Short Essay on the Development of the Shape of the River-Valley

In the upper part of the river’s course i.e. in the mountainous and hilly tracts, where bed-rock erosion is maximum, the channel deepens and forms a valley with vertical sides. But with the aid of other agencies, the sides of the valley are also cut and the valley becomes V-shaped.

If the rocks, in which the valley is cut, arc hard and resistant or porous, the sides will be steep and the valley will be narrow whereas if the rocks are soft and can be easily worn away, the valley will form a wide and open ‘V’.

Besides, the ‘V’ widens out as the valley reaches maturity more and more, where lateral erosion dominates and gets the valley floor more and more flattened as shown in the diagrams given below:

1. Escarpments

These are erosional land forms produced by rivers in regions composed of alternating beds of hard and soft rocks. The differential erosion of rocks give rise to a steep slope, called escarpment.

This is usually developed in dipping beds with harder rocks overlying the soft ones. Such structures are commonly referred to as homoclinal structures. Because of the variation in resistance to erosion, the hard rocks are eroded at a very slow rate whereas the soft ones are eroded comparatively at a rapid rate.

This gives rise to a steep slope on one side of the structure and a gentle slope on the other. The steep side is known as escarpment or scarp face.

There is a number of similar erosional features as follows:

(a) Hogbacks

These are sharp crested often sawtooth ridge formed of the upturned edge of a resistant rock layer of sand­stone, limestone or lava. In these cases, the beds dip at a high angle, roughly in excess of 45°, so that the dip slope becomes almost as steep as the escarpment. Thus the ridges have step slopes on both the sides, as may be seen in the follow diagram

(b) Cuestas

These erosional landforms are developed on resistant strata having low to moderate dip. This has the shape of an asymmetric low ridge or hill belt with a steep scarp on one side and a gentle slope on the other. Majority of the cuestas are associated with coastal plains.

(c) Mesa

In regions of horizontal strata in which isolated portion of land is capped by a hard, erosion-resistant bed, the erosional landforms produced will have an isolated table-land area with steep sides, commonly known as mesa.

(d) Butte

With continued erosion of the sides a mesa is reduced to a smaller flat-topped hill, known as butte. This represents the final remnant of a resistant rock layer in a region of horizontal strata. Such features in South Africa are called Kopjes.

2. Peneplain

Peneplain is the term used for the nearly smooth erosion surface of relatively low relief and altitude which covers a large area. This is a landform which is the ultimate of the erosion cycle and it evolves in a temperate humid climate.

Here the slope is so gentle that the river’s velocity down the slope is not sufficient to move the products of erosion and weathering.

The surface of the peneplains may be dotted here and there by a relatively few, small rounded hills, underlain by resistant rocks. These hillocks made up of harder, durable and resistant rocks on the surface of the peneplains are termed as Monadnocks; which are the remnants of the pre-existing country which could survive the vehemence of fluvial erosion.

3. Pediments

These are the erosional landforms mostly found in the arid regions. The landform is a gently sloping plain worn down by scarp retreat They are usually located in the piedmont area, fringing an upland massif, but sometimes in wider low land zones.

They may or may not be covered by a thin veneer of alluvium but scattered over it are rock fragments, some brought by running water from the adjacent mountains and some derived by weathering from those immediately beneath. Retreat of the mountain slope lengthens the pediment at its upslope edge.

The vast plains formed by the convergence of many pediments are termed as Pediplains. It consists of areas of pediment merging with surrounding alluvial fan and playa surfaces. Pediplains are often considered as analogous to peneplains.

Isolated concave-shaped masses of rock rising above a pediment or pediplain as round-topped mounds are known as Inselbergs (German term meaning island mountain). These are entirely analogous to monadnocks occurring on a peneplain and appear near the ultimate stage in the erosional cycle.

These residual hills are also known as Bornhardts. Typical bornhardts are prominent knobs of massive granite or similar plutonic rock with rounded summit and often showing exfoliation shells.

4. Wadies

These are the channels formed during rains in desert or arid regions and are also known as washes.

5. River-terraces

These are erosional features consisting of several step-like plains along the side of a river valley. As we know, in the absence of tectonic disturbances, the river attains a profile of equilibrium in duo course of time and there will be a marked growth of lateral erosion and deposition; the down-cutting of the valley gets stopped and the valley becomes nearly a plain.

But with the subsequent revival of tectonic movements when the source region is uplifted the gradient of the river increases resulting in the increase of the velocity of the river.

This will again start down-cutting of the valley floor and development of topographic features which are characteristic of a river in its youth stage. This phenomenon of development of youthful topographic features in a landmass having stable features is known as Rejuvenation.

Due to uplift of the source region, when erosion becomes more active, the river cuts through its old flood-plain resulting in the development of another valley beneath the t inner valley floor.

The floor of the older valley is left as a pair of river-terraces. With the subsequent uplift of the source region or lowering of the base-level of erosion the river cuts through the flood-plain again and there develops a new valley and a contemporary flood-plains.

The terraces are thus separated by steep-wall like escarpments. The highest terrace above the flood plain will, accordingly, be the oldest while the lowest will be youngest.

The number of teraces also reflect the number of periods of rejuvenation which have affected the river. There are three types of river-terraces as:

(a) erosion terraces, (b) accumulative terraces (c) composite ter­races. Erosion terraces are composed , almost entirely of bed rock and little or no alluvial veneer. Accumulative terraces are composed of alluvial deposits.

Composite terraces consist of the terrace-bench composed of alluvium while the bed-rock lies at the base of their cusp. The erosion terraces are also known as bed-rock terraces, whereas the accumulative terraces are also known as alluvial-terraces and the composite terraces as rock-defended terraces.

River-terraces are also classified as (i) Cyclic terraces and (ii) Non-cyclic terraces. In the cyclic terraces, the remnants on the opposite sides of the valley are paired whereas the non-paired terraces are characteristic of non-cyclic terraces.

Apart from giving rise to river terraces, rapid uplift of the land gives rise to incised or entrenched meanders and natural rock-bridges.

The meandering course of the river is still maintained but there is rapid deepening of the valley by the increased erosion, consequent on the uplift. As a result, the winding gorge digs deep into the soild rock beneath. Such gorges are called incised meanders.

If the neck of an incised meander is narrow, the intervening ridge of rock may be cut through down below from both the sides forming ultimately, what is called a natural bridge.

6. Badlands

These are rugged land surface of steep slopes, which are developed on weak clay formations of clay-rich regolith by fluvial erosion too rapid to permit plant growth and soil formation.

Clay and silt formation have little permeability, as such there is little infiltra­tion of water. Besides, they are weak and yield readily to the attack of running water.

Therefore, due to pronounced erosion, the terrain constituted of argillaceous rocks are intricately dissected by gullies and ravines. Such terrains are known as badlands.