Landforms and their EvolutionClass 11 Fundamentals of Physical Geography NCERT Solutions

In the youth stage of landscape evolution by running water, streams are vigorous and flow over steep gradients. The text states, "In the early stages, down-cutting dominates during which irregularities such as waterfalls and cascades will be removed."

The text defines a canyon as being "characterised by steep step-like side slopes." It also notes that canyons commonly form in horizontally bedded sedimentary rocks.

The text states that in rocks like limestones or dolomites, "surface water as well as groundwater through the chemical process of solution and precipitation deposition develop varieties of landforms." This process of solution is a form of chemical weathering.

The text describes that after the surface of limestone is eaten away by pits and trenches, it leaves the surface "extremely irregular with a maze of points, grooves and ridges or lapies."

A cirque is defined in the text as a "deep, long and wide troughs or basins with very steep concave to vertically dropping high walls at its head as well as sides." They are the most common landforms in glaciated mountains.

Incised meanders cut deep into hard rocks, indicating that a river with a steep gradient has been actively eroding downwards, a sign of a youthful or rejuvenated river. Meanders in plains of alluvium are formed on gentle slopes by lateral erosion and deposition, indicating a mature or old stage of river development where the river has a low velocity and meanders freely across its floodplain.

Valley sinks, or uvalas, are landforms found in karst (limestone) topography. They evolve when multiple, individual sinkholes and dolines form close to one another. Over time, due to the slumping of materials along their margins or the collapse of the roofs of underlying caves, these separate depressions join together to form a single, elongated, and wide trench.

Underground water flow is more common in limestone areas because limestone is rich in calcium carbonate, which is easily dissolved by carbonated rainwater. This chemical process of solution creates and enlarges cracks, joints, and bedding planes, allowing surface water to easily percolate downwards and flow through an underground network of channels and caves rather than as surface run-off.

Glacial valleys contain several linear depositional forms made of glacial till. These include:

Wind performs its task in deserts through deflation (lifting and removing dust), abrasion (sandblasting rock surfaces), and impact. However, wind is not the only agent responsible for erosional features. The text clearly states that many desert features also owe their formation to mass wasting and the action of running water in the form of torrential, short-lived sheet floods.

Running water is the most dominant geomorphic agent in both humid and arid climates, though its mode of action differs.

In humid regions, which receive abundant rainfall, running water is the primary agent of land degradation. It operates as both overland sheet flow and linear flow in streams and rivers. In its youth, a river's steep gradient allows for vigorous down-cutting, carving V-shaped valleys, gorges, and canyons. As it matures, lateral erosion widens the valleys and creates extensive floodplains. In old age, the river deposits vast amounts of sediment to form deltas and alluvial plains. These processes of erosion and deposition by running water are responsible for creating the most widespread landscapes in humid areas.

In arid climates, although rainfall is scarce, it often occurs as intense, torrential downpours. This leads to powerful but short-lived sheet floods. These floods, flowing over sparsely vegetated land, are highly effective at transporting weathered debris. They play a crucial role in the erosion of mountain fronts and the formation of pediments. Thus, even in deserts where wind is prominent, running water is a key force in shaping the landscape.

Limestones behave differently in humid and arid climates primarily due to the availability of water, which is the key agent in their chemical weathering. Limestone is composed mainly of calcium carbonate, which is highly soluble in carbonated water (rainwater that has absorbed carbon dioxide from the atmosphere). In humid climates, the abundance of water facilitates this chemical reaction, leading to rapid dissolution and the development of distinct landforms. In arid climates, the scarcity of water means that this chemical weathering process is significantly slower and less effective.

The dominant and almost exclusive geomorphic process in limestone areas is solution. This chemical process, along with subsequent precipitation and deposition, creates a unique landscape known as Karst topography.

The results of this process include a variety of erosional and depositional landforms:

Glaciers reduce high mountains into low hills and plains through immense and prolonged erosion. The sheer weight of the ice causes tremendous friction and pressure at its base, enabling it to drastically alter the landscape through two main processes: plucking and abrasion.

This erosional work carves distinctive landforms. At the mountain peaks, glaciers create bowl-shaped basins called cirques. When several cirques erode back-to-back, they form sharp, saw-toothed ridges called arêtes and pointed peaks called horns (like the Matterhorn and Mount Everest). As glaciers flow down pre-existing river valleys, they widen and deepen them into characteristic U-shaped troughs. Over vast periods, this continuous erosion lowers the mountain divides and scrapes away the peaks. Eventually, the mountain slopes are reduced to such a low gradient that the glaciers can no longer move, leaving behind a landscape of low, rounded hills and vast outwash plains formed from the deposits of meltwater streams.

A landscape is a large tract of the earth's surface that is composed of several related landforms. While a landform is a single feature with its own shape, size, and materials, a landscape is the broader picture formed by the combination of these individual landforms.

Based on the text, the two important aspects of the evolution of landforms are:

Complete reduction of the relief of a high landmass is not entirely possible according to the text. Through prolonged stream erosion, the landscape is lowered and flattened into a lowland of faint relief called a peneplain (an 'almost plain'). However, some low, resistant remnants of rock, called monadnocks, may remain standing out in this plain. Therefore, while the relief is drastically reduced, it is not completely eliminated.

Natural levees and point bars differ in their location and formation:

The basic differences between glacial valleys and river valleys are their shape and formation:

River alluvial plains and glacial outwash plains are both depositional landforms but differ in their formative agent and composition:

Till and alluvium are both types of sediment deposits, but they differ based on the agent of deposition and their characteristics:

High rocky coasts and low sedimentary coasts differ significantly in their characteristics, dominant processes, and resulting landforms:

Erosional features in desert landscapes are carved by a combination of wind action and sheet floods. These include:

This is a project-based question that requires local observation and application of the concepts learned in the chapter. The answer will vary depending on your geographical location. To complete this project, you should:

For example, if you live in the Northern Plains of India, you would identify the vast alluvial plains, rivers with meanders and floodplains, and the material would be alluvium. The dominant process would be the depositional work of running water (rivers like the Ganga).