Hillslopes and Slope Evolution: Terminology for UPSC Geography Optional

Hillslopes make up most of the land surface, and every theory of landscape change is, underneath, a theory of how slopes change. This post covers slope-profile elements, the classical models — decline, parallel retreat and replacement — the pediment and pediplain, process–response modelling and the water processes that sculpt slopes, one of UPSC’s most-examined theory clusters.

Each entry opens with a definition, then mechanism, features, examples and a sketch line. UPSC has asked Lester Charles King’s views on slopes (2009), Böschung and Haldenhang in slope replacement (2012), King’s synthesis in pediplanation (2016) and the limits of slope analysis in slope management (2017); those entries are written to answer length.

Quick Revision Table

Term	Meaning in one line	Example
Hillslope	Inclined surface between a divide and a valley floor	Valley sides of the Alaknanda, Garhwal
Slope elements	Waxing slope, free face, constant (debris) slope, waning slope	Bhander plateau scarps, Satna, Madhya Pradesh
Nine-unit land-surface model	Nine process units from divide to channel bed	Dalrymple, Blong and Conacher’s model slope (1968)
Summital convexity and basal concavity	Rounded crest and concave foot of a slope profile	Convexo-concave chalk downs, southern England
Rectilinear slope segment	Straight middle segment at a constant angle	Debris slopes below Western Ghats basalt scarps
Characteristic slope angle	Most frequent angle for a given rock and climate	Uniform maximum angles in Strahler’s California valley studies
Threshold slope	Limiting angle beyond which regolith slopes fail	Clustered valley-side angles, English Pennines
Slope decline	Steepest part flattens with time; convexity and concavity grow	Davis’s model of humid-temperate valley sides
Parallel retreat	Slope keeps its angle while moving back	Drakensberg escarpment, southern Africa
Slope replacement	Gentler lower units extend upslope and consume steeper ones	Penck’s model, Black Forest slopes
Haldenhang and Böschung	Penck’s gentle basal slope and steep slope unit above it	Scarp foot slopes in Penck’s deductive model
Gravity slope and wash slope	Upper slope moved by gravity; lower slope shaped by wash	Free face over pediment, Aravalli piedmont
Pediment	Gently inclined bedrock surface at the foot of a mountain front	Sonoran and Mojave deserts, USA
Pediplain and pediplanation	Plain of coalesced pediments made by scarp retreat	African Surface, southern Africa
Fisher–Lehmann model	Cliff retreat above accumulating scree leaves a convex rock core	Chalk cliffs of Osmond Fisher’s 1866 study
Mono-process and poly-process concepts	One process makes one form versus several processes combining	Gilbert’s creep convexity; Baulig’s creep plus wash
Process–response model	Slope form computed from interacting process rates	Anthony Young’s simulation models (1963)
Slope-evolution vs process–form approach	Historical sequence of forms versus form as a function of process	Davis and King versus Strahler and Young
Overland flow, throughflow, interflow and pipeflow	Surface and subsurface paths of rain water on slopes	Piping in the Chambal ravine lands
Rainsplash, sheetwash, rills and gullies	Stages of water erosion from drop impact to channels	Gullied Siwalik foothills around Hoshiarpur

The Anatomy of a Hillslope

Hillslope

A hillslope is the inclined land surface between a drainage divide or crest and the valley floor or stream channel below it, shaped by weathering, mass movement and running water. Hillslopes cover most of the Earth’s land surface, and as part of the drainage basin they deliver water and sediment to channels, so slope form and river behaviour are linked.

• Genetic types: tectonic slopes (fault scarps, tilted blocks), erosional slopes (valley sides, cliffs cut by rivers, glaciers and waves) and slopes of accumulation (alluvial fans, dunes, moraines, volcanic cones).
• Measurement: profiles are surveyed along the line of steepest descent with an Abney level or pantometer, or read from digital elevation models.
• Classes by angle: Anthony Young grouped slopes from level (under 0.5 degrees) through gentle, moderate and steep to precipitous (over 45 degrees); such classes are used in land-capability and hazard mapping.
• Examples: the steep convex valley sides of the Alaknanda in Garhwal; the long concave footslopes around residual hills of the Chotanagpur plateau.
• Significance: slope angle governs runoff, soil depth, landslide hazard and land use.

