Landforms and their EvolutionClass 11 Fundamentals of Physical Geography Notes

Landforms and their Evolution

Imagine the Earth's surface as a giant sculpture. Over millions of years, it's constantly being reshaped. First, weathering breaks down the rocks. Then, geomorphic agents—like running water, groundwater, wind, glaciers, and waves—get to work. They perform two main actions:

1. Erosion: The process of wearing away and carrying off the weathered rock and soil.
2. Deposition: The process of dropping or laying down the eroded material in a new location.

Both erosion and deposition create and change the features we see on the Earth's surface.

So, what is a landform? In simple terms, a landform is a small to medium-sized piece of the Earth's surface, like a hill, a valley, or a beach. When you put several related landforms together, they create a landscape, which is a much larger area.

Every landform has a story. It has a beginning and evolves over time, changing its shape, size, and even the materials it's made of. This evolution is usually very slow, driven by the steady action of geomorphic agents. Think of the development of a landmass like the stages of a person's life: youth, mature, and old age.

Running Water

In humid regions with heavy rainfall, running water is the most powerful geomorphic agent shaping the land. It works in two ways:

• Overland flow: Water moving across the land surface in a thin sheet, causing sheet erosion.
• Linear flow: Water concentrated in channels, like streams and rivers.

Erosional landforms are most common where young, energetic rivers flow down steep slopes. As these rivers erode the land, their slopes become gentler, their speed decreases, and they begin to deposit more sediment. The gentler the slope, the more deposition occurs. Eventually, this process of erosion and deposition can reduce hills and valleys to nearly flat plains.

This process of landscape reduction happens in stages:

1. Overland flow carves out tiny channels called rills.
2. Rills grow into larger gullies.
3. Gullies deepen and widen, joining together to form a network of valleys.

In the early stages, rivers cut downwards aggressively. In the middle stages, they begin to cut sideways (lateral erosion), widening their valleys. Over a very long time, the landscape can be flattened into a lowland with a few resistant rock remnants called monadnocks. This nearly flat plain created by river erosion is called a peneplain.

Youth Stage

• Streams are few and not well-connected.
• Valleys are shallow and V-shaped, with narrow or no floodplains.
• The land between streams (divides) is broad and flat, often with marshes or lakes.
• Waterfalls and rapids are common where hard rock is exposed.

Mature Stage

• Streams are numerous and well-integrated into a drainage network.
• Valleys are deep and V-shaped, with wider floodplains.
• The divides between streams become sharper and narrower.
• Waterfalls and rapids have mostly disappeared.

Old Stage

• Tributaries are fewer and have gentle gradients.
• Rivers meander (flow in winding curves) freely across vast floodplains.
• Features like oxbow lakes and natural levees are common.
• The landscape is very flat, close to sea level, with broad divides that may have swamps and marshes.

Erosional Landforms Created by Running Water

Valleys

Valleys evolve from small rills to large gullies and finally into valleys. Their shape depends on the rock type.

• Gorge: A deep, narrow valley with very steep, straight sides. It is roughly the same width at the top and bottom. Gorges often form in hard rocks.
• Canyon: A deep valley with steep, step-like side slopes. It is typically wider at the top than at the bottom and commonly forms in horizontally layered sedimentary rocks.

Potholes and Plunge Pools

• Potholes are circular depressions drilled into the rocky bed of a river. They form when pebbles and boulders get trapped in a small depression and are swirled around by the current, grinding the rock away.
• Plunge Pools are very large, deep potholes that form at the base of a waterfall due to the sheer force of the falling water and the abrasion of rotating boulders.

Incised or Entrenched Meanders

While meanders are typical of rivers on gentle slopes, sometimes very deep, wide meanders are found cut into hard rock in areas with steep gradients. These are called incised or entrenched meanders. They indicate a period of rapid uplift of the land, which caused the river to cut down vigorously into its existing meandering path.

River Terraces

These are step-like surfaces on the sides of a valley that represent former floodplain or valley floor levels. River terraces form when a river cuts vertically down into its own deposits, leaving behind remnants of the old floodplain at a higher level. Paired terraces occur at the same elevation on both sides of the river.

Depositional Landforms Created by Running Water

Alluvial Fans

When a fast-flowing stream emerges from a mountain onto a flat plain, its velocity drops suddenly. It can no longer carry its coarse sediment (like gravel and boulders), so it dumps this material at the foot of the slope, forming a fan-shaped deposit called an alluvial fan. Streams often split into many smaller channels called distributaries as they flow over the fan.

Deltas

Deltas are like alluvial fans but form where a river enters a standing body of water, like a sea or lake. The river deposits its load of sediment, which accumulates as a low, cone-shaped landform if it's not washed away by currents. Delta deposits are well-sorted, with the coarsest materials settling first and finer silts and clays carried further out.

