Climatic Geomorphology and Morphogenetic Regions: Terminology for UPSC Geography Optional

Climatic geomorphology asks how climate, working through weathering, runoff, frost, wind and vegetation, zones the processes that shape the land and leaves forms that record it. Its vocabulary covers morphogenetic regions, zonal and azonal processes, relict and polygenetic landforms, duricrusts and the tropical family of etchplains, inselbergs and tors. These terms explain the laterite caps and granite hills of peninsular India.

Read each entry for the definition, then the mechanism, examples and sketch. UPSC has asked for a zonal classification of geomorphic processes (2015) and for the climatically controlled agents of erosion compared by their properties of matter (2011); both entries are written to answer length.

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Climatic Geomorphology and Its Zones

Climatic geomorphology

Climatic geomorphology is the branch of geomorphology that explains landforms through the control climate exerts on geomorphic processes, directly through temperature and moisture and indirectly through vegetation and soils, on the premise that each climatic type produces its own characteristic assemblage of landforms. It grew from German and French exploration of the tropics and was systematised in the 1940s–60s.

• Origins: observations by Ferdinand von Richthofen in China and Siegfried Passarge in Africa; developed by Julius Büdel, Carl Troll and Pierre Birot, and introduced to English readers by Louis C. Peltier (1950).
• Direct controls: temperature decides whether weathering is mechanical (frost, thermal) or chemical, and whether water acts as liquid or ice; rainfall amount, intensity and seasonality set runoff and mass movement.
• Indirect controls: vegetation intercepts rain, binds soil and slows runoff; soils and weathering mantles decide what material agents can move.
• Three working themes: landforms differ between climates; the difference arises from climatic control of processes; and certain diagnostic landforms reveal that relationship despite Quaternary change.
• Critique: David Ross Stoddart (1969) found the evidence impressionistic and argued that climate changed so often in the last two million years that equilibrium forms rarely developed; structure and tectonics often override climate. Renewed interest now comes from measured process rates and global warming.
• Indian contrast: monsoon slopes of the Western Ghats weather deeply, carry laterite and fail in landslides, while Ladakh’s cold desert is dominated by frost shattering, scree and dry alluvial fans.

Morphogenetic region (Louis C. Peltier)

A morphogenetic region is a large area within which a distinctive combination of geomorphic processes, determined by climate, operates and tends to produce a characteristic landform assemblage. Louis C. Peltier (1950) defined nine such regions by plotting mean annual temperature against mean annual rainfall and estimating, for each combination, the intensity of chemical weathering, frost action, mass movement, running water and wind.

• Key features: regions are defined by processes inferred from climate, not by measured landforms.
• Indian illustration: the high Himalaya falls in the glacial and periglacial regions, the wet Western Ghats and north-east approach the selva region, much of the Deccan plateau is savanna and the Thar is semi-arid to arid.
• Critique: boundaries are arbitrary, relict forms are ignored and the process intensities are estimates; the scheme underpinned Peltier’s periglacial cycle.
• Sketch: a graph with temperature on one axis and rainfall on the other, divided into nine labelled fields.

Morphoclimatic zones (Tricart and Cailleux; Büdel)

Morphoclimatic zones are global belts in which present and past climates, acting directly and through vegetation and soils, have produced distinctive process systems and landform associations. The two best-known schemes are those of Jean Tricart and André Cailleux (1965; English translation 1972) and of Julius Büdel (1948, 1963, 1977), who built them from process and history rather than from climate figures alone.

• Tricart and Cailleux: four major zones and their subdivisions: a cold zone (glacial, periglacial); a forested middle-latitude zone (maritime, continental, Mediterranean), where Pleistocene relicts dominate; a dry zone (steppe, semi-arid, desert, further divided by winter temperature); and a humid tropical zone (savanna, rainforest). They insisted that vegetation and palaeoclimate be part of the definition.
• Büdel’s climato-genetic geomorphology: a landscape is a stack of “relief generations” inherited from successive climates. His zones contrast a polar zone of excessive valley cutting, a mid-latitude zone of retarded valley formation dominated by relicts, and a seasonal tropical zone of excessive planation where deep weathering and wash level the land.
• Indian link: Büdel drew many of his seasonal-tropics examples from southern India and Sri Lanka, reading their inselberg-dotted plains as double planation surfaces (see etchplain below).
• Difference from Peltier: Louis C. Peltier’s regions are climatic cells of present process; these zones include inherited forms and so explain more of the real landscape.

