Alan Wood’s Model of Slope Evolution

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 the parent rock. The scree protects the base of the rock face from weathering. The talus continues to grow and finally 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 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 the parent rock because of the presence of interstitial space. This will cause the upward growth of scree faster than the 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 the 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 middle rectilinear slope forms will develop. Gradually the rectilinear slope owing to the extension of the waxing slope from above and the waning slope from below, will disappear. Finally, the relief gradually declines due to wasting.

Wood thought 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.

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