Q. Discuss the concept and landforms associated with the second cycle of erosion.
Rejuvenation is the renewal of a river’s erosive power; the second cycle of erosion is the fresh cycle it opens on a landscape already worn down by an earlier one. In the Davisian frame it is an interruption that halts the march to peneplanation and drives the lower course back to youth while maturity survives above. The product is a polycyclic landscape, legible in one valley cross-section.
Rejuvenation and the Second Cycle
- Definition — a sudden increase in the erosive power of streams, caused by steepening of channel gradient, reopening vertical incision in an already graded system.
- Negative change of base level is the master control: a fall lengthens cyclic time and accelerates erosion, while a rise shortens it and provokes alluviation.
- Interruption, not replacement — Davis (1899) allowed for interrupted cycles; the second is nested inside the unfinished first rather than superseding it.
- Topographic discordance is the diagnostic signature: senile or mature forms in the upper valley, youthful forms below, and no accordance between them.
- Regrading works headward — the new profile is set at the mouth and migrates upstream, so at any instant the lower course belongs to the second cycle and the headwaters to the first.
The Triggers of Rejuvenation
Dynamic Rejuvenation
- Caused by movement of the land itself — epeirogenic uplift, tilting, or lowering of an outlet.
- Uplift raises the basin above base level, adding potential energy and steepening gradient, so the effect is regional and repeatable.
- Tilting produces asymmetric incision and drainage adjustment; faulting produces localised knickpoints.
- The Chota Nagpur highlands record three phases of Tertiary uplift transmitted from the Himalayan orogeny, lifting the Ranchi patlands by some 305 metres.
Eustatic Rejuvenation
- No uplift is required: the base level itself falls, chiefly by glacio-eustatic lowering as ocean water is locked into continental ice sheets.
- Incision begins at the mouth and migrates upstream, so the effect is time-transgressive, not simultaneous across the basin.
- Pleistocene glaciation lowered sea level enough to rejuvenate rivers far into the tropics; the four terraces of the Red River of the United States are classically correlated with the four Pleistocene glacials. Tectono-eustatic change of ocean-basin volume works the same way, far more slowly.
Static Rejuvenation
- No change of base level at all; the disturbance is internal, the system regrading because its own variables have shifted.
- Increased discharge — heavier rainfall, glacial meltwater, or river capture handing the catchment of a beheaded stream to the captor.
- Reduced sediment load — resistant bedrock exposed upstream, or sediment trapped behind a reservoir, leaves surplus energy for corrasion: the “hungry water” effect below large dams.
- Both raise stream power without any movement of land or sea, making static rejuvenation the hardest type to read from landforms alone.
The Diagnostic Landforms
Knickpoints and Rejuvenation Heads
- A knickpoint is the break of slope where the new regraded curve intersects the old profile, marking the point rejuvenation has reached — hence head of rejuvenation.
- It recedes upstream by headward erosion and plunge-pool undercutting, so its position measures how far the second cycle has advanced.
- Indian knickpoint falls: Hundru (about 76 m) on the Subarnarekha, Dassam on the Kanchi and Johna near Ranchi, on the dissected Chota Nagpur edge; Chachai (127 m), Keoti (98 m) and Purwa (75 m) on the Rewa plateau.
- Jog falls on the Sharavati and Chitrakote on the Indravati mark the same break at the Western Ghats escarpment and the Bastar plateau rim; knickpoints in the Bhagirathi and Alaknanda gorges record active Himalayan uplift.
Valley-in-Valley Forms
- Renewed incision cuts a narrow, steep-sided inner valley inside the broad, shallow outer valley of the first cycle — the two-storeyed or valley-in-valley form, flanked by terraces marking the older floor.
- A further interruption adds a third storey — a multi-storeyed or multi-cyclic valley, with a pair of terraces at each level.
- The Damodar at Rajrappa is the classic Indian two-storeyed valley: a senile flat-floored valley with a youthful gorge excavated within it.
- The Narmada at Bheraghat, downstream of Dhuandhar falls near Jabalpur, shows the same discordance cut into marble.
Incised Meanders
- Meanders shaped by lateral erosion on a floodplain and then cut down into bedrock when erosive power is renewed; unlike simple meanders they are always cut in rock.
- Entrenched meanders have uniform slopes on both sides of the loop — symmetrical, gorge-like, the product of rapid incision, as on the Chambal above its Yamuna confluence.
- Ingrown meanders are asymmetrical: the concave bank is undercut into a river cliff, the convex bank carrying a gentle slip-off slope of sand and gravel. Asymmetry implies slower uplift, letting lateral swing continue during downcutting.
- The Narmada gorge at Bheraghat and the Damodar gorge near Rajrappa are the standard Indian examples; the Goosenecks of the San Juan on the Colorado Plateau are the world type-site.
River Terraces
- Abandoned floodplains or valley floors left as benches above the present channel when incision resumes.
- Paired terraces face each other at equal heights and indicate rapid vertical incision; unpaired terraces alternate down the valley and indicate slow incision with continued lateral migration.
- Strath terraces are rock-cut, carrying only a thin veneer of alluvium, and record incision into bedrock; fill terraces are cut into an earlier valley fill and record aggradation followed by fresh downcutting.
- Major Himalayan rivers of the Uttarakhand hills carry three sets of paired terraces, matching three uplift phases; on the plains the Bhangar older alluvium stands as a terrace above the Khadar floodplain of the Ganga and Yamuna, and flights survive along the Damodar and Subarnarekha. The Middle Rhine terraces are the European parallel, combining uplift of the Rhenish massif with glacial sea-level swings.
Hanging Valleys, River Cliffs and Structural Benches
- Tributaries adjust more slowly than the trunk, so their floors are left hanging above the deepened master valley and enter it by a fall — the Bhera joining the Damodar at Rajrappa.
- River cliffs on the undercut limb of ingrown meanders and structural benches, where incision pauses on a resistant bed, add further steps to the cross-profile and are easily mistaken for terraces.
The Polycyclic Long Profile
- A once-rejuvenated river carries two concave graded segments separated by a knickpoint; a twice-rejuvenated one carries three.
- A long profile with several concave segments is therefore the field evidence for a polycyclic landscape, the number of segments being read as the number of cycles.
- At regional scale the same argument rests on uplifted peneplains — the Ranchi patlands, and the Schooley, Harrisburg and Somerville surfaces of the Appalachians.
Critique
- Polycyclic reasoning is difficult to prove without dating. Counting terraces or profile segments counts events only if their ages are known; luminescence and cosmogenic burial dating now supply what field correlation never could.
- Lithological knickpoints mimic rejuvenation knickpoints. A resistant band generates a break of slope that is structural and stationary rather than cyclic and migratory, so many Vindhyan and Deccan falls demand a bedrock check before uplift is inferred.
- Complex response (Schumm, 1973) shows that terraces and alluvial fills can arise inside the system once an intrinsic threshold is crossed, with no base-level change at all — a terrace is not automatically a cycle.
- Hack (1960) attacks the apparatus at its root: accordant summits and multi-level surfaces may express uniform lithology and drainage density, not successive cycles.
- Climate and tectonics are hard to separate, since one terrace flight can record either, and only correlation across several basins decides between them.
Conclusion
The second cycle is best read not as a separate cycle but as an interruption superimposed on an unfinished first, which is why polycyclic, rather than cyclic, describes most real landscapes. Its landforms — knickpoints, valley-in-valley forms, incised meanders and terrace flights — remain the most legible archive geomorphology holds of uplift and sea-level change. Their evidential value now rests on dating them, not on counting them.