Earth Movements, Folds, Faults, Nappes and Structural Landforms

• Earth movements are displacements of the crust driven by endogenetic (internal) forces. They bend rocks into folds, break them along faults, stack them as nappes, and raise or lower whole continents.
• These movements create the primary relief: mountains, plateaus, rift valleys, basins. Exogenetic agents then carve that relief, and the result is structural landforms, whose shape still betrays the folds and faults beneath.
• Structural geomorphology is the study of this link: the tectonic structure sets the first frame, while differential erosion decides which rock stands up and which is worn down.

Earth Movements and Their Classification

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Nature and Origin of Endogenetic Forces

• Source of energy: heat from radioactive decay, primordial heat left from the Earth’s formation, and mantle convection that drives the lithospheric plates.
  • Unequal heating makes rocks expand and contract. The energy is released either slowly (bending, warping) or suddenly (earthquakes, eruptions).
  • The basic process view is set out in endogenic forces; this note builds the structural side on it.
• Time scale: changes are either long-period (visible only over thousands to millions of years) or short-period (seconds to hours).
• Direction: every movement is ultimately vertical (radial) or horizontal (tangential).

Sudden Movements

• Sudden movements are the release of stress built up slowly at depth: earthquakes and volcanic eruptions.
  • They are constructive as well as destructive. Fissure eruptions built the Deccan Traps and the Columbia Plateau; earthquakes open fractures, raise fault scarps and dam lakes.
  • Indian example: the 1819 Rann of Kachchh earthquake raised the Allah Bund, a ridge about 80 km long and about 6 m high, while ground near Sindri sank about 3 m and flooded.

Diastrophic Movements

• Diastrophism covers the slow movements that deform the crust over long spans and build the continental-scale relief.

Epeirogenic Movements (continent-forming)

• Epeirogenic (Greek epeiros, continent) movements are vertical, radial and broad, affecting large parts of continents with little folding.
  • Upward movement (uplift): of a whole continent or of its coasts; the latter gives emergence (raised beaches of Kathiawar and Tamil Nadu).
  • Downward movement: subsidence of land, or submergence of coasts below sea level (submerged forests near Mumbai and Tirunelveli).
  • Cratons such as Peninsular India respond mainly by epeirogeny.

Orogenic Movements (mountain-forming)

• Orogenic (Greek oros, mountain) movements are horizontal, tangential and confined to long, narrow belts, chiefly at convergent plate margins.
• Tensional (divergent) forces pull the crust apart and cause crustal fracture: cracks, joints, normal faults, rift valleys.
• Compressional (convergent) forces push crust together and cause crustal bending:
  • Warping: broad upwarping (domes, swells) or downwarping (basins); over very large areas it is called broad warping.
  • Folding: buckling of strata into anticlines and synclines, and at the extreme, recumbent folds, thrusts and nappes.
• Compression also produces reverse and thrust faults, so fracture is not confined to tension.

Classification at a Glance

• Endogenetic forces
  • Sudden: earthquakes; volcanism
  • Diastrophic
    • Epeirogenic (vertical): uplift (emergence); subsidence (submergence)
    • Orogenic (horizontal)
      • Tensional: crustal fracture, as cracks, joints, faults
      • Compressional: crustal bending, as warping and folding

Folds and Nappes

Anatomy of a Fold

• A fold is a wave-like bend in layered rock produced by compressive stress. The upfold is an anticline (oldest rocks in the core); the downfold is a syncline (youngest rocks in the core).
• Limbs: the two sides of a fold. The limb shared by an anticline and its neighbouring syncline is the middle limb.
• Crest and trough: the highest line of an anticline and the lowest line of a syncline.
• Axial plane: the surface that bisects the fold; its line of intersection with a bed is the fold axis (hinge line).
• Dip: the angle and direction of maximum slope of a bed from the horizontal (measured with a clinometer, written like 60° N).
• Strike: the direction of a horizontal line on a bedding plane. Dip is always at right angles to strike.
• Plunge: the angle by which the fold axis tilts from the horizontal. Plunging folds end in V- or U-shaped outcrop noses.
• These terms are collected in the terminology of earth movements, folds and faults.

