Elements of Slope

2 years ago

Atiqur Rahman

11 minutes

Slopes have several 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 believed that a standard composite slope profile consists of four elements. The uppermost part is a convex slope or crest. Below the crest is a scarp or free face followed by a rectilinear slope, and at last, at the bottom is the concave slope. This is the most common composite slope profile; however, in reality, they are found in several combinations.

All the elements may not occur in a single slope profile. The occurrence of these elements in varying combinations depends on factors like the structure of the rock, the nature of the 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 the 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 the convex profile of the 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 the ‘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.

2. Cliff or Free Face

It is a steep wall-like slope often known as a 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 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 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 in a new depositional feature which is called a talus slope.

The size of the weathered materials determines the angle of the talus or scree slope. If the material is coarse, it would result in a steep slope, but if it is fine gentler slopes would emerge. The consistent rise of the slope due to the 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 a 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 an aggradational slope of a lesser angle than the cliff.

3. Rectilinear Slope

It mostly lies below the cliff or free face. It is also known as a 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 often extends from the 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 the 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 many instances.

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 slopes. Strahler (1950) used the term ‘repose slope’ to refer to such slopes.

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 the 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.

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 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 an infinite number of such combinations. In reality, several combinations occur frequently and appear very common.

The three most common combinations of composite slopes have been discussed in the following section:

a) The convex-rectilinear-concave slope profile has upper convex, middle rectilinear and lowers concave forms. All three slope elements smoothly grade into each other and give a curving slope profile (Small,1978).

This slope profile is most common in regions with weak rock types. The lowland England region is full of these types of slope profiles.

Slope profiles with variations in the length of different slope elements are seen in the landscape. However, in those areas where there is a vast diversity of rock types, where hard and soft rocks alternate, or the region has witnessed several rejuvenations, there is a likelihood of a very complex slope profile.

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 are massive strata, it would give rise to a free face, while weak and thinly bedded rock would result in rectilinear slopes (fig).

c) In arid regions where there is the 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 a 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.

Geomorphologists have put forward different arguments to account for the reasons for the development of specific slope elements in a slope profile. A lot of attempts were made earlier to relate some processes to particular forms of the slope. Processes 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 a thin sheet. The water gets loaded with 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 the slope that are away from the summit, thus causing convexity to develop after a long period.

Fenneman also stated that when the water reaches the lower part of the slope due to an increase in the amount of surface water, it gets concentrated into small channels, which carve out numerous gullies and lead to the formation of a concave curve.

Many geomorphologists opposed the arguments of Fenneman. They argued that Fenneman’s hypothesis does not consider soil creep, which is an important process in shaping 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 sheet flow section corresponds to the slopes’ upper flatter parts. Further down the slope, with the increase in run-off, the 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 the rounding of hilltop summits and the development of summital convexity. However, his ideas and arguments were considered simplistic. Lawson held rainwash as an important process on the upper slope. But he differed from Fenneman, stating that wash is most effective at the sloping summit.

Besides the above scientists and geomorphologists, many others considered the process of soil creep and rainwash as the most important process that determines the slope form. They have come up with their theories based on their understanding to provide a proper explanation for particular slope forms.

Besides soil creep and rainwash, many factors operate, and their interplay is highly complex. So it can be said that there may be a few dominant factors that play a key role in the formation of specific slope forms, but several other factors also play a role in the development of specific slope forms.