Floodplains, Natural Levees, and Point Bars

• A Floodplain is the flat area of land alongside a river, created by the deposition of sediment during floods. The riverbed itself is the active floodplain, while the area above the banks that gets flooded is the inactive floodplain.
• Natural Levees are low ridges of coarse sediment that build up along the banks of a river during floods. When the river overflows, it loses energy and drops its heaviest material first, right next to the channel.
• Point Bars (or meander bars) are deposits of sediment that accumulate on the inside (concave side) of a meander bend, where the water flow is slower.

Meanders

A meander is a loop-like bend in a river's course. It's a channel pattern, not a landform itself, that is very common on large floodplains and deltas. Meanders form because water flowing over a gentle slope tends to erode the outer bank of a curve (where the current is fastest) and deposit sediment on the inner bank (where it's slowest). Over time, these curves become more exaggerated.

• The eroding outer bank is called the cut-off bank.
• The depositing inner bank is a gentle slope.
• Eventually, a meander loop can become so extreme that the river cuts a shorter path across the narrow neck of land, abandoning the old loop, which becomes a crescent-shaped ox-bow lake.

Groundwater

Groundwater's main work is not physical erosion but chemical action. It is most effective in regions with rocks like limestone and dolomite, which are rich in calcium carbonate. Rainwater absorbs carbon dioxide from the air, becoming a weak acid that can easily dissolve these rocks. This creates a unique landscape called Karst topography, named after the Karst region in the Balkans.

Erosional Landforms in Karst Regions

• Sinkholes (or Swallow Holes): These are circular openings on the surface that are often funnel-shaped. They form when limestone is dissolved from the surface downwards (solution sinks) or when the roof of an underground cave collapses (collapse sinks).
• Valley Sinks (Uvalas): When several sinkholes merge, they form long, wide trenches called uvalas.
• Lapies: As the surface is eaten away by pits and trenches, it becomes a highly irregular landscape of sharp points, grooves, and ridges. This terrain is called lapies. Eventually, this can be smoothed into limestone pavements.
• Caves: As groundwater seeps down and moves horizontally along bedding planes in limestone, it dissolves the rock to form underground passages and chambers called caves. A cave with openings at both ends is called a tunnel.

Depositional Landforms in Karst Regions

These features form inside caves when water rich in dissolved calcium carbonate evaporates or loses its carbon dioxide, causing the mineral to be re-deposited.

• Stalactites: These are icicle-like formations that hang from the ceiling of a cave. They are formed by dripping water.
• Stalagmites: These are formations that rise from the floor of a cave, created by water dripping from a stalactite directly above.
• Pillars (or Columns): When a stalactite and a stalagmite meet and fuse together, they form a pillar.

Glaciers

Glaciers are slow-moving masses of ice. They can be vast ice sheets covering a continent (continental glacier) or river-like flows in mountain valleys (valley glaciers). They move under the force of gravity, and their immense weight makes them incredibly powerful agents of erosion through two main processes:

• Plucking: Freezing onto and pulling away large blocks of rock.
• Abrasion: Scraping and grinding the bedrock with the rock fragments embedded in the ice.

Glaciers can transform high mountains into low hills and plains.

Erosional Landforms Created by Glaciers

• Cirque: A deep, bowl-shaped basin with steep, concave walls at the head of a glacial valley. It is the starting point where a glacier forms. After the ice melts, a cirque lake or tarn often remains.
• Horns and Arêtes: When several cirques erode backward toward each other from different sides of a mountain, they can carve out a sharp, pointed peak called a horn. The sharp, knife-like ridges separating cirques or glacial valleys are called arêtes.
• Glacial Valleys/Troughs: Unlike V-shaped river valleys, glacial valleys are characteristically U-shaped, with a broad floor and steep, smooth sides.
  • Hanging Valleys: Smaller tributary glaciers don't cut as deeply as the main glacier. After the ice melts, their valleys are left "hanging" high above the main valley floor, often with a waterfall.
  • Fjords: In coastal regions, deep glacial troughs can be flooded by the sea, creating long, narrow, steep-sided inlets called fjords.

Depositional Landforms Created by Glaciers

• Glacial Till: The unsorted mix of clay, sand, gravel, and boulders deposited directly by melting ice.
• Outwash Deposits: Sediment carried and deposited by meltwater streams from a glacier. This material is sorted and stratified, with rounded rock fragments.
• Moraines: Ridges of glacial till.
  • Lateral Moraines: Form along the sides of a glacier.
  • Medial Moraine: A ridge formed where two glaciers merge and their lateral moraines join.
  • Terminal Moraine: A ridge of till deposited at the very end (toe) of a glacier, marking its furthest advance.
  • Ground Moraine: A thin layer of till deposited over the valley floor as a glacier melts and retreats.
• Eskers: Long, winding ridges of sand and gravel. These are formed by streams that flowed in tunnels underneath a glacier. When the ice melted, the stream bed deposit was left behind as a ridge.
• Outwash Plains: Broad, flat plains of sand and gravel deposited by meltwater streams at the foot of a glacier.
• Drumlins: Smooth, oval-shaped hills made of glacial till. The long axis is parallel to the direction of ice movement. The steeper, blunter end (the stoss end) faces the direction from which the ice came.