What Controls the Style of Folding

• Rock ductility: soft, plastic shales and limestones fold tightly; rigid sandstones and crystalline rocks fold gently or break.
• Intensity of compression: gentle stress gives open symmetrical folds; strong stress gives isoclinal, overturned and recumbent folds.
• Duration of stress and depth (heat and confining pressure make rocks more plastic).

Types of Folds

• Symmetrical fold: both limbs dip equally; an open fold formed by moderate, even compression. Rare in nature.
• Asymmetrical fold: one limb longer and gentle, the other short and steep; the commonest type.
  • By limb dip, an anticline is called gentle below about 40° and steep between 40° and 90°.
• Monocline: a single step-like flexure in otherwise flat or gently dipping beds; one limb steep, the other near horizontal.
  • Often forms as strata drape over a fault in the basement; the steep limb may itself break into a fault.
• Isoclinal fold: compression so strong that both limbs become parallel, dipping the same way at the same angle.
• Overturned fold: one limb is tilted beyond the vertical, so both limbs dip in the same direction; beds on that limb are upside down.
• Recumbent fold: the axial plane is nearly horizontal, so the fold lies on its side; the lower limb is inverted.
• Overthrust fold: a recumbent or overturned fold whose middle limb has ruptured, and the upper part has moved forward along a thrust plane. This is the first step towards a nappe.
• Fan fold: a broad fold in which limbs are overturned outward, so the structure spreads like a fan.
• Open and closed folds: open when the interlimb angle is greater than 90°; closed (tight) when it is acute.
• Plunging fold: as above; common in the Appalachian Valley and Ridge and in the Siwalik belt.

Anticlinorium and Synclinorium

• Anticlinorium: a large anticline carrying many minor anticlines and synclines on its back; synclinorium is the equivalent large syncline.
  • They form where compression is irregular in intensity. Their overall outline gives some authors the name fan fold.
  • Large fold belts, including the Aravalli–Delhi belt of Rajasthan, show this compound style.

Nappes

Definition

• A nappe (French, “sheet” or “tablecloth”) is a large sheet of rock displaced far (usually several to over 100 km) from its place of origin along a low-angle thrust, or as the upper limb of a giant recumbent fold.
  • Its hallmark is older rocks lying on younger rocks, a direct reversal of normal superposition.
  • The broader concept of orogeny behind nappes is covered under theories of mountain building.

Parts and Related Terms

• Root zone: the belt where the nappe was detached from its base; often squeezed, steep and metamorphosed.
• Front (frontal lobe): the leading edge, often folded down.
• Klippe: an isolated erosional outlier of a nappe, resting on younger rocks, cut off from the main sheet by erosion.
• Window (fenster): a hole eroded through the nappe that exposes the rocks beneath, surrounded on all sides by the nappe.
• Autochthon: rocks that have not moved from their site of formation. Parautochthon: moved only a short distance. Allochthon: far-travelled rock, as in a nappe.

Processes that Form Nappes

1. Fold-nappe (recumbent fold to thrust)
   • Compression turns an anticline into an overturned, then a recumbent fold. The stretched middle limb thins and ruptures.
   • The upper limb is then thrust forward along a near-horizontal plane, carried kilometres beyond its root.
   • Typical of hot, ductile rocks deep in the orogen, like the Pennine nappes of the Alps.
2. Thrust-nappe (décollement)
   • A sedimentary cover detaches along a weak layer (shale, salt, gypsum) and slides as a thrust sheet with little folding.
   • Marion King Hubbert and William Walden Rubey (1959) showed that high fluid pressure in the fault zone reduces friction enough to let huge sheets move, solving the old “overthrust paradox”.
   • Examples: the Helvetic nappes, the Moine thrust sheets.
3. Gravity gliding and spreading
   • Once a mountain front is uplifted, sheets may slide downslope under their own weight on a lubricated surface.
   • Early interpreters of the Prealps favoured this idea; today gravity is seen as assisting rather than solely driving nappe movement.
4. Thrust-wedge (critical taper) growth
   • A fold-thrust belt grows like snow in front of a bulldozer: it thickens until it reaches a critical surface slope, then new thrusts form at its front (Dan Davis, John Suppe and F. A. Dahlen, 1983).
   • This explains the forward-younging sequence of Himalayan thrusts.
5. Ductile extrusion (channel flow)
   • Partially molten, low-viscosity middle crust flows towards the surface between a thrust below and a normal fault above, aided by focused erosion on the mountain front.
   • Proposed for the Greater Himalayan crystallines by Christopher Beaumont and co-workers (2001).