Slope elements (waxing slope, free face, debris or constant slope, waning slope)

Slope elements are the distinct segments into which a hillslope profile can be divided by changes in angle. In Alan Wood’s scheme (1942), a fully developed profile has four: a convex waxing slope at the crest, a steep bare-rock free face, a straight constant (debris) slope of fallen debris at its foot, and a concave waning slope at the base.

• Waxing slope: the rounded crest, shaped by creep and splash.
• Free face: an outcrop too steep for debris to rest on; it retreats by rockfall and weathering and supplies the slope below.
• Constant (debris) slope: a straight slope at about the repose angle of debris fed from the free face.
• Waning slope: a concave, low-angle, wash-shaped slope; in dry lands, the pediment.
• Development: without resistant rock and enough relief there is no free face, and a convexo-concave profile forms.
• Examples: the scarps of the Bhander plateau in Satna district, Madhya Pradesh, where massive Vindhyan sandstone forms the free face above shales carrying the debris and waning slopes.
• Don’t confuse with: talus, a rockfall deposit; the constant slope is a profile element.
• Sketch: one hillslope profile labelled crest, waxing slope, free face, constant slope and waning slope.

Nine-unit land-surface model

The nine-unit land-surface model is a hypothetical hillslope, proposed by J. B. Dalrymple, Russell J. Blong and Arthur J. Conacher in 1968, that divides the land surface from divide to stream bed into nine units, each defined by its dominant processes of water movement, mass movement and soil formation rather than by angle alone.

• The units: (1) interfluve, where water moves vertically in the soil; (2) seepage slope, with lateral subsurface flow and leaching; (3) convex creep slope; (4) fall face, steeper than about 45 degrees, with falls and slides; (5) transportational midslope, where material moves by flows, slides and wash; (6) colluvial footslope, where mass-movement debris is redeposited; (7) alluvial toeslope, built by stream deposition; (8) channel wall; (9) channel bed.
• Key idea: any real slope uses only some units, in varying order; units may repeat on stepped slopes.
• Advantage: it ties form to process and soil (catena) and suits soil surveys, land evaluation and engineering.
• Sketch: a profile from interfluve to channel with the nine units numbered along it.

Summital convexity and basal concavity

Summital convexity is the rounded, convex upper part of a hillslope profile, where slope angle increases downslope from the crest. Basal concavity is the concave lower part, where angle decreases downslope towards the valley floor. Together they form the convexo-concave profile typical of soil-covered slopes in humid climates.

• Why convex: Grove Karl Gilbert (1909) argued that creep must move more soil with every metre away from the divide, and it can do so only if the slope steepens downslope. Rainsplash acts the same way.
• Why concave: running water gathers volume downslope, so a gentler gradient can still carry the sediment supplied; fine debris accumulating at the foot also builds depositional concavities.
• Controls: creep-dominated slopes have broad convexities; wash-dominated and dry-land slopes have long concavities and narrow crests.
• Examples: the convexo-concave chalk downs of southern England; rounded laterite-mantled interfluves of the Kerala midlands.
• Sketch: a profile with a convex crest grading into a concave foot, arrows labelled creep at the top and wash at the base.

Rectilinear slope segment

A rectilinear slope segment is a straight part of a hillslope profile in which the angle stays nearly constant, usually lying between the summital convexity (or free face) and the basal concavity. It is often the steepest and most extensive segment of a mature slope, and its angle is commonly controlled by the angle of repose of debris or by the strength of the regolith.

• Formation: debris from a free face resting at its repose angle, or regolith held at a threshold angle by landsliding.
• Richter denudation slope: Eduard Richter (1900) described Alpine rock slopes at the talus angle formed beneath retreating cliffs, a classic rectilinear form.
• Key features: uniform angle, often 25–35 degrees; thin debris cover in transit.
• Examples: debris-mantled straight slopes below the basalt scarps of the Western Ghats in Maharashtra; talus-graded slopes of Himalayan valley walls.
• Significance: a straight, steep segment is where most shallow landslides begin.

Characteristic slope angle

A characteristic slope angle is the angle that occurs most frequently on the hillslopes of an area of uniform rock type and climate, showing up as a sharp peak in the frequency distribution of measured angles. Its existence implies that slopes adjust to an equilibrium angle set by their material and processes rather than by their age.