Waves and Currents

Coastal areas are incredibly dynamic, with waves being the primary force of change. The type of coast determines whether erosion or deposition is the dominant process.

High Rocky Coasts (Submerged Coasts)

These coasts are typically rugged and steep, where the land has been submerged by a rise in sea level.

• Erosion is dominant. Waves crash with great force, carving the land into cliffs.
• Over time, cliffs retreat, leaving behind a wave-cut platform.
• Erosion eventually supplies sediment that can be deposited by longshore currents to form beaches, offshore bars, and spits (ridges of sand connected to the land at one end).
• A lagoon can form when a bay is blocked by a bar.

Low Sedimentary Coasts (Emerged Coasts)

These coasts have a gentle slope and are formed where land has emerged from the sea.

• Deposition is dominant.
• Rivers bring large amounts of sediment, building deltas and coastal plains.
• Waves and currents easily move the bottom sediment to build bars, barrier bars, spits, and lagoons.
• The coastline is generally smooth, with marshes and swamps.

Erosional Landforms Created by Waves

• Cliffs and Terraces: Sea cliffs are steep rock faces formed by wave erosion at their base. A wave-cut terrace is the flat platform at the foot of a cliff, representing the area where the cliff used to be.
• Sea Caves: Wave action can hollow out weaker sections of rock at the base of a cliff to form sea caves.
• Sea Stacks: When a sea cliff retreats, it may leave behind isolated, resistant pillars of rock in the sea, known as sea stacks.

Depositional Landforms Created by Waves

• Beaches and Dunes: Beaches are accumulations of sand or shingle (small pebbles) along the shoreline. They are temporary features, built up and eroded by seasonal changes in wave action. Wind can then transport sand from the beach inland to form sand dunes.
• Bars, Barriers, and Spits: These are ridges of sand or shingle formed in the sea.
  • An off-shore bar is submerged.
  • A barrier bar is exposed above the water.
  • A spit is a bar connected to the land at one end, often extending across the mouth of a bay.

Winds

In hot deserts, wind is a major agent of erosion and deposition. The dry, barren ground heats up quickly, creating turbulent air movements. Wind performs its work through:

• Deflation: Lifting and removing loose particles like dust and sand.
• Abrasion: Using sand particles as tools to sandblast and wear away rock surfaces.
• Impact: The sheer force of wind-blown sand hitting a surface.

However, wind is not the only agent at work in deserts. Occasional torrential rains cause sheet floods that wash away weathered debris and are responsible for much of the mass erosion.

Erosional Landforms Created by Wind

• Pediments and Pediplains: Pediments are gently sloping rocky floors at the base of mountains in a desert. They are formed by erosion of the mountain front. As the mountain front retreats (a process called backwasting), the pediment expands. Over time, mountains can be reduced to isolated remnants (inselbergs) on a vast, low-relief plain called a pediplain.
• Playas: In desert basins, drainage flows towards the center. Sediment deposition creates a nearly level plain. After rain, a temporary, shallow lake called a playa may form. When the water evaporates, it often leaves behind salt deposits, creating an alkali flat.
• Deflation Hollows: These are shallow depressions created where wind has blown away the surface material.
• Mushroom, Table, and Pedestal Rocks: In these formations, wind abrasion has eroded the base of a rock outcrop more than the top, creating shapes that resemble a mushroom, table, or pedestal. This happens because the wind can carry more sand (its cutting tools) closer to the ground.

Depositional Landforms Created by Wind

Wind is an excellent sorting agent, separating particles by size. When the wind slows down, it drops the sediment it carries, creating various types of sand dunes.

• Barchans: Crescent-shaped dunes with their "wings" pointing downwind. They form where the wind direction is constant and there is a moderate supply of sand.
• Parabolic Dunes: Also crescent-shaped but are the reverse of barchans; their wings point upwind. They often form where vegetation partially covers the sand.
• Seif Dunes: Long, single-winged dunes that can form when wind conditions shift.
• Longitudinal Dunes: Long, low ridges of sand aligned parallel to the constant wind direction. They form when the sand supply is poor.
• Transverse Dunes: Long ridges aligned perpendicular to the wind direction. They form where the wind is steady and the sand source is an elongated feature at a right angle to the wind.