Geomorphic Cycles, Landscape Development, Denudation Chronology, Channel Morphology, Erosion Surfaces and Slope Development

How does a young, rugged mountain region slowly turn into a low, almost flat plain? Geomorphologists have tried to answer this question through models of landscape evolution – the cycle of erosion of W.M. Davis, the uplift-based model of Walther Penck, the pediplanation model of L.C. King and the idea of dynamic equilibrium. This fourth part covers these models along with denudation chronology, erosion surfaces, river channel morphology and slope development – the most theory-heavy part of geomorphology, made easy with short tricks.

Geomorphology Series (5 parts):
Part 1 – Basics, Forces, Earth's Crust, Interior and Geomagnetism
Part 2 – Geosynclines, Continental Drift, Isostasy, Plate Tectonics and Mountain Building
Part 3 – Volcanicity, Earthquakes and Tsunamis
Part 4 – Geomorphic Cycles, Denudation Chronology, Channel Morphology, Erosion Surfaces and Slopes
Part 5 – Applied Geomorphology, Economic Geology and Environment

1. Base Level – the Starting Point

Base level is the lowest level down to which a river can erode its bed. The concept was given by J.W. Powell (1875).

• Ultimate (general) base level: sea level.
• Temporary (local) base level: a lake, a hard rock band or a dam that stops downcutting for a while.

A fall in base level (or uplift of land) gives new energy to rivers – this is called rejuvenation.

2. Davis's Geographical Cycle of Erosion (1899)

William Morris Davis, the American geographer, gave the first complete model of landscape evolution, called the "Geographical Cycle" or "Normal Cycle of Erosion" (normal because it is based on running water in a humid temperate climate).

• Basic idea: Landscape is a function of structure, process and stage (time). Davis famously said "structure, process and time".
• Assumption: a land mass is uplifted rapidly, and then remains stable (no further uplift) while erosion wears it down.

Stages of the Davisian Cycle

Short trick: Youth = Vertical + V-valley; Maturity = Maximum relief + Meanders begin; Old age = Ox-bow lakes + peneplain.
Monadnock is named after Mount Monadnock in New Hampshire, USA.

Interruptions of the Cycle

• Rejuvenation due to uplift or a fall in sea level starts a new cycle. Signs of rejuvenation: knickpoints (breaks in the long profile of a river, often with waterfalls), incised (entrenched) meanders, river terraces and valley-in-valley forms.
• Climatic accidents (e.g., glaciation or desert conditions) and volcanic accidents can interrupt the normal cycle.

Criticism

• Rapid uplift followed by a long period of stability is unrealistic – uplift and erosion usually happen together.
• Too much emphasis on time; less on processes.
• Ignores the role of climate in different regions.
• Few true peneplains exist on the Earth today.

3. Walther Penck's Model (1924)

The German geomorphologist Walther Penck presented his ideas in Die morphologische Analyse (published in 1924 after his death). He argued that landforms depend on the ratio between the rate of uplift and the rate of erosion (degradation), not on time alone.

• The initial gently uplifted surface is called the Primärrumpf, and the final low surface is the Endrumpf.
• He described Piedmonttreppen (piedmont benchlands) – step-like surfaces around a slowly expanding dome.
• Penck believed that slopes retreat parallel to themselves and are replaced from below by gentler slopes (slope replacement).

Short trick: Penck's phases = "Wax – Uniform – Wane" → Convex – Straight – Concave ("CSC").
Davis = Down-wearing (time-based); Penck = Proportion of uplift to erosion.

4. L.C. King's Pediplanation Cycle (1948–1953)

Lester Charles King, a South African geomorphologist, based his model on the landscapes of Africa and semi-arid regions.

• Landscapes are shaped by backwearing – parallel retreat of scarps (steep slopes), not by down-wearing.
• As scarps retreat, gently sloping rock surfaces called pediments are left at their base.
• Pediments join together to form a vast plain called a pediplain.
• Isolated residual hills standing above the pediplain are called inselbergs ("island mountains"); dome-shaped ones of granite are called bornhardts.
• King identified four slope elements: waxing slope (crest), free face (scarp), debris slope and waning slope (pediment).

