Development of Slope

2 years ago

Atiqur Rahman

9 minutes

A physical landscape is an assemblage of slopes. Geomorphologists for long been intrigued by the study of the origin and form of slopes, but it posed a major challenge to the study of landforms.

Various theories and models were formulated to provide a rational explanation for its origin and form but all had their flaws. Despite the fact that the slope constitutes the core of the landform study, it has not received due attention and largely remained neglected.

The study of slopes faces several challenges. It becomes difficult to determine its nature, the rate of operation of the processes and its effect on the slope. It is also very difficult to mark the whole trajectory of slope development and trace changes in its form with the passage of time.

In a landform study, there are two aspects which have always remained in the focus- the form and the process. The term ‘form’ indicates the morphology of a given region at a given time. The different shape a landform assumes is the focus of the study while ‘process’ means the actual operation of different agents which bring about changes in the physical environment.

These agents are many and they vary in terms of their role in different regions. The process includes agents such as soil creep, surface wash, weathering etc.

Two approaches were followed to have a proper understanding of slope development. They are:

(a) Historical approach
(b) Process-Form approach

a) Historical Approach

This approach emphasizes the historical evolution of slopes right from their origin to the present form. It, however, suffers from the inherent problem of correct reconstruction of the past forms of the slope.

There is no yardstick to measure or verify the correct reconstruction of the past, so it makes the problem of identifying a proper theory that explains the actual forms of the slopes in the past more complex. Tracing the correct historical development of slope forms is therefore not an easy task.

Many writers and investigators have taken recourse in the assumption that the present-day slopes have developed from near-vertical cliffs which in course of time have weathered back and modified to new forms and gradients.

Such conditions may occasionally occur in nature but in a majority of cases, slopes appear to border river valleys which were never vertical cliffs. It is wrong to generalize that river erosion always produces steep vertical wall-like features due to its high intensity of erosion.

Along with the process of river erosion, there are other processes like weathering and creep that tend to consistently modify the slope edges produced by rivers and transform them into more complex slope forms. “The concept of initial slope is perhaps a unrealistic one”(Small, 1978).

Another commonly encountered problem is determining the age of the slope. There is no standard method which can be universally applied to determine the age of slope with accuracy. Extensive field surveys of different slope profiles can give rather more satisfactory results than any other method of study. Studying numerous profiles would help the investigator easily distinguish profiles in various stages of their evolution and place them in the proper time sequence.

However, this exercise also does not give a completely satisfactory result. Determination of slope age based on its form is by no means an easy exercise. Despite all the problems mentioned above many geomorphologists went ahead and formulated diverse theories explaining the process of slope evolution. But theory formulation and reconstruction of the past forms of the slope are still largely based on speculation.

b) Process-Form Approach

The premise of this approach rests on the assumption that the form and gradient of the slope are an outcome of the causal relationship between weathering, erosion, transportation and deposition.

These processes of denudation operate in different combinations and varying rates giving rise to an immense variety of slope forms with varying steepness. The variation in rock types, climate, vegetation etc has a direct bearing on the types of slope forms produced.

If we take the example of limestone region having an adequate amount of rainfall we observe convex slope as the most common form because of the reason that rainwash is less effective here due to the porous nature of rock which in other regions would have resulted in concavity.

This approach like the historical approach suffers from several difficulties. It becomes very difficult to observe the different processes at work on slopes since the process of weathering, creep, rainwash etc are extremely slow and not perceptible to the eyes. Thus it requires state-of-the-art tools and highly accurate methods for recording the operation of these processes for arriving at accurate results.

It is again very difficult to assert very firmly that slope processes have a direct bearing on the form of the denudational slope. Another problem associated with this process-form approach is the possibility that many slopes of the present times are not an outcome of present-day processes (Small, 1978).

There have been diverse opinions expressed regarding the relationship between slope form and climate. If the slope is controlled by slope processes alone then it is assumed it would give rise to different slope forms in different regions. Some geomorphologists have overstressed this relationship citing several examples in its support.

However, the assumption that a certain type of slope will be formed only in a particular kind of climate is not always true. For a long period, it was held that pediments are found in desert regions but now many scientists and geomorphologists support the view that it may occur anywhere in the world.

In the above-discussed two approaches the historical approach retraces the sequence of events in the past and the process-form approach investigates the processes and their interrelationships that result in diverse slope forms. Both the above slope approaches are helpful in the study and understanding of the slopes and their evolution.

Genetic Classification of Slopes

Slopes are produced by both Endogenetic and Exogeneticprocesses. Based on these two processes they have been broadly divided into- Endogenetic Slopes and Exogenetic Slopes.

a) Endogenetic Slopes

These slopes originate due to the processes which originate within the earth. Different earth movements lead to the formation of folds, faults, rift valleys etc. These slopes are also referred to as tectonic slopes. Fault scarps are often associated with faults and rift valleys.

The volcanic eruption which is also an outcome of the endogenetic process going on inside the earth results in the formation of new features. Volcanic Eruptions cause the accumulation of lava and pyroclastic materials and give rise to different kinds of volcanic hills, plateaus and cones.

The size and steepness of volcanic cones, plateaus and hills depend on the nature of the eruption, the viscosity of the lava and the amount of pyroclastic material released during eruptions.

The features formed due to the volcanic eruptions or tectonic processes undergo modification by sub-aerial processes resulting in various slope forms.

b) Exogenetic Slopes

These slopes are an outcome of external processes originating at or near the earth’s surface. Processes like weathering, mass wasting, erosion and deposition play key roles in fashioning a landscape. These processes consistently operate upon the surface and regularly create and modify slope forms.

The slopes created by exogenetic processes can broadly be divided into two categories:

(i) Erosional or Degradational Slopes and
(ii) Depositional or Aggradational Slopes

(i) Erosional slopes are formed by the action of wind, running water, waves, glaciers etc. Glaciers and running water produce numerous landforms in their valleys. Features like escarpments, watersheds, river terraces, and cliffs (in the coastal regions) are examples of erosional slopes.

(ii) Depositional slopes are again produced by the same agents. Alluvial fans and natural levees are produced by running water; moraines are produced by glaciers; wind produces sand dunes of various shapes and sizes. Deposition along sea coasts gives rise to sand bars, barriers and beaches. These are all different depositional slopes produced by the action of water, wind and glacier.

Development of Slope

a) Historical Approach

b) Process-Form Approach

Genetic Classification of Slopes

a) Endogenetic Slopes

b) Exogenetic Slopes

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a) Historical Approach

b) Process-Form Approach

Genetic Classification of Slopes

a) Endogenetic Slopes

b) Exogenetic Slopes

Read More in Geomorphology

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Related

Share Your ThoughtsCancel reply