Rejuvenation and Polycyclic Landforms

• Rejuvenation is the renewal of a river’s erosive vigour, usually after a fall in base level, which drives the stream back into active downcutting and pushes the landscape back towards a youthful stage.
  • The idea grew out of the Davis cycle of erosion, which assumed long crustal stability that nature seldom allows.
• Polycyclic (multicyclic) landforms are landscapes carrying the imprint of more than one cycle of erosion, complete or incomplete, so that old forms sit above young ones.
  • The core vocabulary (base level, interruption, accident, knickpoint, polycycle) is collected in Cycle of Erosion, Rejuvenation and Polycyclic Relief terminology.
• In India, the Chotanagpur plateau, the Narmada at Bhedaghat and the terraced valleys of the Garhwal Himalaya are the standard field laboratories for both ideas.

Interruptions in the Cycle of Erosion

Geography Optional Notes {PDF}

Complete Notes Package (Paper I & II) | 3000+ Pages | Instant Download

Discounted Price | Buy Now @ ₹1,180 →

Why a cycle is interrupted

• William Morris Davis (1899) described an uplifted landmass passing through youth, maturity and old age to a peneplain.
  • That sequence needs the crust and sea level to stay still for tens of millions of years.
  • Plate motion, ice ages and volcanism make such stillness rare, so most real cycles are interrupted.
• An interruption is any disturbance that upsets the normal run of the cycle. Its causes are:
  • Sea-level change: subsidence or uplift of ocean floors, and the locking up or release of water in ice sheets (the Carboniferous glaciation of Gondwanaland, the Pleistocene glaciation of the northern hemisphere).
  • Uplift or subsidence of the land.
  • Volcanic outpourings and major climatic shifts.
• A cycle broken by interruptions is an interrupted cycle; repeated interruptions leave several cycles (polycycles) recorded in one region.

Base-level interruptions and accidents

Base-level changes

• Base level is the lower limit of vertical erosion; for most rivers it is set by sea level.
• A negative change (base level falls) comes from uplift of the land or a fall of sea level.
  • Channel gradient steepens, the river incises, and the cycle is pushed back (late maturity reverts to youth), so cyclic time lengthens.
  • This is rejuvenation, the most important interruption and the main generator of polycyclic relief.
• A positive change (base level rises) comes from subsidence or a rise of sea level.
  • Gradients slacken, alluviation accelerates and the cycle jumps forward (youth to maturity, maturity to old age), so cyclic time shortens.
  • Example: the Holocene rise of sea level after the last glacial maximum drowned river mouths into estuaries, as along the Narmada and Tapi.
• Sea-level changes are eustatic (worldwide); uplift and subsidence are tectonic and may be local or regional.

Accidents: volcanic and climatic

• An accident does not merely stretch or shorten the cycle; it closes the chapter and a fresh cycle starts on a new surface.
• Volcanic accident: fissure eruptions bury the old relief and obliterate the drainage.
  • A new cycle begins only after the lava cools and new consequent streams form.
  • Indian example: the Cretaceous Deccan lava flows buried the older planation surface over much of the western and central Peninsula, with outliers reaching the western pat country of Chotanagpur; the Cenozoic cycle began on the new surface.
• Climatic accident: a humid region turning arid or glacial abandons the fluvial cycle for an arid or glacial/periglacial one.
  • A minor climatic shift does not close the cycle; a wetter climate raises discharge and accelerates erosion, which is itself a form of rejuvenation.

Rejuvenation

Meaning

• Rejuvenation is the acceleration of fluvial erosion, caused mainly by a negative change of base level, which returns a mature or old river to youthful downcutting.
  • An old river with a gentle gradient, sluggish flow and broad shallow valley is converted into a steep, vigorously incising one.
  • It may begin at the mouth, in the middle course or in the headwaters.
• Because a new cycle starts before the old one ends, the landscape keeps old-stage forms above and young-stage forms below, which is the root of polycyclic relief.

Types and causes of rejuvenation

Dynamic rejuvenation

• Uplift of the landmass raises the river above its base level; tilting steepens gradients down the tilt.
• Lowering of an outlet (for example a lake outlet cutting down) releases extra water and lowers local base level.
• Indian example: the Chotanagpur plateau was raised in Cenozoic (Tertiary) pulses, with the western pat country lifted about 300 m.
  • The North Koel and its tributaries were rejuvenated; knickpoint falls such as Lodh (Burhaghagh) Falls, 143 m on the Burha river, mark the head of that incision.