5. Other Models of Landscape Development

Short trick (end-forms): "Davis Peeps, King Pedals, Crickmay Pans, Wayland Etches, Penck Ends" → Davis – Peneplain; King – Pediplain; Crickmay – Panplain; Wayland – Etchplain; Penck – Endrumpf.

Landscape Development – Key Ideas

• Monocyclic landscapes show only one cycle of erosion; polycyclic landscapes show traces of several cycles (multiple erosion surfaces, terraces). Most real landscapes are polycyclic.
• Climatic geomorphology (Büdel, Peltier, Tricart and Cailleux): each climatic zone – glacial, periglacial, arid, humid temperate, humid tropical – produces its own typical landforms.
• Other special cycles proposed: arid cycle (Davis), karst cycle (Grund, Cvijić), glacial cycle, marine cycle (Johnson).

6. Denudation Chronology

Denudation chronology is the study of the history of erosion of a region – reconstructing the sequence of erosional events (and the changes in base level) that produced the present landscape. It was developed in Britain by S.W. Wooldridge and D.L. Linton in their study of south-east England (Structure, Surface and Drainage in South-East England, 1939).

Methods Used

• Identifying and mapping erosion surfaces at different heights.
• Studying accordant summit levels – hilltops at the same height may be remnants of an old surface.
• Altimetric frequency curves and hypsometric curves (A.N. Strahler, 1952) – showing how land is distributed at different heights.
• Superimposed, projected and composite profiles drawn from contour maps.
• Studying river terraces, knickpoints and long profiles of rivers.
• Dating deposits lying on old surfaces.

Limitation: erosion surfaces are often difficult to date accurately, and similar heights do not always mean the same age.

7. Erosion Surfaces (Planation Surfaces)

An erosion surface is a large, nearly flat surface cut across rocks of different hardness and structure by long-continued erosion. It indicates a long period of stable base level.

Indian examples: The Chotanagpur plateau shows step-like erosion surfaces at different levels, such as the high Pat lands and the Ranchi plateau surface. Erosion surfaces have also been identified in the Western Ghats, Meghalaya plateau and parts of the Deccan.

8. Channel Morphology

Channel morphology studies the shape, size and pattern of river channels and how they adjust to the water and sediment they carry.

Drainage Basin and Stream Order

• A drainage basin is the area drained by a river and its tributaries; it is separated from neighbouring basins by a watershed (divide).
• R.E. Horton (1945) proposed laws of drainage composition, and A.N. Strahler (1952) modified the method of stream ordering:
  • Streams with no tributaries are first order.
  • When two first-order streams join, they form a second-order stream; two second-order streams form a third-order stream, and so on.
  • A lower-order stream joining a higher-order one does not raise its order.
• Bifurcation ratio = number of streams of one order ÷ number of the next higher order (normally 3–5).
• Drainage density = total length of streams ÷ area of the basin. It is high in areas of impermeable rocks, sparse vegetation and heavy rain.

Short trick: Strahler = "Same + Same = Step up" (two streams of the same order make the next order).

River Discharge and Hydraulic Geometry

• Discharge (Q) = Cross-sectional area (A) × Velocity (V), measured in cubic metres per second (cumecs).
• Hydraulic geometry (L.B. Leopold and T. Maddock, 1953): as discharge increases, the river's width, depth and velocity all increase in a regular way.
• Graded river (J.H. Mackin, 1948): a river whose slope is adjusted so that it can just carry the load supplied to it – it neither erodes nor deposits overall.
• The long profile of a graded river is smooth and concave upward – steep near the source and gentle near the mouth.
• The line joining the deepest points of a channel is called the thalweg.

Channel Patterns

Short trick: Meander – Middle-load river, Mild slope; Braided – Big load + Bars. Erosion on the outer bank, deposition on the inner bank ("Out = Off, In = In-fill").

