Landforms and their Evolution

Learning Outcomes

1. Understand the processes of erosion and deposition by geomorphic agents.
2. Explore various landforms formed by running water, glaciers, wind, and groundwater.
3. Analyze the stages of development of landforms like valleys, mountains, and plains.
4. Study the evolution and transformation of landforms over time.

Once weathering processes have altered the surface materials of the earth, geomorphic agents like running water, glaciers, wind, and waves carry out erosion and deposition. These processes change the earth’s surface over time, leading to the formation of landforms—small to medium tracts of the surface. Let us explore the transformation and evolution of these landforms.

Running Water and Its Impact

Running water, particularly in humid regions, plays a key role in landform development. There are two main types of flow: overland flow, which moves as a sheet over land surfaces, and linear flow in streams and rivers, which shape valleys and landscapes.

1. Youthful Stage: In this stage, rivers flow swiftly over steep gradients, leading to the creation of V-shaped valleys and minor waterfalls. The water has high erosive power, leading to the deepening of valleys and the formation of narrow floodplains.
2. Mature Stage: With continued erosion, the river’s slope reduces. The valleys widen, and meanders begin to form. Lateral erosion becomes more significant than downward cutting, causing the valley walls to reduce in slope.
3. Old Age: Rivers meander freely over broad floodplains. Features such as oxbow lakes and natural levees are common. The landscape is reduced to low relief, with most of the terrain close to sea level.

Important Concept
Peneplain: This is an almost flat surface formed due to the continuous erosion by running water. It represents the final stage in landscape evolution.

Erosional Landforms Created by Running Water

1. Valleys: Valleys develop from small rills, which evolve into gullies and eventually form gorges or canyons. Gorges are narrow with steep sides, while canyons have step-like slopes.
2. Potholes and Plunge Pools: Circular depressions form in riverbeds due to abrasion by waterborne sediments. At the base of waterfalls, plunge pools develop from the impact of water and boulder rotation.
3. Incised Meanders: Rivers flowing over hard rocks in steep gradients cut deep meandering paths. These are known as incised or entrenched meanders.
4. River Terraces: River terraces are remnants of old riverbeds formed by the vertical erosion of streams. Paired terraces on either side of a river indicate earlier floodplain levels.

Depositional Landforms Formed by Running Water

1. Alluvial Fans: When rivers flow from higher elevations to plains, they deposit sediments in a cone-shaped formation called an alluvial fan. These deposits are coarse, and the river often splits into several distributaries.
2. Deltas: Unlike alluvial fans, deltas form when rivers deposit sediments at their mouths, where they meet a body of water like a sea. The sediments are well-sorted, with coarser materials settling first.
3. Floodplains: These are formed by the deposition of fine sediments over the riverbed during flooding. The areas between the riverbed and the riverbanks are termed inactive floodplains.
4. Natural Levees and Point Bars: Natural levees are ridges of coarse deposits along riverbanks, while point bars form on the inside of meanders where sediments are deposited in linear patterns.
5. Meanders: Meanders are loop-like curves in a river’s course. They form due to lateral erosion along the outer banks and deposition on the inner banks.

Groundwater and Karst Topography

In areas with permeable rocks like limestone, groundwater plays a major role in shaping landforms through erosion and deposition. The chemical processes of solution and precipitation create unique landforms in regions known as karst topographies.

1. Pools, Sinkholes, and Lapies: Sinkholes are circular depressions formed by the dissolution of limestone. Lapies are ridges created by differential solution activity along joints in limestone.
2. Caves: Groundwater flowing through limestone dissolves the rock, forming caves. When these caves have openings at both ends, they are called tunnels.
3. Stalactites and Stalagmites: These are formed by the deposition of calcium carbonate in caves. Stalactites hang from the ceiling, while stalagmites rise from the floor. When they meet, they form pillars.

