Q. As differential erosion agents act upon Earth’s surface, there is produced an orderly sequence of landforms. Write a critique.
The statement is the seventh of the fundamental concepts set out by W.D. Thornbury in Principles of Geomorphology (1954), and it is Davisian in descent: it presumes that each erosional agent, given time and stability, drives a landscape through a predictable succession of forms. The proposition is defensible as a statement about process-specific assemblages, but fails as a claim about temporal succession — structure, climate, thresholds and tectonics all break the sequence.
Locating the Proposition in the Geographical Cycle
- W.M. Davis (“The Geographical Cycle”, 1899) made landscape a function of structure, process and stage, evolving through youth, maturity and old age toward a peneplain dotted with residual monadnocks, with rejuvenation restarting the run.
- Thornbury codified the Davisian inheritance into two linked concepts: that each geomorphic process leaves a distinctive imprint and develops its own characteristic assemblage of landforms, and that the different agents accordingly produce an orderly sequence.
- The model rests on two stated conditions — rapid uplift followed by prolonged tectonic stillstand, and a single dominant agent operating on structurally uniform ground. The critique turns almost entirely on how rarely both hold.
What the Proposition Gets Right
- Assemblages are genuinely diagnostic. Glacial erosion reliably yields cirque, arête, horn and U-shaped trough; karst solution yields sinkhole, uvala and polje; aeolian action yields yardang, zeugen and barchan. An agent does impose a recognisable signature.
- Direction of change is real over long time. Under prolonged stability, relief does decline and rivers do approach grade. The Peninsular erosion surfaces of Chota Nagpur and Bundelkhand record exactly this tendency toward planation.
- The scheme is a powerful ordering device, and denudation chronology — the reading of a region’s history from its surviving surfaces — remains a working method built directly on it.
Structure Refuses to Surrender
- J.T. Hack (1960) argued from the Appalachian ridge-and-valley and Shenandoah country that relief expresses the balance between erosional resistance and process intensity. Ridges stand because their rock is strong, not because the landscape is young; form is time-independent.
- Deccan trap step-topography makes the point on Indian ground: benches and flat mesas follow near-horizontal resistant flows, not any stage of a cycle. The Bhander plateau caprock slopes behave identically.
- Aravalli monadnocks and Nilgiri inselbergs persist on quartzite and resistant crystallines through what should, on a cyclic reading, be an advanced old age. Lithology, not elapsed time, holds them up.
Climate Sets the Sequence, Not Time
- Peltier (1950) defined morphogenetic regions on the basis of mean annual temperature and precipitation, showing that the dominant process — and therefore the landform assemblage — is climatically determined. There is no single sequence to be orderly about.
- Julius Büdel’s climato-genetic geomorphology went further: most landscapes are relict, carrying forms inherited from climates no longer operating. Peninsular India’s deep lateritic weathering profiles are a tropical etch inheritance being stripped under a different present regime.
- Davis’s “normal” cycle was drawn from the humid temperate margins of North America and then universalised. Under a climatic reading it is one regional case among several, not the standard against which others are deviations.
Landscapes Are Polycyclic and Polygenetic
- Thornbury’s own next concept concedes the ground: complexity of geomorphic evolution is more common than simplicity. The orderly sequence is thus qualified by its own author.
- Polycyclic landscapes carry surfaces from repeated uplift, as the stepped Peninsular surfaces and the multiple planation levels of the Western Ghats hinterland do. Polygenetic landscapes carry forms from successive process regimes.
- Quaternary climatic oscillation guarantees polygenesis. No mid-latitude or monsoonal landscape has experienced one uninterrupted regime long enough for a cycle to run to completion.
Thresholds and Complex Response Deny Orderliness
- S.A. Schumm (1973, 1979) showed that adjustment is episodic. Once a geomorphic threshold is crossed, a system reorganises abruptly, and a single external change produces a complex response of alternating incision and aggradation rather than a smooth transition.
- The Kosi megafan and its westward avulsions are the textbook Indian case: a channel that shifts catastrophically rather than ageing gracefully. Brahmaputra braiding at Majuli varies with sediment supply and seismic disturbance, not with stage.
- Intrinsic thresholds mean change can occur with no external trigger at all — a possibility a strictly sequential, externally driven model cannot accommodate.
Uplift and Erosion Are Coupled, Not Sequential
- Walther Penck (Die morphologische Analyse, 1924) held that form expresses the ratio of uplift rate to denudation rate, both acting simultaneously. Davis’s stillstand is the exception, not the rule.
- Isostatic compensation makes the coupling automatic: erosional unloading provokes uplift, which renews erosion. Himalayan knickpoints and river captures record continuous tectonic forcing that resets any sequence long before it can complete.
- Modern tectonic-climatic coupling research treats precipitation, denudation and rock uplift as mutually reinforcing — a feedback loop, not a one-way progression from initial form to end form.
The Deeper Logical Objection
- “Orderly sequence” conflates spatial arrangement with temporal succession. Davis inferred a chronology from landforms observed side by side, substituting space for time.
- A basin showing steep headwater slopes and a broad lower valley displays a spatial gradient in process intensity, not three ages of one landscape. Steep slopes may be young, or merely resistant, or merely undercut.
- The inference is therefore unfalsifiable in the form stated: any landscape can be assigned a stage after the fact, which is why Strahler (1952) demanded measurable, testable propositions in its place.
Conclusion
The proposition survives as a statement about process signatures and as a heuristic for reading long-term denudational tendency, but not as a law of landscape development. Structure persists, climate varies, thresholds intervene, and uplift never obligingly pauses — so real landscapes are polycyclic, polygenetic and episodic rather than orderly. Its enduring value is that in failing so instructively it produced its own successors: Penck’s rate ratios, Hack’s equilibrium, Peltier’s morphogenetic regions and Schumm’s thresholds were all framed against it.