Wood (1942) began his evolution of slope taking cliff as the initial form which emerged either due to erosion or earth movements.
The process of weathering would push back the cliff (free face). In other words, weathering causes the free face to retreat parallel to itself.
Weathered material would collect at the foot of the face (scarp); the scree accumulates and slowly buries the lower parts of the free face thus reducing its height.
Wood regarded the foot of the scarp as the local base level for weathering process. He assumed an ideal case of accumulated talus which is not subjected to weathering and has the same volume as parent rock.
The scree provides protection to the base of the rock face from weathering. The talus continues to grow and finally, it completely buries the free face (Fig). The retreating face above leaves behind protrusion under the scree.
The surface of the scree which accumulates at a constant angle is termed as the constant slope (wood, 1942).
Beneath the scree will lie buried the convex slope. While this is an ideal case but in nature, such a process would be highly complex as there are various factors that affect the evolution of slopes.
In nature, the volume of the scree will never be the same as the parent rock rather the volume would be more than parent rock because of the presence of interstitial space. This will cause the upward growth of scree faster than ideal case as a result the buried face will become steeper while still retaining the convex form.
Similarly, if the removal of scree takes place due to the washing out of the fine materials it will have an effect in the opposite direction. The rate of growth of scree will be slowed down and the slope of the buried face will become gentler. It can also be added that the production of more coarse debris will lead to the rapid growth of scree as compared to the rock producing finer debris.
As stated earlier the lower part of the constant slope which is formed by accumulated scree in nature will be weathered and carried by rainwash away from the foot of the hill slope, resulting in the gradual reduction of the slope and assuming a concave upward form known as waning slope.
The recession of the hill continues till the free face disappears and the constant slope keeps extending upwards. The upper part will then result in a waxing slope. The upper convex, lower concave, and middlerectilinear slope form will develop. Gradually the rectilinear slope owing to the extension of waxing slope from above and waning slope from below will disappear. Finally, the relief gradually declines due to wasting.
Wood was of the opinion that the manner in which the slopes evolve is not the same for all as a lot depends on climate, structure, and conditions observed at the slope base.
Strahler’s View on Slope Development
The work of Strahler is statistical in nature. His work is based on the data collected from fieldwork in parts of California. Strahler (1950) collected data “with a view to determining
If differences in underlying rock types are associated with differences in slope angle.
If differences in directional exposure to sunlight and other meteorological factors produce differences in slope angles, and
If slopes decline in angle when left to weathering and erosion processes and not accompanied by basal erosion and removal”.
He conducted measurements of maximum angles attained by slope and carefully identified the area and ensured it should have uniformity in terms of climate, vegetation, relief, and tectonic history. Lithological factors, however, were not the same.
He calculated the mean maximum slope angle for the study area, then assessed the deviation of slope from the mean slope by comparing the data of slope collected at different points in the study area.
Strahler argued that if a large number of slopes show very little variation from the mean slope it means that the slopes have developed at approximately the same angle for the reason that this is the angle allowing the steady and efficient removal of the slope debris by slumping, creep and wash. Such slopes are in a delicate state of equilibrium (Small,1978).
According to Strahler (1950) “under the equilibrium, slopes maintain an equilibrium angle proportional to the channel gradient of the drainage system and are so adjusted as to permit a steady state to be maintained by the process of erosion and transportation under prevailing conditions of climate, vegetation, soils, bedrock, and initial relief”.
Thus one can infer that the equilibrium slopes are governed by different slope controlling factors and change in any of the factors can cause readjustment of the equilibrium angle.
Strahler during the course of his field observation also studied the relationship between valley side slopes and the stream gradient and noted that with the reduction of landmass there is a reduction in stream gradient and slopes. They gradually regrade towards the maintenance of equilibrium.
He confirmed the correlation between slope angle and channel gradient (slope adjusts in proportion to the debris obtained from the valley side slopes) where the valley sides slope is steep it will have a steep channel slope and where it is gentle it will have a gentle channel slope. However, there are exceptions to this rule in that the side slopes do not steepen with an increase of channel gradient very near the head of streams (Sparks,1986).
Strahler also noted that a bare slope would contribute a greater amount of load than a vegetated slope at a given angle. This will result in a steeper channel gradient beneath the bare slope than that below vegetated. This example displays how both the angle of the stream channel and valley side slope modify with the change in controlling factor (i.e.vegetation cover).
Through careful measurements of slopes, Strahler discovered that where the river was closer to the foot of the slope it formed a steeper slope because of the removal of debris. But when the stream was away from the slopes they were protected from the basal cutting and had lower angles
Conclusion
The chapter gives an insight into the diverse hypotheses put forward by different geomorphologists.
The term ‘Slope Decline’ is used by Davis to indicate the process of slope evolution where the steepest part of the slope declines with the development of convexity and concavity.
Penck, on the other hand, emphasized ‘Slope Replacement’where maximum angle declines due to its replacement by gentler slopes from below causing the greater part of the slope profile to become concave.
L. C. King discussed ‘Parallel Retreat’where the maximum angle remains constant but the concavity gradually increases in length.
Wood defined elements of slope and discussed the development of hillside slope.
Strahler used statistical techniques in his work and analyzed factors affecting slope evolution.
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