Important Note
Karst Topography: Karst landscapes are characterized by the presence of sinkholes, caves, and underground rivers, typically formed in regions with soluble rocks like limestone.

Glacial Landforms

Glaciers are massive bodies of ice that move over land. Glaciers sculpt landforms through processes of abrasion and plucking, and they leave behind distinctive erosional and depositional features.

Erosional Landforms by Glaciers

1. Cirques: These are deep basins formed at the heads of glacial valleys. They often hold lakes after the glaciers melt, called tarn lakes.
2. Horns and Serrated Ridges: When glaciers erode the surrounding land from several directions, they leave sharp peaks known as horns. Serrated ridges or arêtes form between adjacent glaciers.
3. Glacial Valleys/Troughs: These are U-shaped valleys carved by glaciers. The steep sides and broad floors of these valleys often feature hanging valleys or truncated spurs.

Depositional Landforms by Glaciers

1. Moraines: These are ridges of debris deposited by glaciers. Terminal moraines form at the glacier’s end, while lateral moraines form along its sides.
2. Eskers: Eskers are sinuous ridges formed by streams flowing beneath glaciers. They are composed of sand and gravel deposited by meltwater.
3. Drumlins: These are elongated, oval-shaped hills composed of glacial till. The blunt end faces the glacier, while the tapered end points in the direction of movement.
4. Outwash Plains: Formed beyond the glaciers, these plains are made up of glacio-fluvial deposits like sand and gravel.

Coastal Landforms

The action of waves and currents along coastlines leads to both erosion and deposition, shaping a variety of coastal landforms.

Erosional Coastal Landforms

1. Cliffs and Terraces: Wave-cut cliffs form due to the constant pounding of waves against the land. The erosion leaves behind wave-cut terraces.
2. Sea Caves and Stacks: Erosion by waves creates sea caves at the base of cliffs. When these caves collapse, they form isolated rock remnants called stacks.

Depositional Coastal Landforms

1. Beaches and Dunes: Beaches are formed by the deposition of sand and pebbles along the shoreline. Dunes form inland from the beach, as wind transports and deposits sand.
2. Bars and Spits: Offshore bars are ridges of sand and gravel formed parallel to the coastline. When these bars rise above sea level, they form barrier bars. A spit is a narrow landform that extends from the shore into the sea.

Important Note
Barrier Bars and Spits: These features provide protection to the coast by absorbing the energy of waves and storms, reducing the risk of coastal erosion.

Wind and Desert Landforms

In deserts, wind is the primary agent of erosion and deposition, shaping various landforms. Deflation, abrasion, and the impact of wind-carried sediments create unique features in arid regions.

Erosional Desert Landforms

1. Pediments and Pediplains: These are gently sloping rocky surfaces formed by the erosion of mountains. As the mountains retreat, they leave behind pediplains, low-lying plains with isolated remnants of the mountain known as inselbergs.
2. Deflation Hollows and Caves: Deflation creates shallow depressions in the desert floor, known as deflation hollows. Continued wind erosion forms caves in resistant rocks.
3. Mushroom and Pedestal Rocks: Wind erosion creates unique rock formations like mushroom rocks, which have a narrow stalk and a broad cap, and pedestal rocks, which are elevated remnants of erosion.

Depositional Desert Landforms

1. Sand Dunes: These are formed by the deposition of windblown sand. Barchan dunes are crescent-shaped and form where the wind direction is constant. Seif dunes are long, narrow dunes that form due to shifting winds.
2. Loess Deposits: Loess is a fine, silt-like sediment deposited by the wind. These deposits cover large areas in desert and semi-arid regions.

Note
Deflation refers to the removal of fine particles from the desert surface by wind, leaving behind a stony or pebbly surface.

Comparative Table: High Rocky Coast vs. Low Sedimentary Coast

MCQ
Which of the following is a depositional landform created by running water?
(a) Canyon
(b) Cirque
(c) Delta
Answer: (c) Delta