Morphoclimatic equilibrium

Morphoclimatic equilibrium is the state in which the landforms of a region are fully adjusted to its present climate and process system, so that regional landforms reflect regional climate. It is the end-state assumed by morphogenetic-region schemes, but it requires the climate to stay constant for longer than the time the landscape needs to adjust.

• Mechanism: processes set by climate reshape slopes, channels and deposits until forms stop changing in character; fast-acting systems reach it quickly, slow ones seldom.
• Where it is approached: active glacial landscapes of Antarctica and Greenland, periglacial slopes of the Arctic and mobile dune fields, where processes are rapid relative to climatic change.
• Where it fails: Quaternary glacial–interglacial swings recurred on 41,000- and later 100,000-year rhythms, faster than cratonic landscapes can adjust, so most mid-latitude and tropical landscapes are in disequilibrium.
• Indian example: vegetated, reddened older dunes on the eastern margins of the Thar are stabilised relicts of drier phases, not in equilibrium with today’s rainfall.
• Don’t confuse with: dynamic equilibrium, which balances erosive energy against rock resistance rather than landform against climate.

Zonal, azonal and extrazonal processes

A zonal classification of geomorphic processes groups them by their relation to climatic zones. Zonal processes operate mainly within one climatic zone and produce its distinctive landforms; azonal processes act in all zones, their intensity merely modified by climate; extrazonal processes are zonal processes operating outside their home zone because local conditions reproduce its climate.

Zonal processes and their zones

• Azonal processes: running water, mass movement, waves and tides, groundwater solution, and the non-climatic tectonic and volcanic processes act in every zone. Their style is climatically tinted: rivers carry nival floods and gravel in periglacial lands, fine suspended load in the humid tropics and flash floods in deserts.
• Extrazonal processes: glaciers and frost action on high mountains in low latitudes, as on Kilimanjaro and in the Himalaya, where the Gangotri glacier lies near 31°N; coastal dunes built by onshore winds in humid zones, as along the Odisha coast near Puri; frost action on north-facing slopes in otherwise temperate valleys.
• Use: the scheme identifies which processes to expect in a zone and flags anomalies, which usually point to relict climates or overriding structure.
• Critique: zone boundaries are transitional; relict forms blur the picture; tectonics and lithology cut across zones; and human action has become a global azonal agent (see anthropogenic geomorphology).
• Sketch: a profile from the equator to the pole, with a Himalayan peak inserted at 30°N showing an extrazonal ice cap above the snowline.

Climatically controlled agents of erosion

Climatically controlled agents of erosion are the mobile media whose presence and power depend on climate: running water, glacier ice and wind, with waves and groundwater also shaped by climate. They differ in state of matter, since water is a liquid, ice a solid that flows plastically and air a gas, and those properties govern how each erodes, transports and deposits.

• Why the state of matter matters: a liquid follows the steepest path, so rivers concentrate energy in narrow channels and cut down; ice creeps slowly as a thick solid mass that grinds its whole bed, widening valleys into troughs and deepening basins below base level; air, about eight hundred times less dense than water, lifts only fine particles but works in any direction.
• Waves and groundwater: waves deliver liquid energy along a line, the shore, moving sediment to and fro; groundwater moves slowly and erodes mainly by solution, producing karst.
• Climatic control: running water dominates humid lands, ice the polar and high-mountain zones, wind the deserts; frozen seas suppress wave work at high latitudes.
• Indian examples: the V-shaped Alaknanda gorge (see fluvial landforms); the U-shaped trough below the Gangotri glacier (see glacial erosion); barchans near Jaisalmer (see arid landforms); cliffs and platforms of the Konkan (see coastal landforms); solution caves at Borra, Andhra Pradesh (see karst).
• Sketch: three cross-sections side by side, a V-valley with sorted gravels, a U-trough with unsorted till and a desert basin with sorted dunes, each labelled with its medium’s state and density.

Reading Past Climates in Landforms

Diagnostic landforms

Diagnostic landforms are landforms thought to form only under a particular climate, so that their presence identifies that climate, present or past. They are the main evidence of climatic geomorphology and include cirques and till, ice-wedge casts and pingo scars, desert dunes and ventifacts, and the more contested duricrusts, inselbergs, pediments and tors.

• Reliable indicators: glacial forms (cirques, moraines), periglacial structures (ice-wedge casts, involutions) and aeolian forms (dunes, ventifacts) are tied closely to one process and climate.
• Contested indicators: inselbergs, pediments and tors occur from humid tropics to deserts and cold uplands, so they indicate rock and history as much as climate.
• Examples: cirques of the Kolahoi massif, Kashmir, mark past glaciation; laterite crusts sandwiched between Palaeogene lavas in County Antrim, Northern Ireland, record a tropical weathering climate at a latitude that is temperate today.
• Critique: equifinality (see equifinality) means that a form can arise by more than one route, so diagnosis needs deposits and dates as well as shape.

