| Stages of Davis | Stages of Penck | Landform Character |
| Youth | Aufsteigende Entwickelung | Accelerating |
| Mature | Gleichformige Entwickelung | Uniform development |
| Old | Absteigende Entwickelung | Waning or decellerating |
Davis, through several articles and essays, postulated his concept of the cycle of erosion. His work on the cycle of erosion was published in the year 1899. Later through a number of papers and articles modified his work several times.
Davis envisaged that all the landforms of the world pass through an evolutionary sequence during which denudation processes act upon them to fashion different landforms in different stages of their evolution. Change in landform with the passage of time formed the cornerstone of his cyclic concept. The landform would change from ‘initial’ to ‘ultimate form’. The crest (hilltops) will not remain stable for a long period of time rather their height and slope will decline with the passage of time.
His cycle of landform development was thus dynamic in nature. Davis argued that all physical landforms can be analysed in terms of the three variables- structure, process and stage (Small,1978).
Structure: The term structure included more than what its literal meaning. It included the manner of disposition of underlying rocks, level of hardness, porosity, folds and faults etc.
Process: It includes all types of weathering rivers, wind and glacial erosion, mass movements etc.
Stage: It means the time duration during which the processes operate on a structure.
Davis used the terminology of youth, mature and old age to mark different phases of the evolution of landforms. He drew an analogy between landscape and living beings and therefore compared the life cycle of a living being with the life cycle of landforms.
He argued every landform undergoes sequential changes through the process of evolution where it passes through youth, maturity and old age. Davis talked about all these stages of the life cycle in relative terms.
In other words, there is no fixed time duration for youth, maturity or old age because the time duration for each stage will depend on many factors. In regions of highly resistant rocks, the duration of this cycle will be fairly long compared with relatively weaker, less resistant rock types.
Davis made some assumptions regarding the upliftment of landforms. He visualised the initial landform as a mass of land uplifted from the sea. The process of upliftment was faster, and the denudation processes acted upon it from the time it almost became a stable mass.
His normal cycle represents the case of a humid temperate landform in which several consequent streams would emerge due to rainfall. The surface gradient would control the velocities and stream direction.
Stages of the Cycle of Erosion
Youthful Stage
The streams, after their emergence, will begin rapid downward erosion resulting in the formation of deep valleys bounded by very steep slopes. In other words, a V-shaped valley would be formed, which will sustain its nature all through its youth. The divide summits are preserved in this stage.
During the early beginning of youth, the valley side slopes are steep, which by the close of the youthful stage, gradually diminishes. The rivers extend their valleys by continuous headward erosion.
The youthful stage is marked by the development of features like waterfalls, rapids and gorges etc. The long profile of the river bears these features; hence Davis referred to such profiles as degraded.
Stage of Maturity
Here the rate of valley deepening declines, and lateral erosion predominates. In this phase begins the lowering of interfluves summit, thereby reducing the relief of the landform. The vertical height between the valley floor and the summit declines faster.
The end of the mature stage sees a decline in slope angles because of wasting divides. The landscape will have relatively smooth slopes.
According to Davis, this is a stage where slopes reach the condition of grade, a state where there exists a balance between the rate of production of weathered material and the rate of its removal. While vertical erosion was dominant in the youthful stage, lateral erosion that slowly erodes the interfluves dominates in maturity.
Stage of Old Age
In this stage, the process of landform evolution goes at a very slow pace. There is a marked reduction in the river gradients. The angle of valley side slopes declines continuously; there is also a decline in creep and wash.
The slope gets covered with detritus and thereby protected from mechanical weathering. The detritus conceals everything beneath it. The streams move on a very gentle gradient and often wander over their floodplain.
This stage lasts longer than the previous two stages. The slopes continue their wasting till they get to a very low angle.
At the end of the third stage, the relief is almost destroyed, and the land surface appears as a flat, featureless plain known as a ‘peneplain’. The peneplain may have a few hills that have still not lost their existence despite wasting. Davis called these isolated hills ‘Monadnocks’.
Davis on Slopes
Davis held that the upper convex part of the slope is produced by soil creep. He was of the view that creep produces ‘rounded contours’. As one comes down the slope, the volume of surface wash increases but near the divide summit ratio of creep to wash is large (Young,1972).
He also put forward the concept of graded waste sheets and graded valley sides (Young, 1972). According to him both rivers and waste sheets are mixtures of water and rock debris in different proportions. The graded condition is first established at the lowest part of the slope and then moves upward.
He has equated the graded slope with the graded waste sheet. A graded waste sheet is one on which there is equality between the supply and the removal of debris while a graded slope has a continuous soil cover.
In the words of Davis (1899) “just as graded rivers slowly degrade their courses after the period of maximum load is passed, so graded waste sheets adopt the gentler and gentler slopes. When the graded slopes are first developed they are steep, and the waste covering them is coarse and of moderate thickness. In a more advanced stage of the cycle, the graded slopes are moderate, and waste that covers them is of finer texture and greater depth than before”.
Davis further added in his work in 1932 that the retreat of the valley side slope is accompanied by a decline in the steepness of the slope and the development of a convex and concave profile at the top and the base, respectively a.
With the advance of the cycle, both the convexity and concavity extend and assume a larger radius of curvature. He has also stated that mountains and hills will be worn to gentler and gentler declivities, owing to faster downwash of soil from their upper convex slopes than the removal from the slope base.
Analysis of Davis Cycle
There have been diverse views expressed by different geomorphologists on the concept of ‘peneplain’ put forward by Davis. Some regarded the peneplain as a theoretical landform because they considered that for Davis’ cycle to run its full course, it requires the landform should remain stable for a very long time. This possibility was very rare in a reality where both endogenetic and exogenetic forces operate continuously, which may obstruct the cycle’s smooth course.
It is also argued that during the period when the river is eroding its valley, the removal of the overlying load is compensated by the addition of more material to its root as per the principle of Isostatic adjustment. This will give a push to the overlying landform, thus keeping the process of upliftment continue for an infinite time. Thus the attainment of the peneplain stage is questioned as it contradicts the view of isostatic adjustment.
Most geomorphologists also believe that Davis’ idea of the sequential change of landforms is a too simplistic presentation of landform evolution. The evolution of landforms, in reality, is a far more complex process.
Although the Davis cycle has faced a lot of criticism, it still holds its place in the work pertaining to slope evolution owing to its wide appeal and manner of presentation.
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