Landforms: Fluvial, Arid and Glacial

Agents of Erosion and the Concept of Geomorphic Cycles

Landforms produced by erosion and deposition reflect the interaction of structure, process, and time. The dominant external agents of erosion are running water, glaciers, wind, and waves, each producing a characteristic suite of landforms. Running water is the most universal agent: it operates not only in humid fluvial environments but also contributes to landform development in glacial and desert settings. The landforms produced by any agent pass through a sequence of stages broadly described as youthful, mature, and old age, corresponding to progressively lower gradients, reduced erosional energy, and increasing dominance of deposition. Rejuvenation - triggered by tectonic uplift or a fall in sea level - reintroduces youthful characteristics into an old landscape, producing distinctive landforms that record the disrupted cycle.

Karst Topography

Karst topography develops in regions underlain by limestone, chalk, or other soluble carbonate rocks. The name derives from the Karst region of Yugoslavia bordering the Adriatic Sea, which is the type locality for this landscape. Limestone is an extremely weak rock when exposed to water carrying dissolved carbon dioxide, which forms weak carbonic acid capable of dissolving calcium carbonate. The defining characteristic of karst regions is the near-total absence of surface drainage: rivers disappear underground through joints and bedding planes enlarged by chemical solution, flow through cave systems, and reappear as springs where they meet impermeable country rock. Karst topography is most intensely developed in humid tropical and subtropical climates where rainfall is abundant and temperatures are high enough to accelerate chemical weathering.

Surface features of karst begin with sinkholes or dolines, which are roughly circular depressions formed where surface water concentrates and dissolves the limestone beneath, widening joints into funnels. A dolina is a larger and more developed depression. When several dolinas merge through continued solution, the composite depression is called a uvala. The largest elongated karst depressions, often associated with structural features such as faults, are called poljes and may be tens of kilometres in extent. Limestone that is exposed at the surface is characterised by a pavement of rectangular blocks called clints, separated by solution-enlarged joint gaps called grikes. The combined surface of clints and grikes constitutes limestone pavement.

Underground, the dissolution of limestone by percolating water creates an extensive system of caves, passages, and chambers. Water dripping from cave ceilings loses carbon dioxide and deposits calcium carbonate on the roof, building downward-growing spires called stalactites. Where the calcium carbonate-laden water drips to the cave floor, it builds upward-growing columns called stalagmites. When a stalactite and a stalagmite meet and fuse, they form a limestone pillar. Stalactites tend to be thin and pointed while stalagmites are typically thicker and more rounded. Notable karst regions include the original Karst of Yugoslavia, the Batu Caves of Malaysia, and the Nokrek Biosphere Reserve in the Garo Hills of Meghalaya, which is also famous as the habitat of the hoolock gibbon, the only ape found in India.

Fluvial Topography: Mechanisms of River Erosion

Running water erodes through four distinct mechanisms that operate simultaneously. Solution or corrosion is the chemical dissolution of soluble minerals directly into river water, particularly important in limestone regions. Abrasion or corrasion is the mechanical grinding of the river bed and banks by the sediment load carried by the water, and is divided into lateral abrasion, which widens valleys, and vertical abrasion, which deepens them. Hydraulic action is the sheer force of moving water that loosens and detaches rock particles from the channel, particularly effective along joints and cracks. Attrition is the mutual wearing down of sediment particles as they collide and rub against each other during transport, progressively reducing their size and rounding their edges.

Youthful Stage: Landforms of Active Downcutting

In the youthful stage the river flows on steep slopes with high velocity, giving it a large carrying capacity sufficient to transport coarse bedload material including boulders. Vertical erosion is dominant. Eddy currents within the river create circular motion in pebbles and boulders that drill into the bedrock, forming deep cylindrical depressions called potholes. Continued vertical erosion produces narrow, deep V-shaped valleys called gorges. An extended gorge developed in arid and semi-arid regions where resistant rock is exposed to incision by a river is called a canyon. The Grand Canyon of the United States, carved by the Colorado River as it crosses the Sonoran Desert of Arizona, is the classic example, while the Kali Gandaki gorge in the Himalayas, approximately 7,000 metres deep, is considered the deepest gorge in the world.

Waterfalls form where a river crosses a band of resistant rock overlying weaker rock, or at a fault scarp. Differential erosion removes the softer rock faster, creating a vertical drop. Smaller waterfalls are called rapids; larger ones are called cataracts. The five major cataracts on the River Nile historically impeded navigation on the upper Nile. At the base of waterfalls, the hydraulic impact of the falling water creates a deep depression called a plunge pool, which is a tourist attraction and marks the point of maximum erosive energy at that site. The highest waterfall in India by height is Jog Falls on the Sharavati River in Karnataka, while the largest by volume is Dhuandhar Falls on the Narmada at Jabalpur.