Relict (palaeo-) landforms

Relict landforms are landforms produced by processes or climates that no longer operate at the site, surviving because present processes are too weak or too slow to erase them; buried examples are called fossil landforms. Julius Büdel estimated that the great majority of middle-latitude landforms are relicts of earlier climates.

• Recognition: the form is out of adjustment with present process, carries old weathering or soils, or rests on dated deposits of another climate.
• Examples: periglacial blockfields and solifluction sheets on Dartmoor and in central Europe, far from present permafrost; vegetated, reddened dunes on the eastern Thar margin and the Aravalli piedmont; laterite caps on Western Ghats plateaus; misfit rivers in oversized valleys.
• Quaternary cases: pluvial lake shorelines and raised beaches are relicts treated in the Quaternary post.
• Significance: relicts are archives of palaeoclimate, and relict landslide masses or dunes can be reactivated by clearing or drought.
• Don’t confuse with: palimpsest topography, the overall written-over landscape in which relicts sit.

Polygenetic landforms

Polygenetic landforms are landforms shaped by two or more different processes, usually under different climates, acting in succession, each leaving part of its imprint on the final form. An inselberg first isolated by deep weathering and then trimmed by sheetwash under a drier climate is a typical case.

• Mechanism: a change of climate or base level replaces the process system before the old form is erased, so the new process modifies rather than replaces it.
• Examples: tors prepared by tropical-style weathering and exposed by periglacial solifluction on Dartmoor; the Deccan plateau, built by flood basalt, laterised under humid Palaeogene climates, cut back by rivers into the Western Ghats scarp and now scarred by monsoon landslides.
• Distinctions: polycyclic forms repeat the same process through several cycles (see polycyclic relief); polygenetic forms combine different processes; equifinality is different routes arriving at similar forms.
• Sketch: a hill profile labelled with successive stages: weathered mantle, stripped dome and later talus apron.

Climatic accident (climatic interruption)

A climatic accident is William Morris Davis’s term for a change of climate that interrupts a running cycle of erosion by replacing its process system, for example the onset of glaciation or aridity in a humid landscape, so that the landforms of the old cycle are modified or buried by those of a new one.

• Glacial accident: Pleistocene ice sheets overrode fluvial landscapes in northern Europe and North America, replacing valleys with troughs, lake basins and till plains.
• Arid accident: desiccation turns perennial rivers into ephemeral washes and lets dunes invade valleys; parts of the Thar carry river channels abandoned as the climate dried.
• Humid accident: a return of rain rejuvenates rivers, dissects pediments and stabilises dunes under vegetation.
• Reinterpretation: climatic geomorphology treats such changes as normal rather than accidental, since the Quaternary alone brought dozens of glacial–interglacial swings; Julius Büdel’s relief generations replace the single interrupted cycle.
• Don’t confuse with: base-level and volcanic interruptions, which are treated with the other interruptions of the cycle.

Duricrust (laterite, bauxite, calcrete, silcrete, ferricrete)

A duricrust is a hard, indurated layer formed at or near the ground surface by the accumulation and cementation of iron and aluminium oxides, silica, calcium carbonate or gypsum within a weathering profile or soil. Duricrusts resist erosion, so they cap plateaus, protect old erosion surfaces and form escarpments called breakaways.

• Formation: relative accumulation leaves resistant iron and aluminium behind as silica and bases are leached, as in laterite and bauxite; absolute accumulation brings in material by groundwater or capillary rise and precipitates it, as in calcrete and silcrete.
• Laterite: named by Francis Buchanan-Hamilton in 1807 from the Latin later, “brick”, because it hardens on exposure and is cut for building; the type locality at Angadipuram, Malappuram district, Kerala, is a national geological monument.
• Geomorphic role: laterite caps the flat tops of the Western Ghats (Mahabaleshwar and Panchgani tablelands) and the pats of the Chotanagpur plateau, where bauxite is mined; ferricrete formed in old valleys can later stand as ridges, an inversion of relief.
• Relict status: many Indian laterites record Palaeogene–Neogene weathering and are no longer forming at their present sites (see dating of erosion surfaces); their economic value is covered under residual mineral deposits.

Tropical Deep Weathering and Residual Hills

Etchplain and etchplanation (double planation surface)

An etchplain is a planation surface produced when bedrock is deeply weathered and the weathered mantle is later stripped away, so that the land is lowered along the weathering front rather than by slope retreat or downwasting; etchplanation is the process. E. J. Wayland named it in Uganda in 1934, and Julius Büdel’s double planation surface (1957) describes its two levels.