Nappes of the Alps

• History: Arnold Escher von der Linth saw that older Verrucano rocks overlie Jurassic–Palaeogene beds at Glarus but described them as two overturned folds (1866). Marcel Bertrand (1884) reinterpreted Glarus as one great overthrust.
  • Hans Schardt then argued (1890s) that the Prealps are allochthonous, having travelled from farther south. The nappe concept was born here.
  • The Glarus thrust is part of the Swiss Tectonic Arena Sardona, a UNESCO World Heritage Site since 2008.
• Nappe systems (from lowest to highest):
  • Helvetic nappes: European-margin cover sediments, moved over 100 km north.
  • Penninic nappes: basement-cored fold-nappes from the ocean basins and microcontinents between Europe and Africa.
  • Austroalpine nappes: the highest, derived from the Adriatic (African) plate.
  • South of the Periadriatic Line lie the Southern Alps, which link to the Dinarides and are not part of the northward-stacked pile.
• Windows and klippen: the Tauern (Hohe Tauern) window and Engadine window expose Penninic rocks beneath the Austroalpine cover; the Prealps are a group of large klippen.

Himalayan Nappes and Thrusts

• The Himalaya is a south-verging stack of thrust sheets formed by India’s collision with Asia. The thrusts are north-dipping and young southward.

• All these thrusts are thought to merge at depth into one gently dipping décollement, the Main Himalayan Thrust. The Gorkha earthquake of 2015 ruptured it.
• Pioneer work:
  • Darashaw Nosherwan Wadia mapped the Kashmir nappe and the syntaxis of the north-west Himalaya.
  • Guy Ellcock Pilgrim and William Dixon West worked out the thrust sheets of the Shimla hills (Jutogh and Chail units).
  • John Bicknell Auden traced the thrust structure of the Garhwal Himalaya and the Krol belt.
  • Arnold Heim and Augusto Gansser (1939), after the Swiss expedition of 1936, defined the Main Central Thrust in Kumaun–Garhwal along the Kali and Alaknanda.
• Klippen: the Almora klippe (Almora nappe) of Kumaun is a sheet of Greater Himalayan-type crystallines, bounded by the North and South Almora thrusts, resting on Lesser Himalayan sediments far south of the MCT. The Kathmandu nappe of Nepal is similar.
• Windows: the Kishtwar window (Jammu and Kashmir) and the Larji–Kullu–Rampur window (Himachal Pradesh) expose Lesser Himalayan rocks through the overlying crystalline sheets.
• Hazard link: Joshimath, close to the MCT zone and built on old landslide debris, sank about 5.4 cm in 12 days (27 December 2022 to 8 January 2023) per ISRO’s NRSC. Crushed thrust-zone rock weakens slopes along the whole belt.

The Moine Thrust, Scotland

• In the North-West Highlands, older Moine schists overlie younger Cambrian–Ordovician rocks along a thrust traced for about 190 km.
• Ben Peach and John Horne mapped it in 1883–84 (with Charles Lapworth reaching the same view); it was among the first thrust belts recognised. Movement dates to the Caledonian (Scandian) orogeny, around 430 million years ago; Knockan Crag shows the contact.

Evaluation

• The classical recumbent-fold model explains Pennine-type nappes but not the thin, far-travelled sedimentary sheets; fluid pressure, décollement and critical-taper mechanics fill that gap.
• Plate tectonics supplies the driving force that contraction theories lacked; see recent views on mountain building.
• Channel flow remains debated. Many workers prefer critical-taper wedge growth, or combinations of both, for the Himalaya.

Faults, Rift Valleys and Horsts

Joints and Faults

• Crustal fracture depends on rock strength and stress intensity.
  • A joint is a fracture with no appreciable displacement.
  • A fault is a fracture along which rocks have moved parallel to the fracture. Total throw may reach hundreds of metres and horizontal slip many kilometres, but it builds up a few metres at a time, earthquake by earthquake.
  • Faults are long-lived weak zones, repeatedly reactivated.