• Origin of the idea: Arthur Newell Strahler (1950) found that maximum valley-side angles within a uniform area cluster tightly around a mean, and called them equilibrium slopes; Anthony Young (1961) developed the concept of characteristic and limiting angles.
• Key features: different lithologies give different modal angles; resistant rock gives steeper ones.
• Interpretation: a strong mode suggests parallel retreat, because slopes of different ages share one angle.
• Examples: Strahler’s measured valley-side slopes in southern California; clustered straight slopes on uniform Deccan basalt.
• Don’t confuse with: the threshold slope, which is a limiting maximum; the characteristic angle is a mode.

Threshold slope

A threshold slope is the maximum angle at which a regolith-covered hillslope can remain stable against shallow landsliding. Slopes steeper than it fail until they are reduced to it, so hillslopes in rapidly eroding terrain cluster at a few threshold angles set by the strength of their material and by pore-water pressure.

• Mechanism: for dry cohesionless debris, the threshold equals the angle of internal friction; with groundwater seeping parallel to the surface, pore pressure lowers the stable angle to about half of that value in tangent terms.
• Evidence: Michael A. Carson and D. J. Petley (1970) found valley-side slopes in the English Pennines clustering at a few distinct angles, which they read as threshold angles for regolith of different types, steepest on coarse, rubbly debris and lowest on clay-rich mantles.
• Significance: in fast-rising mountains such as the Himalaya, slopes sit at threshold angles, so extra uplift produces more landslides rather than steeper slopes, as on Lesser Himalayan valley sides.
• Don’t confuse with: the angle of repose, which applies to loose, cohesionless debris heaps only.

Classical Models of Slope Evolution

Slope decline

Slope decline is the model of slope evolution, associated with William Morris Davis, in which the steepest part of a hillslope progressively decreases in angle through time, while a summital convexity and a basal concavity grow at its expense. Relief is lowered from above by downwasting, and the slope ends as a very gentle convexo-concave profile.

• Mechanism: Davis reasoned that once rivers stop cutting down, debris supply at the base exceeds removal; the lower slope is protected while the upper slope continues to waste, so the whole profile flattens.
• Graded waste sheet: Davis’s term (1899) for a mantle whose transporting processes can just remove the waste supplied at every point; grading spreads upslope from the base as the cycle ages.
• Stages: steep, convex slopes in youth; graded, smoothly curved slopes in maturity; broad concave slopes of a few degrees in old age.
• Where it fits: soil-mantled slopes on weak rocks in humid climates, and slopes whose base is no longer cleared of debris.
• Criticism: it assumes long stillstands and was reasoned, not measured, though Anthony Young found it internally consistent and testable.
• Examples: convexo-concave slopes of the humid-temperate Appalachians; low, rolling slopes of the central Ranchi plateau.
• Contrast in one line: in decline the maximum angle falls; in parallel retreat it stays the same; in replacement it is lowered from below.
• Sketch: successive profiles 1 to 5 from one base point, each gentler than the last, crest lowering.

Parallel retreat

Parallel retreat is the model of slope evolution in which a hillslope, or its steep elements, moves back while keeping a constant angle, so the maximum slope angle does not change through time. As the upper slope retreats, a low-angle basal slope — the pediment — lengthens at its foot. Lester Charles King (1953) made it the central mechanism of landscape evolution.

King’s view of slope development

• The standard hillslope: King adopted Alan Wood’s four elements as crest (waxing slope), scarp (free face), debris slope and pediment (waning slope), and held that this profile develops wherever there is enough relief and resistant rock, in all climates short of the frigid and the extremely arid.
• How retreat works: the scarp retreats by weathering, rockfall and gullying; the debris slope keeps its angle because debris is removed as fast as it arrives; the crest also stays constant; so both retreat parallel, and the pediment extends headward at the expense of the upland.
• End-form: retreating scarps from neighbouring valleys consume the uplands, leaving inselbergs, and coalescing pediments form a pediplain.
• Where scarps are absent: on weak rock and low relief, King accepted slope decline and convexo-concave profiles, and he regarded the convexo-concave slopes of the northern mid-latitudes as relict periglacial forms rather than “normal” slopes.
• Criticism: Anthony Young judged King’s views to rest on reconnaissance rather than measurement and to be hypotheses for testing, not proofs.

