17 Slope Elements and Slope Evolution

Dr. Khusro Moin

 

   Objectives

To know about different slope elements/forms and the dominant processes in their evolution.

Keywords

Creep, wash slope, Process-form approach of slope study, rainwash.

    Physical landscape is an assemblage of slopes. Geomorphologists for longbeen intrigued by the study of origin and form of slopes, it posed a major challenge to the study of landforms. Various theories and models were formulated to provide a rational explanation to its origin and form but all had their own flaws. Despite the fact that slope constitutes the core of landform study, it has not received due attention and largely remained neglected. The study of slopes faces a number of 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 manyand they vary in terms of their role in different regions. Process includes agents such as soil creep, surface wash, weathering etc.

Two approaches were followed to have a proper understanding of the slope development. They are (a) Historical approach (b) Process-Form approach

(a) Historical Approach: This approachemphasizes on historical evolution of slopes right from its origin to the present form.It, however, suffers from the inherent problem ofcorrect reconstruction of the past forms of slope. There is no yardstick to measure or verify the correct reconstruction of the past, so it makes the problem of identifyinga 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 cliff 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 feature due to its high intensity of erosion. Along with the process of river erosion there are other processes like weathering and creep that tends to consistently modify the slope edges produced by river and transform them into more complex slope forms. “The concept of initial slope is perhaps a totally 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 survey 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 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 geomorphologist went ahead and formulated diverse theories explaining the process of slope evolution.But theory formulation and reconstructing the past forms of slope is still largely based on speculation.

(b) The process-form approach: The premise of this approach rests on the assumption that the form and gradient of slope is an outcome of causal relationship between weathering, erosion, transportation and deposition. These processes of denudation operate in different combinations and in varying rates giving rise to immense variety of slope forms with varying steepness. The variation in rock types, climate, vegetation etc has direct bearing on the types of slope forms produced. If we take the example of limestone region having adequate amount of rainfall we observe convex slope as the most common form because of the reason thatrainwash 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 verydifficult 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 of the operation of these processes for arriving at accurate results.

It is again very difficultto assert very firmlythat slope processes have direct bearingon the form of 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 slope is controlled by slope processesalone then it is assumed it would give rise to different slope forms in different regions. Some geomorphologist have overstressed this relationship citing several examples in its support. However, the assumption that certain type of slope will be formed only in a particular kind of climate is not always true. For a long period of time it was held that pediments are found in desert regions but now many scientists and geomorphologist 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 results in diverse slope forms. Both the above slope approaches are helpful in the study and understanding of the slopes and their evolution.

GeneticClassification 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 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. Volcanic eruption which is also an outcome of endogenetic process going on inside the earth results in the formation of new features. Volcanic Eruptions cause 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 depends on the nature of eruption, viscosity of lava and 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 into various slope forms.

(b)  Exogenetic slopes– These slopes are an outcome of external processes originating at or near the earth’s surface. Processess like weathering, mass wasting, erosion and deposition play key role 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(a) Erosional or degradational slopes and (b) Depositional or aggradational slopes. Erosional slopes are formed by the action of wind , runningwater, waves , glaciers etc. Glaciers and running water produce numerous landforms in their valleys. Features like escarpment, watersheds, river terraces, cliffs ( in coastal region) are examples of erosional slopes.

Depositional slopes are again produced by the same agents. Alluvial fans, 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 givesrise to sand bars , barriers and beaches. These are all different depositional slopes produced by the action of water, wind and glacier.

Elements of slope

Slopes have a number of forms or elements. A slope profile commonly consists of convex (crest),rectilinear and concave slope forms. Convex slopes are commonly found at the top and concave slopes lie at the base of the hillslope.L. C. King and A.Wood were of the belief that a standard composite slope profile consist of four elements. The uppermost part is convex slope or crest. Below the crest is scarp or free face followed by rectilinear slope and at last at the bottom is the concave slope. This is the most common composite slope profile, however, in reality theyare found in several combinations. All the elements may not occur in a single slope profile. Occurrence of these elements in varying combination depends on factors like structure of rock, nature of rock and the processes that operate upon the surface. The four main types of slope elements (forms) have been discussed below in detail.