Fault Terminology

• Fault plane: the surface of movement; vertical, inclined or curved. A fault zone is a band of many closely spaced planes.
• Fault dip: angle between the fault plane and the horizontal. Hade is its complement, measured from the vertical.
• Hanging wall: the block above an inclined fault. Footwall: the block below it.
• Upthrown and downthrown sides: the relatively raised and lowered blocks (which block actually moved is often uncertain).
• Throw: vertical displacement. Heave: horizontal displacement across the fault.
• Fault scarp: the steep slope formed directly by fault movement.

Types of Faults

• Faults are grouped by the direction of slip: dip-slip (along the dip) or strike-slip (along the strike).
• Normal fault: the hanging wall moves down; formed by tension; typically steep (commonly around 60°); lengthens the crust.
• Reverse fault: the hanging wall moves up; formed by compression; shortens the crust. A reverse fault dipping less than 45° is a thrust; a very low-angle thrust with large displacement is an overthrust.
• Strike-slip (wrench, tear, transcurrent) fault: blocks slide horizontally. Dextral (right-lateral) if the far block moves right, sinistral (left-lateral) if it moves left. Such faults make little or no scarp.
  • Where a strike-slip fault bends or steps, pull-apart basins open, such as the Dead Sea on the left-lateral Dead Sea Transform.
• Oblique-slip fault: combines dip-slip and strike-slip motion.
• Step faults: a series of parallel faults whose downthrown blocks all drop in the same direction, like a staircase.

Rift Valleys, Graben and Horst

• A graben is a block dropped down between two roughly parallel normal faults; a horst is a block left standing high between them.
• A rift valley is a long, narrow, regional-scale trough of such faults, often with volcanism and earthquakes. Some authors treat graben and rift as synonyms; others keep “rift” for the large, plate-scale structure.
  • Many rifts are half-graben, bounded by a major fault on one side only.
• Two ways of forming: the middle block sinks while the sides stay put, or the side blocks rise while the middle stays put.
• World examples:
  • Upper Rhine graben: about 350 km long and 50 km wide, from Basel to the Frankfurt area, formed mainly in the Oligocene. It is flanked by the Vosges and Palatinate Forest on the west and the Black Forest and Odenwald on the east, classic horsts (block mountains).
  • Afro-Arabian rift system (traditionally the “Great Rift Valley”): about 6,500 km from Syria through the Red Sea to Mozambique. It joins the Red Sea and Gulf of Aden rifts at the Afar triple junction, where Africa is splitting.
  • Lake Baikal rift in Siberia; the Midland Valley of Scotland; Death Valley in California.
• Indian examples:
  • Narmada–Tapi grabens and the Satpura horst: steep faults with 500–800 m of cumulative throw bound the Narmada graben. The Satpura block stands between the Narmada and Tapi grabens as a horst, uplifted again in the Late Quaternary.
    • The zone has hot springs, dyke swarms and earthquakes, such as the 1997 Jabalpur earthquake. It is part of the Son–Narmada–Tapi (SONATA) lineament, a Precambrian weakness reactivated many times.
  • Damodar valley: its coalfields (Raniganj, Jharia, Bokaro) lie in Gondwana half-grabens preserved by faulting.
  • The rift origin of the Narmada valley itself was long debated. Deep seismic and gravity work now supports a faulted rift system.
• Floors below sea level: the deepest continental rifts sink below sea level.
  • The Dead Sea surface, in the Jordan rift, lies about 440 m below sea level and is still falling; it is the lowest land on Earth.
  • Badwater Basin in Death Valley lies about 86 m below sea level.

Origin of Rift Valleys

• Tensional (keystone) hypothesis
  • Stretching opens two parallel fractures and the central block drops like the keystone of a cracked arch.
  • John Walter Gregory, who studied the East African rift in 1893 and published The Great Rift Valley (1896), described it as subsidence between parallel rents.
  • Objections: there is no empty space under the crust for a block to fall into. It would have to displace magma, so rifting should always bring volcanism, which it does not.
• Compressional hypothesis
  • Edward James Wayland (Lake Albert–Ruwenzori), Bailey Willis (Dead Sea and East Africa) and Edward Crisp Bullard (1936), using gravity data, argued that the side blocks are thrust up and over the central block, which is held down beneath them.
  • Such a “ramp valley” would be wedge-shaped, narrow above and broad below. It fitted the negative gravity anomalies over the rift floors.
• Current view
  • Seismic, drilling and GPS evidence shows rifts are bounded by normal faults and are zones of lithospheric extension and thinning. They are driven by plate divergence, often above mantle upwelling.
  • East Africa is an incipient divergent plate boundary. The Red Sea marks the next stage, a young ocean; see plate tectonics.
  • Local compression does occur where rifts are later inverted, as in Kachchh, where old rift faults now move in reverse.