Drainage Patterns

• Antecedent drainage: the river existed before the uplift of the land and kept cutting through it as it rose, forming deep gorges – Indus, Sutlej and Brahmaputra across the Himalaya.
• Superimposed drainage: a river developed on a younger rock cover and, after the cover was removed, cut into the older rocks below without changing its course – Damodar, Subarnarekha, Chambal, Banas.

Short trick: Antecedent = Ante (before) uplift; Superimposed = from a super (upper) cover.

9. Slope Development

Slopes cover most of the land surface, so the way they form and change is central to landscape development.

Elements of a Hill Slope (A. Wood, 1942)

Short trick: "We Find Cool Water" → Waxing – Free face – Constant – Waning (top to bottom).

Theories of Slope Development

Short trick: "Davis Declines, Penck Replaces, King Keeps the angle (parallel retreat)."

Slope Forms and Processes

• Convex slopes are linked with soil creep – explained by G.K. Gilbert (1909).
• Concave slopes are linked with surface wash (rain wash), which increases downslope.
• Straight (rectilinear) slopes form where debris lies at its angle of rest (about 30–35°).
• Slope form depends on rock type, structure, climate, vegetation, base level and mass wasting processes (creep, slides, flows, falls).

Quick Revision Points

• Base level: Powell (1875). Normal cycle of erosion: Davis (1899).
• Davis's stages: youth (V-valley), maturity (max relief), old age (peneplain, monadnocks).
• Penck: waxing (convex), uniform (straight), waning (concave); Endrumpf.
• King: parallel retreat, pediments, pediplain, inselbergs.
• Hack: dynamic equilibrium (1960); Chorley: open system.
• Denudation chronology: Wooldridge and Linton (1939).
• Stream order: Horton (1945), Strahler (1952).
• Braided river: Brahmaputra; meander cut-off: ox-bow lake.
• Antecedent: Indus, Sutlej, Brahmaputra; Superimposed: Damodar, Subarnarekha, Chambal.
• Slope elements (Wood): waxing, free face, constant, waning.

Practice MCQs

1. The concept of the "Normal Cycle of Erosion" was given by:
(a) W. Penck
(b) W.M. Davis
(c) L.C. King
(d) J.T. Hack

2. According to Davis, the final landform of the cycle of erosion is:
(a) Pediplain
(b) Peneplain
(c) Panplain
(d) Etchplain

3. Isolated hills of hard rock standing above a peneplain are called:
(a) Inselbergs
(b) Monadnocks
(c) Mesas
(d) Buttes

4. In Penck's model, the waxing development produces which type of slope?
(a) Concave
(b) Straight
(c) Convex
(d) Vertical

5. The concept of pediplanation was given by:
(a) L.C. King
(b) Crickmay
(c) Wooldridge
(d) Strahler

6. The concept of dynamic equilibrium in geomorphology is associated with:
(a) Davis
(b) Hack
(c) Powell
(d) Penck

7. Which river shows a typical braided channel?
(a) Narmada
(b) Brahmaputra
(c) Tapi
(d) Mahanadi

8. Radial drainage pattern is found around:
(a) Amarkantak
(b) Loktak Lake
(c) Ganga plain
(d) Vindhyan range

9. The Indus, Sutlej and Brahmaputra are examples of:
(a) Superimposed drainage
(b) Antecedent drainage
(c) Consequent drainage
(d) Deranged drainage

10. "Denudation chronology" was developed by:
(a) Horton and Strahler
(b) Wooldridge and Linton
(c) Leopold and Maddock
(d) Davis and Penck

Answers: 1 – (b) | 2 – (b) | 3 – (b) | 4 – (c) | 5 – (a) | 6 – (b) | 7 – (b) | 8 – (a) | 9 – (b) | 10 – (b)

Reference: Savindra Singh, Geomorphology; W.D. Thornbury, Principles of Geomorphology; NCERT Fundamentals of Physical Geography and India: Physical Environment (Class XI).