(1) Convex Slope: At times an entire slope may assume a convex form but it is most commonly observed on the upper parts of the slope. Convex slope profile is a result of denudational process; they are assumed to be the characteristic of the humid temperate region. Some rock types like chalk and limestone are also associated with convex profile of slope. However, this is not a rule as they can easily be observed in many other regions having a variety of rock types other than limestone or chalk.In the upper parts of the hillslope they are often referred to as ‘crest’ or ‘summit slope’. The angle of the slope increases downslope from the crest. Weathering and soil creep are believed to be the two most active processes which have caused the formation of the summital convexity. The term summital convexity is often referred to as ‘waxing slope’ after it was used and popularized by a well known German geomorphologist W. Penck.

Fig 1.Convex slope

(2) Cliff or Free Face: It is a steep wall like slope often known as scarp or free face. It is mostly bare because of its steepness. No regolith or debris can accumulate at such a steep slope therefore all the material falls down and accumulate at the foot of the cliff. Since it remains free of any detritus or debris many geomorphologists call it ‘Free Face’. Cliff develops along the coast (due to undercutting by sea waves), in river valleys, glacial regions, in faulted landscapes and many other places. As previously mentioned weathered material falls down or slides and accumulates at the base of the free face. This accumulated feature if left untouched by transporting agents will grow in size and result into a new depositional feature which is called a talus slope. The angle of talus or scree slope is determined by the size of the weathered materials. If the material is course it would result in a steep slope but if it is fine gentler slopes would emerge. The consistent rise of the slope due to continuous supply of weathered material would slowly cover the lower parts of the free face and hence protect it from weathering. The talus slope would gradually grow higher causing reduction in the length of the free face. Eventually, a time would arrive when the entire cliff or free face would disappear and will be replaced by aggradational slope of lesser angle than the cliff.

Fig 2. Cliff or Free Face

Rectilinear slope: Itmostly lies below the cliff or free face. It is also known as constant slope since the slope angle largely remains constant. The slope is straight in profile. This element varies in its dimension and may also dominate the entire slope. This slope section is often found extending from summit to the bottom of a valley. On many other slope profiles the rectilinear section lies at the centre of the profile between a broader convexity on the upper part and a larger concavity on the lower section. The angle of rectilinear slope is determined by the angle of repose of the weathered fragments derived from the underlying rock and occupying its surface (Small,1978). Many geomorphologists are of the view that rectilinear slopes develop due to aggradation only but this is not the case in manyinstances. In the words of Small ( 1978)‘Rectilinear slopes can be essentially denudational forms, underlain by solid rock and bearing only a veneer of detritus,either at rest or moving very slowly downhill owing to disturbance by frost and other agencies’.These slopes are also referred to as debris controlled slope. Strahler (1950) used the term ‘repose slope’ to refer to such slopes.

Fig 3. Rectilinear slope

(4) Concave slope: The concave slope is observed at the lowest part of the slope profile. It is located at the bottom of a hillslope and extends further down to the river valley. It is usually covered with a layer of debris. The accumulated scree due to rainwash spreads the finer particles farther than the coarser ones leading to development of concavity. Penck used the term waning slope for such slopes. In arid and semi arid regions these slopes display a sharp break of gradient between the lower concave section and the steeper slopes above. In contrast, in humid conditions the basal concavity grades smoothly into the higher slopes.

Fig 4.Concave slope

The above discussed elements are assumed to be present in a standard hillslope but as has been pointed out by many geomorphologists and thinkers that all the four elements may not be noticed in a hillslope. One or more than one element may be missing from the slope profile owing to various reasons. A slope profile may have different combinations of elements and one can theoretically assume infinite number of such combinations. In reality, there are several combinations which occur frequently and appearvery common.