Eustatic rejuvenation

• Diastrophic eustasy: subsidence of ocean floors (or uplift of coastal land) lowers sea level relative to the continent.
• Glacio-eustasy: water locked in ice sheets lowers sea level worldwide.
  • At the last glacial maximum (about 20,000 years ago) sea level stood roughly 120 m lower; rivers cut valleys across the exposed continental shelves, later drowned by the Holocene rise.
  • Repeated glacial-interglacial swings left terrace flights along many rivers of the formerly glaciated middle latitudes; the classical four-glaciation scheme has since been replaced by dozens of dated cycles, as explained in Quaternary geomorphology and Pleistocene glaciation.

Static rejuvenation

• No change of base level is involved; the river’s energy budget changes instead.
  • Decrease in load: an underloaded river spends its surplus energy on the bed.
  • Increase in discharge from wetter climate or meltwater.
  • River capture: the captor gains the captured stream’s water and cuts down, while the beheaded stream is left misfit in an oversized valley.
• Indian example: luminescence-dated terraces of the Son and Belan valleys (2006) show incision during strong-monsoon, high-discharge phases and aggradation during weak-monsoon phases, far upstream of any sea-level effect.

How a rejuvenated river regrades

• A graded river is a delicate balance of discharge, load and gradient; rejuvenation upsets it and the river works to a regraded profile. The graded curve itself is explained in River Valleys, Graded River and Profile of Equilibrium.

• The reverse also happens: delta growth lengthens the river and lowers its gradient, so it aggrades and regrades at a higher level; a load-heavy young tributary does the same below its confluence.

Landforms of Rejuvenation

• Rejuvenation superimposes youthful erosional forms on mature or old ones; the general catalogue of river forms is in fluvial landforms.

Knickpoints and knickpoint falls

• A knickpoint (nick point) is a break of slope in the long profile where the new, steeper profile meets the old graded one.
  • It is called the head of rejuvenation because incision has advanced only up to it.
  • Knickpoints migrate upstream by headward erosion until the whole profile is regraded and they vanish.
• Where the break is abrupt, water falls vertically, giving knickpoint falls, followed downstream by gorges and plunge pools.
  • Falls retreat by backwasting (headward recession) and lose height by downwasting.
  • Niagara Falls has receded about 11 km since the last ice age; its crest averaged about 1 m a year historically but only about 0.1 m a year now, after flow diversion and crest management.
• Indian knickpoint falls:
  • Ranchi plateau (eastern scarp): Hundru Falls (98 m) on the Subarnarekha, Jonha (Gautamdhara) Falls (43 m) where the Gunga hangs over the Raru, and Dassam Falls (44 m, two cascades) on the Kanchi. The three lie on a north-east to south-west line, read as the line of the latest uplift.
  • Palamau and western pats: Lodh Falls (143 m) on the Burha, a North Koel tributary, with further falls on the Sankh and other pat-edge streams.
  • Rewa plateau: Chachai (130 m) on the Bihad, Keoti (98 m) on the Mahana and Purwa (70 m) on the Tamsa (Tons).
  • Narmada: Dhuandhar Falls (30 m) at Bhedaghat, near Jabalpur.
• Potholes are drilled rapidly in rejuvenated rocky beds, as in the channels of Chotanagpur and the Narmada near Jabalpur.

Valley-in-valley and multi-storeyed valleys

• Topographic discordance (unconformity): older, mature or old-stage forms above and younger, youthful forms below in one valley section.
• Valley-in-valley (two-storeyed or two-cycle) valley: a rejuvenated river cuts a narrow, deep inner valley into the floor of its broad, flat older valley.
  • Remnants of the old floor survive as benches or terraces on both sides.
  • In time the inner valley widens to a new flat floor; a second rejuvenation cuts a third storey, giving three-storeyed and multi-storeyed valleys.
• Indian examples:
  • Damodar at Rajrappa (Ramgarh district, Jharkhand): a broad old valley with a narrow incised gorge inside it; the Bhairavi (Bhera) enters by a 9 m fall, a hanging valley left behind by the trunk’s faster incision.
  • Narmada at Bhedaghat: a broad upper valley with the marble gorge cut below the Dhuandhar knickpoint.
  • Himalayan rivers of Uttarakhand: many carry several terrace levels on each side.