River capture or river piracy occurs when a more energetic river erodes headward into the watershed of a neighbouring drainage basin, breaching the divide and diverting the headwaters of the captured river into its own channel. The point at which capture occurs is called the elbow of capture, and the gap left in the former watershed after capture is called a windgap or wind gap, which can serve as a mountain pass for roads and railways. The beheaded river downstream of the capture point is called a misfit, flowing in a valley too large for its reduced discharge. In old age, river capture can also occur through the intersection of meanders, as the narrow neck of land between two meander loops is eroded through and the river straightens its course.

Mature Stage: Landforms of Lateral Erosion and Deposition

In the mature stage the gradient decreases, velocity falls, and the river shifts from dominant vertical cutting to lateral or sideways erosion. The narrow V-shaped valley of the youthful stage widens into a broader, shallower valley. At the foot of mountains the river, suddenly losing velocity as it crosses from steep to gentle slopes, drops its coarsest sediment in the form of an alluvial fan: a cone-shaped depositional landform composed of poorly sorted gravels, sands, and boulders spreading outward from the mountain front. In the Himalayan foothills of India, the alluvial fan zone is divided into the Bhabhar belt, where rivers lose themselves in the highly porous coarse gravels and flow largely underground with only trees with deep root systems surviving, and the Terai belt immediately south, where the underground water re-emerges as springs creating a marshy, waterlogged zone of dense forest, high biodiversity, and fertile soil. The Terai belt once extended continuously along the Himalayan foothills and hosts important national parks including Jim Corbett in Uttarakhand, Dudhwa in Uttar Pradesh, Kaziranga in Assam, and Manas in Assam.

Interlocking spurs are the projecting ridges of high ground that alternate on either side of a valley in the mature stage, giving the valley a zigzag appearance when viewed from above or below. Meanders are sinuous bends developed in the longitudinal profile of a river as it flows in increasingly winding loops across a low-gradient surface. Though meanders can occur at any stage, they are most fully developed in the mature and old age stages. Within a meander, the outer bank experiences erosion and forms a steep, concave river cliff while the inner bank experiences deposition and forms a gentle, convex slip-off slope. The asymmetry of deposition and erosion within each bend continuously reinforces and amplifies the sinuosity of the channel.

Old Age Stage: Landforms of Dominant Deposition

In the old age stage the river flows across a nearly flat floodplain with very low velocity. Vertical erosion has essentially ceased and deposition is the dominant process. Ox-bow lakes form when the neck of a meander loop is cut through and the river adopts a straight course, leaving behind a curved, crescent-shaped abandoned channel filled with water. Kanwar Lake in Bihar is the largest ox-bow lake in India, formed by river capture. Wular Lake and Dal Lake in Kashmir occupy ox-bow-like depressions formed by local topographic factors rather than meander cut-off.

The floodplain is an extensive, nearly level depositional surface flanking the river, built up over time by the periodic deposition of alluvium during floods. Natural levees are ridges of coarser sediment deposited immediately adjacent to the channel when floodwaters spread over the floodplain and rapidly lose velocity. They naturally raise the river bed above the surrounding plain in many rivers, including the Ganga and the Brahmaputra, increasing flood hazard. Artificial embankments are typically constructed on top of natural levees to contain flood events. In India the floodplain is divided into bangar, composed of older alluvium deposited during earlier flood events, and khadar, made of newer, finer alluvium deposited by more recent flooding and being the more fertile of the two.

A delta is a wholly depositional landform built at the mouth of a river where it enters the sea or a large water body, as the sudden drop in velocity causes the river to deposit its sediment load. The river channel is divided into multiple distributaries as sediment chokes and blocks the main channel. Three major types of delta are recognised. An arcuate or fan-shaped delta forms when river water and seawater are of similar density, causing the river to begin depositing immediately at its mouth and fan outward symmetrically. The Nile delta, the Ganga-Brahmaputra delta, the Indus delta, the Godavari delta, the Mekong delta, and the Rhine delta are arcuate deltas. A bird-foot or finger delta forms when river water is less dense than seawater, allowing the river to flow into the sea as a fast-moving underflow before beginning to deposit, with the resulting distributary system resembling the spread toes of a bird. The Mississippi delta is the classic example. An estuary forms where the mouth of a river is drowned by a rise in sea level or coastal submergence, producing a funnel-shaped tidal inlet rather than a delta. The Narmada, the Tapi, and the Amazon form estuaries rather than deltas.

Conditions favouring delta formation include high sediment supply, which is greatest in rivers draining steep mountain ranges such as the Himalayas, a long river course that allows the accumulation of sediment, a shallow and sheltered coastline that prevents wave action from dispersing deposited material, geological stability of the coastal zone without recent submergence, sediment of intermediate grain size that is neither too fine to be swept away by tides nor too coarse to be carried to the coast, and the absence of large riparian lakes that would act as sediment traps. East-flowing rivers of India form large deltas because they are longer, drain the soft alluvium of the Indo-Gangetic plain, and their coasts are sheltered and shallow. West-flowing peninsular rivers are short, swift, drain hard rock, and their coasts face high-energy monsoon waves, leading them to form estuaries rather than deltas.