Three most common combinations of composite slopes have been discussed in the following section

a) The convexo-rectilinear-concave slope profile has upper convex, middle rectilinear and lower concave form. All three slope elements smoothly grade into each other and give a curving slope profile( Small,1978). This kind of slope profile is most common in the regions having weak rock type. The lowland England region is full of these types of slope profiles. Slope profileswith variations in the length of different slope elements are seen in the landscape. However, in those areas where there is vast diversity of rock types, where hard and soft rocks alternate or the region has witnessed several rejuvenations there is likelihood of very complex slope profile.

Composite slope form

Fig 5. 1 (a)Composite slope form

b) In regions where massive and thinly bedded weak strata alternate, where relief is high ,valleys are very deep and weathering is active, a very different composite profile will be seen. The profile will have several free faces and rectilinear slopes while summital convexity and basal concavity will be very limited in extent or completely absent (Small,1978). Where there is massive strata it would give rise to free face while weak and thinly bedded rock would result in rectilinear slopes(fig 5.1 b ) .

c) In arid regions where there is occurrence of hard crystalline rocks,a composite slope profile develops which consists of a free face on the upper section with a slope of 40° or more, a mid-section boulder controlled slope with slope angle of 25° or more(littered by rocks of different sizes) and a concave (pediment) slope in the lower section. The concave slope which lies at the bottom is very gentle with angles below 7 degrees.

Composite slope form

Fig 5. 1 (c)Composite slope form

Different arguments have been put forward by geomorphologist to account for the reasons of development of specific slope elements in a slope profile. A lot of attempts were made earlier to relate some processes to particular forms of slope. Processess like rainwash and soil creep are related to the development of convex and concave slope forms. N. M. Fenneman, an American physiographer explained the most common convexo- concave profiles in terms of the action of running water. He stated that the upper slopes have lesser runoff during rainfall and that the water would move as thin sheet. The water gets loaded with the particles as it moves down the slope. There is an increase of surface water downslope because of the addition of run-off from higher up the slope to that received from rainfall at lower sections of the slope. It can easily be imagined that there is greater erosion in the sections of slope that are away from the summit thus causing convexity to develop after a long period of time.Fenneman also stated that when the water reaches the lower part of the slope due to increase in the amount of surface water it gets concentrated into small channels which carve out numerous gullies and lead to the formation of concave curve.These arguments of Fenneman were opposed by many geomorphologists. They argued that Fenneman’s hypothesis does not take into account soil creep which is an important process in shaping the slopes. However, his hypothesis got support from the works of Horton (1945). Horton stated that on the upper section of the slope there is a certain distance from the crest where erosion by wash is absent because run-off lacks the required energy to erode. This section of sheet flow corresponds to the upper flatter parts of the slopes. Further down the slope with the increase in run-offthe section of no erosion is left behind and erosive action by sheet wash assumes importance.

Gilbert (1909) attributed soil creep as a major factor that causes rounding of hilltop summits and development of summital convexity. However,his ideas and arguments were consideredsimplistic. Lawson held rainwash as an important process on the upper slope. But he differed from Fenneman and stated that wash is most effective at the slope summit.

Besides the above scientists and geomorphologiststhere were many others who considered the process of soil creep and rainwash as the most important process that determines the slope form. They have come up with their own theories based on their understanding to provide proper explanation for particular slope forms. Besides soil creep and rainwash there are many factors which operate and their interplay is highly complex. So it can be said that there may be a few dominant factors that play key role in the formation of specific slope forms but there are a number of other factors that too play a role in the development of specific slope form.

References

• Bloom, A.L. 2012, Geomorphology: A Systematic Analysis of Late Cenozoic Landforms, Rawat Publication,New Delhi
• King,L.C. 1957. The Uniformitarian Nature of Hillslopes. Trans. Roy. Geol. Soc. Edinburg
• Strahler,A.N, 1950. Equilibrium theory of slopes approached by frequency distribution analysis. American Journal of Science, 248, 800-14
• Small, R.J.1978, The Study of Landforms, Cambridge university Press, Cambridge
• Sparks, B.W. 1986, Geomorphology,Longman, New York
• Wood, A.1942. The Development of Hillside Slopes, Proc.Geol. Ass. 53 Young, A.1972, Slopes , Oliver and Boyd, Edinburg