• Double planation surface: an upper wash surface on the soil, lowered by sheetwash in the wet season, and a lower basal weathering surface in rock, lowered by chemical attack; both descend together in the seasonal tropics.
• Mechanism: deep weathering, often tens of metres, advances fastest along closely jointed rock, leaving massive compartments as buried domes and corestones; when uplift, base-level fall or drier climate speeds erosion, the saprolite is stripped and the irregular basal surface emerges with tors and bornhardts.
• Types: mantled etchplains keep their weathered cover; stripped etchplains expose the basal surface; intermediate forms are partly stripped.
• Examples: the plains of Uganda and Zimbabwe; in India, the Karnataka plateau around Bengaluru and the Hosur–Dharmapuri uplands, where tors and bornhardts stand on stripped surfaces cut across granite and gneiss, and parts of the Chotanagpur plateau.
• Contrast: a peneplain is lowered by slope decline, a pediplain by scarp retreat, an etchplain by the weathering front.
• Sketch: two sections, the first showing a wash surface above an irregular weathering front with corestones, the second the stripped surface with tors and a dome.

Inselberg

An inselberg (“island mountain”) is an isolated, steep-sided residual hill or mountain rising abruptly from a surrounding plain of low relief, usually of resistant crystalline rock such as granite or gneiss. The German term was applied by the geologist Wilhelm Bornhardt (1900) to the hills of the East African plains.

• Formation: Lester Charles King saw inselbergs as remnants left by parallel scarp retreat and pediplanation; the two-stage school sees them as massive, sparsely jointed compartments of the basal weathering surface exposed when an etchplain is stripped. Sparse jointing, not climate, is the common factor.
• Types: bornhardts (bare domes), castle koppies (block-strewn) and small boulder-covered nubbins.
• Climatic range: once treated as diagnostic of arid lands and the end-form of the arid cycle, inselbergs occur from rainforest to desert, and many predate the present climate.
• Examples: Uluru, central Australia (arkose); in India, Savandurga near Magadi, Karnataka, one of the largest granite monoliths in Asia; the granite hills of the Bundelkhand massif around Jhansi; the Gingee hills, Tamil Nadu.
• Don’t confuse with: a monadnock, a gentle residual of downwasting on a peneplain.

Bornhardt

A bornhardt is a dome-shaped inselberg of massive, sparsely jointed granite or gneiss with bare, steep, convex flanks and a smooth rounded summit, named after the geologist Wilhelm Bornhardt. Its form is maintained by sheet jointing, curved fractures parallel to the surface along which slabs peel off.

• Formation: differential deep weathering leaves a massive compartment standing as a buried dome on the weathering front; stripping exposes it; pressure release then opens sheet joints that keep the dome smooth (see exfoliation).
• Key features: bare rock surfaces, exfoliation slabs, weathering pits and hollows, and a sharp junction with the surrounding plain.
• Dome-on-dome forms: repeated weathering and stripping can expose a younger dome at the foot of an older one.
• Examples: Sugarloaf Mountain, Rio de Janeiro; Stone Mountain, Georgia, USA; in India, Savandurga, Karnataka, and the granite-gneiss domes of the Ranchi plateau, Jharkhand.
• Sketch: a smooth dome with curved sheet joints parallel to its surface rising from a flat plain.

Castle koppie (kopje)

A castle koppie (Afrikaans kopje, “little head”) is a small, steep residual hill crowned and flanked by piles of large joint-bounded blocks, resembling a ruined castle. It forms when a bornhardt or inselberg breaks down by the collapse of joint blocks and exfoliation sheets, leaving a core of blocks above a talus apron.

• Formation: where joints are closer than in a bornhardt, weathering along them isolates blocks that topple and pile up; Lester Charles King regarded koppies as the last stage in the consumption of inselbergs.
• Key features: stacked rounded boulders, open clefts, a boulder apron and a small base area.
• Examples: the kopjes of the Serengeti plains, Tanzania; the Matobo Hills, Zimbabwe; in India, the boulder hills around Hampi, Karnataka.
• Don’t confuse with: a tor, a smaller outcrop of corestones on a hilltop or valley side rather than a hill-sized residual.

Tor (two-stage origin)

A tor is a small, isolated stack of joint-bounded, often rounded rock blocks (corestones) standing above the surrounding surface on a hilltop or valley side, typically in granite. David Leslie Linton’s (1955) two-stage theory explains it as the product of deep chemical weathering along joints followed by stripping of the weathered mantle.