Evidence

• Field support: R. A. G. Savigear (1952) showed at Carmarthen Bay, South Wales, that former sea cliffs retreated parallel where waves cleared the base, and declined where debris accumulated once the sea withdrew — both models can operate side by side.
• Arthur Newell Strahler’s evidence: uniform maximum angles on slopes of different ages point to parallel retreat.
• Examples: the Drakensberg escarpment of southern Africa; mesa and butte scarps of the Colorado Plateau; in India, the sandstone scarps of the Bhander and Kaimur plateaus, Madhya Pradesh, with detached flat-topped outliers.
• Sketch: a scarp shown at three successive positions, each with the same angle, and a lengthening pediment below.

Slope replacement

Slope replacement is Walther Penck’s model of slope evolution in which the maximum slope angle is reduced by replacement from below: a gentler slope unit forms at the base of a steeper one and extends upslope at its expense, until the steeper unit is consumed. Flattening therefore always proceeds from the bottom upward, and the profile becomes concave.

• Relation to other models: each steep unit retreats parallel while it lasts, as in parallel retreat, but it shortens as the gentler unit rises beneath it, so the overall angle falls, as in decline — yet from the base rather than from the crest.
• History: early English readers took Penck to mean parallel retreat; later translations showed he meant replacement.
• Link to river incision: Penck argued that the rate of downcutting at the slope foot governs the form — accelerating incision gives convex slopes, constant incision straight ones and decelerating incision concave ones (his wider stages are covered under Penck’s Entwicklung).
• Examples: concave footslopes spreading up the valley sides of the Black Forest, south-west Germany, Penck’s field area; concave pediment-fringed slopes of old shield landscapes.
• Sketch: a steep face with a stack of successively gentler basal units growing upward at its expense.

Haldenhang and Böschung

Böschung is Walther Penck’s term for a steep slope unit, in English rendered as the gravity slope, whose surface loses debris as soon as it is loosened. Haldenhang (literally “heap” or “talus” slope, often rendered basal slope or wash slope) is the gentler slope unit that forms at the foot of a Böschung and grows upward at its expense, which is the mechanism of slope replacement.

How the Haldenhang replaces the Böschung

1. Starting condition: a steep rock face of uniform rock stands above a river that removes all debris but no longer cuts down, so the base level at the foot is fixed.
2. Uniform weathering: in each unit of time a layer of the same thickness is loosened over the whole face; on the steep Böschung the loosened fragments are too steep to rest and fall away, so the face retreats parallel to itself.
3. The lowest strip cannot retreat: at the very foot the face meets the fixed base level, and there is no gradient below to carry debris off; so a small, gentler unit is left at the base — the first Haldenhang — at just the angle needed to move the finer debris resting on it.
4. Replacement from below: as the Böschung retreats further, the Haldenhang extends upward along the line of its lower end. The Böschung keeps its angle but loses height, because the gentle unit climbs beneath it.
5. Repetition: the Haldenhang itself retreats, more slowly because it is gentler, and an even gentler unit forms at its foot; successive units merge into a concave profile.
6. End-point: when the Böschung is wholly consumed, the upper slope is replaced by Haldenhang; where Böschungen from neighbouring valleys meet, divides are lowered and the land approaches Penck’s Endrumpf.

• Why the angle falls: gentler units need finer, more mobile debris to move, and debris becomes finer as it rests longer, so each lower unit is gentler than the one above.
• Criticism: Anthony Young noted that Penck wrongly assumed equal exposure of the whole basal slope to weathering and applied rockfall logic to regolith slopes; the stepped diagram is an artefact of dividing time into intervals, and real profiles are smoothly concave.
• Sketch: a steep Böschung above a gentle Haldenhang, with successive profiles showing the break of slope migrating upward.

Gravity slope and wash slope

A gravity slope is the steep upper part of a hillslope — the free face and the debris slope below it — where debris moves chiefly by gravity through falling, sliding and rolling. A wash slope is the gentler lower part, the waning slope or pediment, where debris is moved mainly by running water in sheets and rills.

• Key features: gravity slopes stand at or above the angle of repose; wash slopes are concave, at a few degrees; in dry lands a sharp piedmont angle separates them.
• Relation to other terms: they correspond broadly to Walther Penck’s Böschung and Haldenhang and to Lester Charles King’s scarp and pediment.
• Examples: bare free faces of the Aravalli ridges in Rajasthan above gently sloping pediments; mountain fronts of the Mojave Desert.
• Sketch: a mountain front with a steep gravity slope, a sharp break and a gently inclined wash slope.