Bullard’s Compressional Model

• Edward Crisp Bullard, from his gravity surveys in East Africa (1933–34), held that a rift block cannot sink under gravity like a keystone. It must be forced down by compression from both sides, in stages:
  • Stage 1, buckling and first crack: lateral compression buckles the rigid plateau crust until the stress exceeds rock strength and it cracks at one point.
  • Stage 2, first thrust: one side overrides along the crack (upthrust) while the other is pushed down (downthrust). The load of the rising block cracks the downthrust block again at its highest point.
  • Stage 3, second thrust: the far part of the downthrust block overrides along the second crack. The strip between the two cracks becomes the rift block, held down between two upthrust blocks.
• The width of the rift depends on the elasticity of the rocks, the depth of the valley and the density of the substratum; the deeper the rift, the wider it is.

Other Indian Fault Structures

• Shillong plateau pop-up: the plateau is a crustal block raised between two reverse faults, the Oldham fault on the north (dipping south at about 57°) and the Dauki fault on the south.
  • In the great 1897 Assam earthquake (about Mw 8.1), its northern edge rose more than 11 m on the Oldham fault (Roger Bilham and Philip England, 2001).
• Allah Bund: a fresh fault scarp from reverse faulting on a reactivated rift fault in the Kachchh basin.
• Great Boundary Fault: separates the Vindhyan basin from the Aravalli–Bundelkhand rocks in south-east Rajasthan (Bundi–Chittorgarh), with scarps along the Chambal region.

Structural Landforms

Structure and Landform

• Pure tectonic landforms (fresh scarps, domes, grabens) are mostly young. Older ones have been reshaped by denudation, so most relief is structural rather than purely tectonic.
  • Arthur L. Bloom (1978) argued that, in a sense, all subaerial relief is tectonic, because uplift must first raise land above sea level.
  • Jean Tricart (1974) distinguished tectostatic relief (the present disposition of strata) from tectodynamic relief (active deformation).
  • Tricart also held that structure dominates at the regional scale, and climate-controlled processes at the detailed scale.
  • Cliff D. Ollier (1981) separated tectonics (mountain ranges, fold belts, island arcs) from structural geology (folds, faults, joints).
• How lithology and structure steer landform evolution is covered under factors controlling landform development. Definitions of the landforms below are listed in the terminology of structural landforms.

Uniclinal (Homoclinal) Structures

• Uniclinal (homoclinal) structures are sedimentary beds all dipping one way at a uniform angle because of regional tilt.
  • They form by uplift of off-lapping coastal-plain sediments, or as one limb of a large dome or fold.
• Differential erosion of alternating hard and soft beds gives scarp-and-vale topography and trellis drainage (drainage patterns).
  • Strike (subsequent) streams cut strike vales along soft beds; resistant beds stand up as ridges.
  • Dip streams on the dip slope are long, gentle and widely spaced, with sustained flow. Anti-dip (obsequent) streams on the scarp face are short, steep and closely spaced (higher drainage density).
• Homoclinal (monoclinal) shifting: streams in the strike vales slide down-dip as they cut down, so the whole ridge-and-vale system migrates laterally. Grove Karl Gilbert (1877) first described it in the Henry Mountains.

Cuesta, Hogback and Homoclinal Ridge

• Cuesta: an asymmetrical ridge on gently dipping beds.
  • It has a steep scarp face (often above 30°, with bare rock) cut across the bed ends, and a long, gentle dip slope (a “backslope” where the surface departs from the true dip).
  • Tricart called cuestas a half-inverted relief.
• Hogback: a sharp ridge on steeply dipping beds, with nearly equal slopes on both sides. Cuestas grade into hogbacks as dips steepen past roughly 30–45°.
• Homoclinal ridge: the general term for ridges of intermediate dip.
• Twin cuestas: a stream incises deeply into the backslope, producing two parallel scarps in the same bed.
• Double cuesta: two superposed cuestas of different beds on one slope, showing unequal scarp retreat.
• Butte and mesa: flat-topped outliers left in front of a retreating cuesta, capped by the resistant bed.
• Indian example: the Kaimur range and the margins of the Bhander plateau (Vindhyan sandstone over shale and limestone). They form continuous scarps in places and scarps broken by embayments where anti-dip streams erode actively.