1. Weathering stage: groundwater penetrates the joints; closely jointed rock rots to grus while widely jointed compartments survive as corestones below the surface.
2. Stripping stage: a change of climate or base level removes the grus by rivers, sheetwash or, on Dartmoor in Linton’s view, periglacial solifluction, and the corestones stand out as a tor.

• Rival view: Jack Palmer and R. A. Nielson (1962) derived the Dartmoor tors from frost shattering of exposed rock and removal of debris by solifluction in one cold stage, leaving clitter slopes of blocks; cosmogenic exposure ages suggest many Dartmoor tor surfaces emerged within the last glacial cycle, which fits periglacial stripping of pre-weathered rock.
• Examples: Haytor, Dartmoor, England; in India, the granite tors of the Ranchi plateau, the Hosur–Krishnagiri uplands and Hampi.
• Significance: the classic case of equifinality, and a link to spheroidal weathering and corestones.
• Sketch: two panels, corestones inside a weathered mantle above an irregular weathering front, then the same corestones exposed as a tor with the mantle removed.

Savanna cycle of erosion

The savanna cycle of erosion is a model of landscape development for the seasonally wet and dry tropics in which deep chemical weathering and the stripping of the weathered mantle by sheetwash and streams produce extensive plains studded with inselbergs, castle koppies and tors. J. C. Pugh (1966) set it out from Nigeria.

1. Deep weathering: an initial surface is rotted to depth in the hot, wet season, leaving an undulating basal weathering surface.
2. Stripping: a fall of base level lets streams remove the regolith, exposing domes of the basal surface as incipient inselbergs.
3. Extension: backwearing widens the lowered plain and consumes small domes.
4. Renewal: uplift starts a second round of weathering and stripping, producing dome-on-dome inselbergs and stepped plains.

• Rival readings: Lester Charles King attributed the same plains to scarp retreat and pedimentation; Michael F. Thomas (1966) read them as etchplains produced by etching and removal of weathered rock, not pediplains.
• Contrast: the arid cycle works by mechanical weathering, sheetflood and wind; the savanna cycle depends on chemical weathering in a wet season followed by wash.
• Examples: the inselberg plains of northern Nigeria and Uganda; the granite plains of Karnataka and Tamil Nadu are often compared with them.
• Evaluation: it explains inselberg-and-plain country better than downwasting, but many such plains are relicts of long Tertiary weathering rather than products of the present climate.

PYQs Built on These Terms

• Attempt a classification of geomorphic process from the zonal point of view. (2015)
• “Present-day landforms bear more complexity than simplicity”. Elucidate. (2015)
• Name the climatically controlled agents of erosion. Explain how they differ in terms of properties of matter. Compare the landforms produced by each one of them. (2011)

Frequently Asked Questions

What is the main idea of climatic geomorphology?

Its central claim is that climate decides which geomorphic processes dominate a region, and therefore which landforms develop. Frost and ice rule cold lands, deep chemical weathering the humid tropics, wind and sheetflood the deserts. Because climate has changed repeatedly, most landscapes also carry relict forms from earlier climates.

What are Peltier’s morphogenetic regions?

Louis C. Peltier (1950) divided the world into nine regions, glacial, periglacial, boreal, maritime, selva, moderate, savanna, semi-arid and arid, by combining mean annual temperature and rainfall and estimating the resulting strength of weathering, frost, mass movement, running water and wind in each.

What is the difference between zonal and azonal processes?

Zonal processes are tied to one climatic zone, such as frost action in the tundra or deflation in deserts. Azonal processes, such as running water, mass movement and waves, work in every zone, with their style modified by climate. Extrazonal processes are zonal ones appearing elsewhere, such as glaciers on tropical mountains.

How are tors formed?

The widely taught two-stage explanation holds that granite first weathers deeply along its joints, leaving sound corestones inside rotted rock, and that the rotted material is later stripped away, exposing the corestones as a tor. A rival view credits frost shattering and solifluction in a single cold stage.

What is the difference between an inselberg and a bornhardt?

An inselberg is any isolated steep-sided hill rising from a plain, whatever its shape or rock. A bornhardt is one kind of inselberg, a bare smooth dome of massive granite or gneiss shaped by sheet jointing. A castle koppie is a third kind, crowned with collapsing blocks.

Why are laterite caps found on plateaus in India?

Laterite forms when intense tropical weathering leaches silica and bases and leaves iron and aluminium oxides that harden into a crust. On the Western Ghats and the Chotanagpur pats these crusts formed on old erosion surfaces and, being resistant, now protect them as flat-topped tablelands.