Pediments, Pediplains and Cliff Models

Pediment

A pediment is a gently inclined erosional surface cut across bedrock at the foot of a mountain front or escarpment, usually covered by a thin veneer of alluvium in transit. It slopes away from the upland at about 0.5 to 7 degrees, is concave in profile, and meets the steep slope above at a sharp piedmont angle.

• Definition in the literature: Kirk Bryan (1922) called pediments “slopes of transportation” cut on bedrock.
• Theories of formation: lateral planation by streams swinging across the piedmont (Grove Karl Gilbert; Douglas Wilson Johnson, 1932); erosion by sheetfloods (William John McGee, 1897); parallel retreat of the mountain front by weathering and wash (Lester Charles King); and subsurface weathering followed by stripping.
• Types: rock pediments, mantled pediments, pediment passes where pediments from both sides of a range meet, and dissected pediments after uplift.
• Where found: best developed in dry lands, but also in savanna and humid tropics.
• Examples: the pediments of the Sonoran and Mojave deserts, USA, including Cima Dome; pediments flanking the Aravalli ridges of central Rajasthan and the residual hills of the Chotanagpur plateau.
• Don’t confuse with: a bajada, which is depositional; a pediment is cut in rock.
• Sketch: mountain front, piedmont angle, pediment with thin alluvium, and bajada beyond.

Pediplain and pediplanation

A pediplain is an extensive, low-relief erosion surface formed by the coalescence of many pediments as the scarps between them retreat, leaving a multi-concave plain studded with residual inselbergs. Pediplanation is the process — twin action of parallel scarp retreat and pedimentation — by which Lester Charles King (1948, 1953) explained the planation of continents.

The cycle of pediplanation

• Youth: uplift of an older plain starts incision; small pediments appear in valley floors.
• Maturity: scarps retreat parallel, pediments widen and interfluves shrink to inselbergs and castle koppies.
• Old age: pediments merge into a pediplain; a new uplift starts a new scarp that eats back into it.

What King took from Davis, Penck and Wood

• From William Morris Davis: the cyclic framework — rapid uplift followed by a long stillstand, and stages of youth, maturity and old age ending in a low surface of planation.
• From Walther Penck: the retreat of slopes rather than their decline, and the idea that new, lower surfaces are cut at the foot of retreating uplands, so stepped surfaces grow headward, much like Penck’s piedmont treppen.
• From Alan Wood: the four slope elements — waxing slope, free face, constant slope and waning slope — which King renamed crest, scarp, debris slope and pediment and made his “standard hillslope”.
• King’s own ideas: the pediment as the key element, cut by sheetwash; semi-arid and savanna lands, not humid-temperate ones, as the “normal” setting; climatic uniformitarianism, with the same processes in all but frigid and extremely arid climates; and global cyclic surfaces correlated between continents (covered under King’s global surfaces).

Evaluation

• Strengths and weaknesses: it explains the scarp-and-plain landscapes of Africa, Brazil and Australia better than slope decline, but rests on reconnaissance, assumes long stillstands and relies on poorly known process rates.
• Examples: the African Surface of southern Africa; in India, the inselberg-dotted granite-gneiss plains of Bundelkhand are often interpreted as pediplains.
• Don’t confuse with: the peneplain, a rolling surface made by downwasting and slope decline, with convex residual hills rather than steep-sided inselbergs.
• Sketch: a block diagram showing retreating scarps, widening pediments and inselbergs on a pediplain.

Fisher–Lehmann model (buried rock core)

The Fisher–Lehmann model is a model of cliff recession in which a free face retreats parallel to itself by uniform weathering while the scree it sheds accumulates at its foot and is not removed. The scree progressively buries the lower cliff, and the bedrock surface preserved beneath it takes a convex form — a buried rock core.

• Authors: first stated by Osmond Fisher (1866) for a chalk cliff, and developed mathematically by Otto Lehmann (1933), who added the cliff angle, the scree angle and the rock-to-scree volume ratio.
• Mechanism: each layer weathered from the face falls to the base as scree, which occupies more volume than the solid rock; the free face shortens as the scree rises, until it disappears beneath a scree slope at the angle of repose.
• Significance: it explains convex rock profiles found beneath talus and anticipates Alan Wood’s constant slope.
• Sketch: a cliff with successive positions, scree wedges accumulating at the base and a convex rock core beneath.