Factors Controlling Cuesta Form

• Lithology: a thick, durable caprock over thick weak beds gives a high, bold cuesta. The profile becomes concave: a sharp crest in the hard bed and long curved slopes in the soft bed.
• Dip: low dips give broad cuestas with long dip slopes; steep dips give narrow, symmetrical ridges. On Clinch Mountain in the Appalachians, dip-slope stream length and basin area fall as dip rises.
• Scarp retreat: mass movement, spring sapping at the scarp foot, stream undercutting and down-dip shifting of strike streams all push the scarp back.
• Bevelling: a recently planed surface cuts all cuestas to one summit level. With time, differences in rock thickness, resistance and dip reassert themselves.

Folded Structures

Relief on Young Folds

• In young, simply folded belts, relief mirrors structure: anticlines are ridges and synclines are valleys.
• The Jura Mountains gave the classic terms:
  • mont: anticlinal ridge
  • val: synclinal valley
  • cluse: a gorge cut across a mont
  • combe: a hollow eroded along the crest of a mont
  • ruz: a hollow cut into its flank
• Indian example: the Siwalik hills have anticlinal ridges (such as the Mohand anticline) and longitudinal structural valleys, the duns such as Dehra Dun, between the Siwalik and Lesser Himalayan ranges.

Inversion of Relief

• Inversion of relief is the reversal of the original fold relief by erosion: anticlines become valleys and synclines become ridges.
  • The crest of an anticline is stretched and cracked by tension, so it weathers fast. Once its hard cap is breached, the soft core is exposed at a higher level than the same soft beds in the syncline.
  • Streams on the anticline are higher and steeper, so they cut down faster than the synclinal streams and eventually capture them.
  • The syncline, its hard beds compressed and protected in the trough, is left standing as a synclinal ridge (perched syncline).
• Examples:
  • Ridge and Valley Appalachians of the USA: Sideling Hill in Maryland is a classic synclinal ridge.
  • The Weald of south-east England: a breached anticline whose chalk has been stripped from the centre.
  • Among the eroded Proterozoic folds of the Aravalli range, ridges follow resistant quartzite rather than the original anticlines.

Cycle of Erosion on Folded Structure

• The classical model, drawn by Oscar Diedrich von Engeln (1942) on the Davisian cycle, assumes simple open folds of alternating hard and soft beds, uplifted once and then left stable.
• Youth
  • Longitudinal (synclinal) consequents flow along the synclines. Lateral (transverse) consequents run down the anticline flanks to join them.
  • Lateral consequents are steeper, so they cut down fast and extend headward onto the anticlinal crests. There they breach the caprock and cut notches (water gaps).
  • Subsequent streams develop along the soft core of the anticlines. By river capture they unite into anticlinal strike streams.
• Maturity
  • Anticlinal subsequents deepen fastest. They have a greater height and gradient, soft beds nearer the surface, and tension-fractured crests.
  • They capture the synclinal streams, and the original master consequent is dismembered. Inversion of relief is now complete: anticlinal valleys and synclinal ridges.
  • Where anticlinal streams reach hard beds below, they shift down-dip by homoclinal shifting into the soft beds of the former syncline. They become resequent streams, “new consequents” flowing in the original direction at a lower level.
• Old age
  • Relief is worn down to a peneplain, and streams lose their adjustment to structure under a waste cover.
  • Renewed uplift starts a second cycle: rejuvenated streams etch out parallel ridges and valleys again, with the hard beds as ridges whatever their fold position.
• Debate: Sidney William Wooldridge and R. S. Morgan placed resequent streams in a second cycle. Armin Kohl Lobeck (1939) found them already in the mature stage of the first. The timing depends on rock resistance and local conditions.
• Evaluation: real fold belts are rarely uplifted once and then left still. Active belts like the Himalaya fold, uplift and erode at the same time, so steady-state and tectonic-geomorphic models have replaced the staged cycle in explaining them.