River Rejuvenation

River rejuvenation occurs when a river's base level falls due to tectonic uplift of the land, a eustatic fall in sea level, or a reduction in the volume of ice or water reservoirs. Rejuvenation reinvigorates the river's erosive capacity, producing a new set of youthful landforms superimposed on the old-age landscape. Incised or entrenched meanders form when a rejuvenated river cuts vertically downward into its existing meander pattern, preserving the sinuous plan form of the meander in a deeply incised valley. River terraces are flat benches cut into the valley sides as the river erodes downward through its own former floodplain, leaving elevated remnants of the old valley floor on either side of the new, lower channel. Uplifted peneplains are former nearly flat erosional surfaces that have been tectonically raised above the current base level and now stand as elevated plateaus. The presence of multiple terraces at different heights records multiple episodes of rejuvenation.

Arid and Desert Landforms

Deserts are regions of extremely low precipitation, typically receiving less than 250 millimetres annually. Hot deserts develop in the subtropical high-pressure belts between roughly 20 and 35 degrees latitude where descending air is compressed, warmed, and dried, reinforced by the influence of cold offshore ocean currents that prevent moisture from reaching the coast and trade winds that blow from the hot interior toward the coasts. Cold deserts develop in rain shadow zones on the leeward side of major mountain ranges where descending air has lost most of its moisture. Wind is the most active agent of erosion in deserts because the absence of vegetation and moisture leaves the surface exposed to the full force of the wind.

Wind erodes through three mechanisms. Deflation is the lifting and removal of loose fine particles by the wind, leaving behind a surface of coarser material called desert pavement or reg and creating shallow depressions called deflation hollows. Where deflation exposes groundwater at the surface, oases form. The Qattara Depression at minus 133 metres in Egypt is the deepest deflation hollow on earth. Abrasion or sandblasting is the mechanical erosion of rock surfaces by wind-driven sand particles, which act as natural sandpaper and are most effective close to the ground where sand is heaviest. Attrition is the mutual wearing down of sand particles against each other during transport, rounding them and reducing their size.

Erosional landforms in deserts include deflation hollows, mushroom rocks, yardangs, zuegens, mesas, and ventifacts. Mushroom rocks form because wind-driven sand is most concentrated close to the ground, eroding the base of isolated rocks faster than their tops, producing a narrow-waisted, top-heavy mushroom profile. Yardangs develop where alternating bands of hard and soft rock are oriented vertically along the prevailing wind direction, with the softer rock eroded into furrows and the harder rock standing as ridges. The furrows between yardangs serve as natural corridors or highways across the desert. Zuegens form where horizontal bands of hard and soft rock are exposed to wind and differential erosion, producing an irregular tabular relief. Mesas are flat-topped, steep-sided erosional remnants capped by a resistant rock layer, typically basaltic, that protects the weaker rocks beneath from erosion. As mesas are progressively reduced they become smaller flat-topped remnants called buttes. Mesas and buttes in the Deccan plateau region of India are separated by canyons. Ventifacts are individual rock fragments shaped and polished by wind abrasion into flat, angular faces; a rock with three wind-polished faces is specifically called a dreikanter.

Desert depositional landforms are dominated by sand dunes, which are mounds and ridges of wind-blown sand in characteristic shapes determined by wind direction and speed. Seifs or longitudinal dunes are dunes that lie parallel to the prevailing wind direction, formed when high-velocity winds pick up sand and deposit it on the flanks, producing long, knife-edged ridges named after the Arabic word for sword. The sand-free corridor between adjacent seifs is called a gassi. Barchans or barchan dunes are crescent-shaped dunes that form transverse to the wind direction around a localised obstruction. Their windward side is convex and gently sloping while the lee side is concave, steep, and characterised by two projecting horns. Barchans are live or mobile dunes that migrate downwind, contributing to desertification as they encroach on neighbouring land. Both seifs and barchans are found in western Rajasthan. Star dunes form where winds blow from multiple directions, producing a central peak with radiating arms.

Glacial Landforms

Glaciers are bodies of ice that form where snowfall over many years exceeds melting, and that move under their own weight by plastic deformation and basal sliding. They are powerful erosional agents, capable of excavating deep valleys, rounding and polishing bedrock, and transporting enormous quantities of rock debris. Glacial erosion operates through abrasion, where rock fragments embedded in the base of the glacier scrape and grind the underlying bedrock, and plucking or quarrying, where the glacier freezes to bedrock and wrenches away blocks along joints. The combined effect produces a landscape of great topographic diversity.