Process-Based Approaches

Mono-process and poly-process concepts

Mono-process concepts hold that each slope form is produced by one dominant process — convexity by soil creep, concavity by rainwash. Poly-process concepts hold that several processes act together or on different parts of a slope, and that the balance between them, changing through time, shapes the profile.

• Mono-process views: Grove Karl Gilbert (1909) explained summital convexity by creep; Nevin Melancthon Fenneman (1908) explained convexo-concave profiles by runoff that erodes little at the crest and much at the base; Andrew Cowper Lawson (1932) argued the opposite, with rainwash most effective near the crest and deposition forming the basal concavity.
• Poly-process view: Henri Baulig (1950) argued that creep dominates the upper slope and wash the lower slope; as relief falls, creep weakens and the concavity extends upslope.
• Current view: most slopes are poly-process, as on the creep-rounded crests and wash-cut foots of Western Ghats foothills.
• Sketch: a convexo-concave profile labelled creep-dominated above and wash-dominated below.

Process–response model of slope evolution

A process–response model of slope evolution is a quantitative model in which the form of a hillslope is calculated step by step from assumed rates of weathering and transport, so that each change in form alters the processes, which in turn change the form. Anthony Young (1963) and Frank Ahnert (1966, 1970) developed the first computer simulations of this kind.

• Principle: the slope obeys continuity of mass — at any point, ground is lowered where more debris leaves than arrives, and raised where more arrives than leaves.
• Results: creep, with transport proportional to slope gradient, produces convex profiles; wash, increasing with distance from the crest, produces concavities; basal removal of debris gives parallel retreat, while accumulation at the base gives decline.
• Weathering-limited and transport-limited slopes: where transport can remove more than weathering supplies, slopes are bare, steep and rock-controlled; where weathering supplies more than can be removed, slopes are soil-covered and shaped by the transport process. Michael A. Carson and Michael J. Kirkby (1972) made this distinction central.
• Significance: it tests the classical models quantitatively; Young’s runs needed hundreds of thousands to millions of years to reshape modest slopes.
• Sketch: a flow chart linking weathering, regolith thickness, transport and slope angle in a feedback loop.

Slope-evolution approach versus process–form approach

The slope-evolution approach studies hillslopes historically, reconstructing how forms change through time from an assumed initial slope, as in the models of William Morris Davis, Walther Penck, Alan Wood and Lester Charles King. The process–form approach studies the relationship between present-day processes and slope forms, treating form as a function of process, material and climate rather than of age.

The two approaches compared

Aspect	Slope-evolution approach	Process–form approach
Question	How has the slope changed over time?	How do present processes produce this form?
Time	Time-dependent; stages matter	Time-independent; forms adjust to processes
Method	Deduction, ergodic reasoning, comparison of profiles	Field measurement of angles and process rates, modelling

Limits of slope analysis in slope management

• Equifinality: different processes and histories can produce the same profile, so form alone cannot identify cause.
• Process rates: creep and wash are so slow that short measurement records rarely capture the rare, large events that matter for failure.
• Relict forms: many slopes were shaped under past climates, so present processes do not explain them.
• Scale mismatch: slope analysis works on whole profiles and regions, whereas management acts on a single cut, block or slip surface.
• Form is not stability: safety depends on hidden factors — joints, pore pressure, shear strength — that surface angle does not reveal.

Where slope analysis is applied

• Road cuts: cut angles, benching and drainage are chosen from characteristic and threshold angles of each rock type, and weathering-limited faces are distinguished from transport-limited regolith slopes.
• Hill-town planning: slope-category maps set limits on building density and height; failures such as the Summer Hill landslide in Shimla on 14 August 2023 show the cost of ignoring them.
• Terracing and land capability: slope classes guide bench terracing, contour bunding and afforestation on Himalayan and Western Ghats farmland.
• Hazard mapping: slope angle is a core layer of landslide hazard zonation.

Water on Slopes

Overland flow, throughflow, interflow and pipeflow

Overland flow is rainwater that moves across the ground surface as a thin sheet because it cannot infiltrate. Throughflow, also called interflow, is water moving laterally downslope within the soil above a less permeable layer. Pipeflow is concentrated subsurface flow through natural soil pipes and tunnels. Together they decide how much erosion a slope suffers and where.