Faulted Structures

Fault Scarp and Fault-Line Scarp

• A fault scarp is made directly by fault movement; the scarp face is the exhumed fault plane, little changed by erosion.
• A fault-line scarp is made by differential erosion along an old fault, where hard and soft rocks have been brought side by side. It may face the same way as the original scarp or the opposite way.
• Since fault-line scarps depend on contrasts in resistance, they occur mainly on stable platforms of alternating sedimentary rocks. Their evolution depends on the relation between the fault’s throw and the thickness of hard and soft beds.

Tectonic Scarps

• Original (active) fault scarp: formed by the upthrown block during faulting.
• Residual fault scarp: an original scarp that has been worn back by erosion during a quiet phase.
• Composite fault scarp: produced where a fault is intermittently active; a fresh fault face appears at the base of an older, eroded scarp.

Fault-Line Scarps

• Consequent (normal) fault-line scarp: soft rocks on the downthrown block are stripped, so the scarp faces the same way as the original.
• Obsequent (reversed, opposed) fault-line scarp: a fall in base level exposes soft rock on the upthrown block. Erosion lowers it below the hard rock of the downthrown block, so the scarp faces the opposite way.
• Resequent fault-line scarp: after further erosion and base-level fall, the scarp faces the original direction again. It comes later than the consequent scarp and stands at a lower level.
• Subdued and exaggerated fault-line scarps: lowered or heightened by erosion relative to the throw.
• Exhumed (resurrected) fault scarp: a scarp buried under sediment and later re-exposed by erosion, usually smaller than the throw.
• Composite fault-line scarp (Charles Andrew Cotton): part faulted, part eroded.
  • A scarp is planed away, re-etched as a resequent fault-line scarp, then the fault moves again. The result is an eroded upper face above a fresh faulted lower face, or the reverse.
• Inversion in a graben: long erosion can lower the flanking horsts below a rift floor of resistant rock, leaving the old graben as high ground.

Block Relief and the Western Ghats Debate

• Block mountains (horsts), tilt blocks and graben lowlands make up faulted relief: the Vosges and Black Forest, the Satpura, the basin-and-range of Nevada.
• The Western Ghats escarpment was long read as a fault scarp of the west coast, often linked to down-faulting of the Konkan.
  • It is now seen mainly as an erosional great escarpment on an uplifted rift flank. The flank was raised when India separated from the Seychelles around the time of the Deccan eruptions (about 66 million years ago), and the scarp then retreated inland.
  • The escarpment shows no stratigraphic offset along most of its length, though normal faults exist offshore and in parts of the coast.

Domed Structures

• Domes form by upwarping of strata or by the intrusion of magma (laccoliths). The latter is seen in the Henry Mountains of Utah, where Grove Karl Gilbert described laccoliths.
• Erosion strips the cover and exposes concentric rings of cuestas, hogbacks and strike vales around a resistant core. Most domes today carry polycyclic relief.

Cycle of Erosion on a Dome

• Youth: consequent streams radiate from the crest, giving radial (centrifugal) drainage. Headward erosion breaches the crest, opening a central basin ringed by inward-facing scarps that retreat outward.
• Maturity: the soft beds are removed down to the crystalline core. Subsequent streams develop along soft rings at right angles to the consequents, giving annular drainage, while obsequent and resequent streams appear later.
  • Alternating beds form rings of hogbacks (steep dips) and cuestas (gentle dips), with strike valleys between.
  • A large, resistant core stands up as a dissected upland. A small or weak core is hollowed into a basin.
• Old age: lateral erosion wears down ridges and core alike to a peneplain, if the area stays stable long enough.
• Examples:
  • Black Hills (South Dakota), raised in the Laramide phase: a granite core (about 1.8 billion years old) and a limestone plateau, ringed by the Red Valley on soft red Spearfish shale and an outer hogback of Lakota–Fall River sandstone.
  • The Weald: a breached anticlinal dome of Miocene Alpine age. Inward-facing chalk escarpments of the North and South Downs, a Greensand ridge and clay vales (Low Weald, Vale of Holmesdale) surround it.

Landforms Produced by Endogenetic Forces

• Endogenetic forces make first- and second-order relief directly, and set the frame for every structural landform.