River terraces

• River terraces are flat steps along a valley side, the remnants of former valley floors or floodplains abandoned when the river cut down.
• Sequence: a graded river builds a wide floodplain, is rejuvenated, incises it, leaves a pair of terraces, widens a new floodplain at the lower level, and repeats; each rejuvenation adds a lower terrace.

• By material:
  • Rock (strath or erosional) terraces: a bedrock platform with a thin gravel veneer.
  • Fill (aggradational or depositional) terraces: cut into a thick valley fill.
  • Cut-and-fill terraces record alternating aggradation and incision.
• Structural benches differ: they arise from differential erosion of hard and soft beds and record lithology, not rejuvenation.
• Himalayan example: in the Alaknanda valley at Srinagar (Garhwal), luminescence dating shows aggradation about 21,000–18,000 and 13,000–9,000 years ago, and incision after about 10,000 years ago, when a stronger monsoon combined with tectonic steepening; bedrock was cut at about 1.4–2.3 mm a year, leaving a cut terrace and a fill terrace above the present bed.

Incised meanders

• Incised (inclosed) meanders are meander loops cut down into bedrock, inherited from a river that meandered on an older, gentle surface and then incised after rejuvenation.
  • The terms incised, entrenched, intrenched, inclosed and ingrown are all used; entrenched and ingrown are the two genetic types.
  • John Lyon Rich (1914) introduced ingrown meanders for asymmetric incised loops.

• World example: the Goosenecks of the San Juan, Utah, are loops cut more than 300 m deep as the Monument Upwarp rose slowly beneath the river.
• Indian examples: the Damodar gorge near Rajrappa, the Narmada gorge at Bhedaghat, and the Karo below its fall at the southern edge of the Ranchi plateau.

Uplifted peneplains and accordant summits

• An uplifted peneplain is an old-stage planation surface raised by uplift and then dissected; its remnants appear as accordant summit levels standing above the younger surface. The wider family of such surfaces is covered in erosion surfaces.
• Appalachians: the Schooley, Harrisburg and Somerville surfaces, from oldest and highest to youngest and lowest, were read as three successive uplifts and cycles.
• Indian example: the pat lands of the western Ranchi plateau and Palamau are a raised older peneplain dissected into mesas and buttes, such as Netarhat, Khamar, Rudni, Jamira and Bagru pats.

Gorges, hanging valleys and rejuvenated drainage

• Gorges and canyons within broad valleys mark the stretch between the mouth and the knickpoint.
• Hanging tributaries result where the trunk incises faster than its tributaries (Bhairavi at Rajrappa, Gunga at Jonha).
• Drainage adjustments:
  • Antecedent rivers keep their course across a rising barrier by incising as fast as it rises (the Indus, Sutlej and Brahmaputra gorges across the Himalaya).
  • Superimposed drainage is let down from a cover (lava or laterite) onto discordant older rocks.
  • Headward erosion from knickpoints favours river capture.
  • On dissected scarps radial and annular patterns develop, with braided channels on the flats below, as on the eastern lower Ranchi plateau.

Landforms of the second cycle of erosion

• A second cycle starts when rejuvenation interrupts the first; its landscape mixes relict first-cycle forms with young second-cycle forms.

• Other second-cycle signatures:
  • Hanging valleys and misfit streams.
  • Fresh alluvial fans at scarp feet.
  • Accelerated gullying of the old regolith.

Polycyclic Landforms

Meaning

• Polycyclic (multicyclic) relief results when several cycles of erosion, complete or incomplete, have left their forms in one region.
  • If cycles follow one another they are successive cycles; the result is a staircase of erosion surfaces joined by scarps.
  • The landscape is a palimpsest: each cycle partly erases and partly preserves the work of the last.
• Reading such staircases to reconstruct regional history is the method of denudation chronology.
• Classic regions: the Appalachians and, in India, the Chotanagpur plateau.