• Infiltration-excess (Hortonian) overland flow: Robert Elmer Horton (1945) showed that overland flow begins when rainfall intensity exceeds the soil’s infiltration capacity. Near the divide the flow is too shallow to erode, creating a belt of no erosion; below a critical distance it cuts rills.
• Saturation overland flow: in humid, vegetated catchments, throughflow saturates the soil near the slope foot and channels, and rain falling there runs off; Thomas Dunne and Richard D. Black (1970) demonstrated this.
• Effects: throughflow removes solutes and raises pore pressure at the slope base; pipes enlarge, their roofs collapse and gullies form.
• Examples: Hortonian flow on crusted slopes of semi-arid Rajasthan; saturation flow in forested Western Ghats hollows; piping in the Chambal ravines.
• Sketch: a slope section showing rain, infiltration, overland flow, throughflow above an impermeable layer, a pipe and a seepage zone.

Rainsplash, sheetwash, rills and gullies

Rainsplash is the detachment and short-distance throwing of soil particles by the impact of raindrops. Sheetwash is the removal of soil by unconcentrated overland flow moving as a thin sheet. Rills are small, shallow channels cut by concentrated runoff, and gullies are larger, steep-sided channels too deep to be erased by ploughing.

• Rainsplash: most effective on bare ground; on a slope more particles are thrown downhill than uphill, giving net transport, and splash seals the surface, increasing runoff.
• Sheetwash: removes thin layers evenly, often unnoticed until roots stand exposed.
• Rills: form in parallel sets below Robert Elmer Horton’s belt of no erosion; cross-grading between them builds a drainage network.
• Gullies: grow by headcut retreat, side-wall slumping and piping.
• Examples: gullies of the Chinese Loess Plateau; gullied Siwalik foothills around Hoshiarpur, Punjab, drained by seasonal torrents called choes. Human-triggered gullying and the ravine lands are covered under man-induced soil erosion.
• Sketch: a slope showing splash at the top, sheetwash, then parallel rills and a gully head near the base.

PYQs Built on These Terms

• “In explaining the concept of ‘Pediplanation’, King combined the ideas of Davis, Penck and Wood with his own.” Elaborate. (2016)
• Explain how Bosche and Haldenhang lead to the Theory of Slope Replacement. (2012)
• Discuss views on slope development provided by L.C. King. (2009)
• “The knowledge of slope analysis has limited field application in the slope management.” Explain. (2017)
• Analyse the differences in the models of slope evolution proposed by Davis and Penck. (1990)

Frequently Asked Questions

What is the difference between slope decline, parallel retreat and slope replacement?

In slope decline, associated with William Morris Davis, the steepest part of a slope flattens with time as convexity and concavity grow. In parallel retreat, championed by Lester Charles King, the slope keeps its angle and moves back while a pediment lengthens below. In Walther Penck’s slope replacement, gentler basal units grow upward and consume steeper ones, so flattening proceeds from the bottom up.

What are the four elements of a hillslope?

The four elements, defined by Alan Wood in 1942, are the convex waxing slope at the crest, the steep bare-rock free face, the straight constant or debris slope below it at about the angle of repose, and the concave waning slope at the base. Lester Charles King called them crest, scarp, debris slope and pediment.

What is the difference between a pediment and a pediplain?

A pediment is a single, gently inclined bedrock surface at the foot of one mountain front or scarp. A pediplain is a much larger plain formed when many pediments merge as the scarps between them retreat, leaving only scattered inselbergs. The pediment is a landform; the pediplain is the end-product of Lester Charles King’s cycle of pediplanation.

What is the difference between a pediplain and a peneplain?

Both are low surfaces at the end of an erosion cycle, but they form differently. A peneplain results from downwasting and slope decline, giving a gently rolling plain with low, convex residual hills. A pediplain results from parallel scarp retreat and the merging of pediments, giving a multi-concave plain with steep-sided inselbergs rising abruptly from it.

What does Haldenhang mean in geomorphology?

Haldenhang is Walther Penck’s term for the gentle basal slope that forms at the foot of a steep slope unit, the Böschung. Because the lowest part of a retreating face cannot move back against a fixed base level, a gentler unit develops there and extends upslope, consuming the steeper face. This is the mechanism of Penck’s slope replacement.

Why does slope analysis have limited use in slope management?

Slope analysis describes the form of a slope, but stability depends on hidden factors such as joints, pore-water pressure and shear strength. Different processes can give one form, rates are too slow to measure easily, many slopes are relict, and management works on single cuts. It still guides road cuts, hill-town planning and terracing.