] The first study of China's topography using the model was published in 1907 by B. Willis and co-workers.[25] The idea of the cycle of erosion was disseminated among college and university students with a series of textbooks published in the 1890s and 1900s.[20] According to Sheldon Judson up toSecond World War reports onregional geology tended to include brief mentions of the local cycle of erosion and ofpeneplanization.[20]
While the model was at first widely accepted amongAnglo-Saxon scholars,[7] inContinental Europe it met some resistance with German scholarsAlbrecht andWalther Penck,Siegfried Passarge andAlfred Hettner standing out as early opponents to the model.[26] Despite Davis's efforts, which included translating his work into German, being a visiting professor at Berlin and touring much of Europe, the cycle of erosion never did take firm root in Germany.[18] Walther Penck came to originate an alternative model with its own set of tenets. Walther's ideas were published posthumously in 1924 by his father, Albrecht. In Walther's model, relief is shaped as a function of the denudation/uplift rate.[18] Davis, a friend of Walther's father,[27] was critical of Walther's new model. Davis's review of Walther Penck's second publication on the subject left a distorted view of Penck's ideas among later workers. This is because Davis misunderstood and mis-translated parts of the paper, therefore Davis is not an adequate critic of Penck's work.[28]
In France the cycle of erosion theory was first spread byAlbert de Lapparent. In general its reception in France was mixed withÉmile Haug giving it limited attention andPaul Vidal de La Blache adopting it without reservations.[29] Other notable French geographers influenced by Davis ideas on the cycle of erosion areEmmanuel de Martonne andHenri Baulig.[29] Geomorphology in France developed into two schools, one based on geographers who adopted Davisian thought and another based onengineering geology with connections tohydraulics andmining.[29]
In 1960 geomorphologist Sheldon Judson noted that American geographers had largely abandoned the concept and moved to study landscape and landforms from aprocess andstatistical point of view.[20]Eiju Yatsu opined, however, that despite the intention of many geomorphologists to abandon Davisian ideas after the Second World War, many ended up returning to them with some modifications.[31]
An alternative cycle theory was proposed byLester Charles King in the 1960s.[1] 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.[7][32] This criticism has been called "Davis bashing" byCliff Ollier and constitutes to Ollier the ridicule of cyclical theories in geomorphology without any alternative model being proposed.[32] Since Davis' ideas were being discredited other areas of research, like that ofclimatic geomorphology, were attacked by their perceived association to it.[33]
The notions of time, uplift,slope anddrainage density evolution in the erosion cycle have been criticized.[21] Further the validity of some whole concepts associated with the cycle of erosion have been questioned includingstream grade,[21]slope decline,[D]base level[21] and most of all that ofpeneplains.[21][E]
Writing in 1971 geomorphologist Ronald Flemal summarized the situation as follows:[21]
Despite considerable criticism the cycle of erosion model has remained part of the science ofgeomorphology.[36] The model or theory has never been proved wrong,[36] but neither has it been proven.[37] The inherent difficulties of the model have instead made geomorphological research to advance along other lines.[36] In contrast to its disputed status in geomorphology, the cycle of erosion model is a common approach used to establishdenudation chronologies, and is thus an important concept in the science ofhistorical geology.[38] While acknowledging its shortcomings modern geomorphologistsAndrew Goudie andKarna Lidmar-Bergström have praised it for its elegance and pedagogical value respectively.[1][7] Writing in 2007 Anthony Orme evaluates that:[18]