Identifying polycyclic relief

• Summit accordance: hilltops rising to common levels across different rocks.
• Benches and stepped surfaces separated by scarps.
• Knickpoints at similar heights on neighbouring streams.
• Terrace flights that can be correlated along a valley.
• Topographic discordance: flat, mature upper valleys above narrow young gorges.
• Correlated deposits and weathering crusts: laterite caps, lava covers and offshore sediment wedges that date each surface.

Regional study: the Chotanagpur plateau

The stepped surfaces

Evolution

• Older planation: the granite-gneiss basement was worn to an extensive peneplain in the Mesozoic.
• Volcanic accident: in the west, Cretaceous basalt covered part of this surface; weathered in a humid tropical climate it now forms laterite caps.
• Cenozoic uplift in pulses, broadly contemporary with Himalayan orogeny, raised the western block by about 300 m and tilted the plateau.
  • Each pulse rejuvenated the rivers, which cut new surfaces at lower levels and left the older ones as pats and accordant ridges.
• Northern edge: the Damodar valley is a faulted trough (Gondwana graben); the plateau’s northern scarp falls from about 600 m to 300 m within some 10 km, with steep straight tributaries and a cascade on the Damodar’s headwater.

Landform evidence

• Central Ranchi plateau:
  • Wide, shallow, terraced valleys with meandering channels.
  • Residual hills, exfoliation domes and tors, the survivors of earlier cycles.
  • Its step-like valley-side “terraces” are debated; one reading treats them as rejuvenation terraces later smoothed by slope wash.
• Eastern and southern ghats: rivers leave graded upper reaches, plunge over Hundru, Jonha and Dassam, and pass into narrow gorges, a classic topographic discordance.
  • Below the ghats they regain wide, flat, meandering valleys on the lower surface.
  • The southern plateau descends through three steps, each marked by falls, rapids and gorges on the South Koel and Karo.
• Pat lands: rejuvenated North Koel tributaries dissect the old surface into mesas and buttes with high falls such as Lodh.
• Damodar at Rajrappa: a two-storeyed valley with an incised gorge and a hanging tributary.
• Genetic reading of the plateau:
  • Endogenetic: uplifted peneplains, fault scarps, escarpments, structural falls.
  • Exogenetic: peneplain remnants, monadnocks, pediments, terraces.
• A 2023 DEM-based landform mapping of the plateau confirmed extensive peneplain remnants, lateritic surfaces and rejuvenation falls tied to the roughly 300 m Cenozoic uplift.

Other Indian polycyclic landscapes

• Narmada at Bhedaghat: a broad upper valley, the Dhuandhar knickpoint and a marble gorge with incised loops, about 20 km from Jabalpur.
• Garhwal Himalaya: flights of cut and fill terraces along the Alaknanda and Bhagirathi, best seen in the wide Srinagar valley of the Alaknanda.
• Rewa and Kaimur plateaus: high falls on the Tons system where rivers leave the plateau for the Ganga plain.

Critical evaluation

• Not every knickpoint is a head of rejuvenation.
  • Lithological knickpoints stay fixed at hard beds or fault lines; transient (rejuvenation) knickpoints migrate upstream and tend to occur at similar elevations across neighbouring tributaries.
  • The Rewa falls, for instance, sit where rivers cross resistant Kaimur sandstone, so lithology and base-level fall act together.
• Terraces need not mean uplift.
  • Climate-driven changes in discharge and load produce aggradation-incision cycles (Son, Belan, Alaknanda).
  • Stanley Alfred Schumm (1973) showed through complex response that a single base-level fall can produce several terraces.
• Accordant summits need not be uplifted peneplains.
  • John Tilton Hack (1960) explained Appalachian ridge-crest accordance as dynamic equilibrium with rock resistance.
  • The Jurassic or Cretaceous age once given to the Schooley surface has been questioned, and an Oligocene–Miocene age has been proposed since the 2000s.
• Uplift is often continuous, not episodic, as Walther Penck argued; rejuvenation may be an ongoing adjustment rather than a series of neat cycles.
• Current view: rejuvenation is recast as the transient response of a river network to base-level fall.
  • Eric Kirby and Kelin Whipple (2012) set out how channel steepness indices and knickpoint patterns reveal uplift rates, while cautioning that lithology and climate allow more than one interpretation.
  • OSL and cosmogenic-nuclide dating now give terraces and surfaces real ages, turning Davis’s qualitative stages into measurable histories.