Erosional landforms produced by mountain or alpine glaciers begin at the head of the glacier where snow accumulates in a bowl-shaped hollow called a cirque or corrie, formed by rotational erosion of the back wall and floor. The floor of a cirque is often occupied by a small lake called a tarn after the glacier retreats. Where two or more cirques erode into a ridge from opposite sides, the intervening wall is sharpened into a narrow, knife-edged ridge called an arête. Where three or more cirques cut into a mountain from all sides, the central summit is reduced to a steep, pyramidal peak called a horn, of which the Matterhorn in the Alps is the archetypal example. Glacial troughs are the U-shaped valleys carved by valley glaciers moving downslope, characterised by steep, nearly vertical sides and a flat floor resulting from the glacier's tendency to widen as well as deepen the valley. When the glacier retreats, smaller tributary valleys are left hanging high on the walls of the main trough, called hanging valleys, often with waterfalls descending to the main valley floor. Where glacial troughs are flooded by the sea following the melting of the ice, they form deep, narrow coastal inlets called fjords, found along the coasts of Norway, Chile, New Zealand, and British Columbia.

Depositional landforms result from the dumping of the unsorted mixture of rock fragments, clay, sand, and boulders called till or boulder clay that the glacier carries. Ground moraine is a layer of till deposited beneath the glacier as it moves. Terminal or end moraines are ridges of till dumped at the snout of a glacier at the maximum extent of its advance, forming low, curved ridges across the landscape. Lateral moraines are ridges of debris accumulating along the sides of a valley glacier from rockfall off the valley walls. When two valley glaciers merge, their inner lateral moraines combine to form a medial moraine running down the centre of the combined glacier. Drumlins are streamlined, egg-shaped mounds of till, always with their blunt end facing the direction from which the glacier came and their tapered end pointing in the direction of ice movement; they typically occur in swarms across formerly glaciated lowlands. Erratics are isolated boulders of a rock type foreign to the local bedrock, transported by glaciers from their source area and deposited far from their origin when the ice melted, serving as evidence of former glacier extent and direction. Eskers are long, sinuous ridges of sorted fluvio-glacial sand and gravel deposited by meltwater streams flowing in tunnels within or beneath the glacier, preserved after the ice melts as winding ridges crossing the landscape. Kettles are depressions formed where blocks of stagnant ice were buried by outwash sediment and then melted, leaving behind a hollow that may be filled with water to form a kettle lake.

Coastal Landforms

Coastal landforms result from the interaction of wave energy, tidal action, and the geological structure of the coastline. Waves are generated by wind friction on the ocean surface and release their energy at the coast. The zone of breaking waves is where most erosion and deposition occur. Wave erosion operates through hydraulic action, where the sheer force of waves compresses air in cracks and explosively dislodges rock; abrasion, where wave-carried rock fragments grind the cliff face; attrition, where rock fragments wear each other down; and solution, particularly effective on calcareous rocks.

Erosional coastal landforms develop where waves attack a rock cliff. The base of the cliff is undercut by hydraulic action and abrasion, forming a wave-cut notch, until the overhanging cliff face collapses. Repeated undercutting and collapse retreats the cliff face landward, leaving behind a gently sloping bedrock platform called a wave-cut platform or shore platform extending seaward from the base of the cliff. Where alternate bands of hard and soft rock approach the coast at right angles, differential erosion produces sea caves in the soft rock flanked by headlands of hard rock. When a cave is cut through a headland it forms a natural arch. Further erosion causes the roof of the arch to collapse, leaving a detached pillar of rock called a stack. Stacks are eventually reduced to wave-washed stumps. Geos are narrow, steep-sided inlets cut along vertical joints by wave action.

Depositional coastal landforms include beaches, spits, bars, and tombolos. Beaches are accumulations of sand or pebbles in the nearshore zone, deposited by waves and longshore drift, the process by which sediment is transported along the coast by waves approaching at an oblique angle. A spit is an elongated ridge of sand or gravel projecting from the coast into the sea or across the mouth of a bay, built by longshore drift and typically with a curved or hooked end where waves refract around its tip. A bar is a ridge of sediment connecting a spit to the opposite shore or lying submerged offshore. A tombolo is a bar of sediment connecting the mainland to an offshore island. A lagoon is the sheltered water body enclosed between a barrier spit or bar and the mainland coast. Emergent coastlines, where the land has risen relative to the sea, expose former wave-cut platforms as raised beaches above the current shoreline. Submergent coastlines, where the land has sunk or the sea has risen, drown former valleys to create rias and former glacial troughs to form fjords. The eastern coast of India is an emerged coastline that has been built up by long-term sediment deposition, while the western coast is a submerged coastline where river valleys have been drowned by the sea, explaining why western rivers form estuaries rather than deltas and why the western coast lacks extensive continental shelf compared to the east.