Q. Davis’s concept of slope evolution must be taken in the context of his normal cycle of erosion. Justify.
Davis’s treatment of hillslopes is not a free-standing theory of slope mechanics but a corollary of the geographical cycle he set out in 1899. Slope decline — the progressive flattening of the steepest element until no part of the profile is steep — follows only if uplift is rapid and brief, stillstand long, the climate humid temperate and rivers the sole sculptors. Detach the slope scheme from those premises and its central prediction loses its warrant.
The Cycle Supplies Every Premise the Slope Model Rests On
- The “normal” climate — Davis generalised from the humid temperate uplands of New England, where a vegetated regolith, soil creep and rainwash dominate. Such slopes are transport-limited: form is governed by the removal of waste, not by bedrock strength.
- Rapid uplift followed by long stillstand — only a prolonged tectonic pause allows relief, gradient and slope angle to fall monotonically. Continuing uplift would keep regenerating steepness and abolish the decline it predicts.
- Fluvial dominance and a fixed base level — valley-side slopes are grafted on to the long profile of the trunk stream, so as downcutting wanes the slope loses the agent that kept it steep.
- Structure, process, stage — Davis’s trio makes stage the master variable. Structure and lithology are conceded early influence but held to fade as the surface nears base level, so slope angle reads as a clock.
Slope Decline Read Stage by Stage
Davis built the argument across “The convex profile of badland divides” (1892), “The grading of mountain slopes” (1898) and “The Geographical Cycle” (1899). The profile is a waste sheet whose thickness and mobility are set by the stage of the valley below.
Youth
- Angles steepen rather than decline — incision deepens narrow V-shaped valleys faster than divides can be lowered, so the maximum angle actually increases. Rockfall and sliding dominate thin, patchy debris and the slope is weathering-limited.
- Any free face inherited from structure or incision is at its largest now, and slope retreat is negligible.
Maturity
- Lateral erosion supersedes downcutting — valleys widen, divides are consumed by downwasting, and the maximum angle begins to fall.
- The slope becomes graded: every point carries just the gradient needed to move the debris delivered from above — the hillslope analogue of Mackin’s (1948) graded river and Gilbert’s (1877) reasoning in the Henry Mountains.
- Summital convexity is attributed to soil creep, whose ratio to wash is greatest near the divide, while wash increasing downslope excavates the basal concavity. The free face, no longer undercut and buried by its own talus, is consumed early.
Old Age
- A convexo-concave profile throughout — the rectilinear element disappears, convexity and concavity meet, and surviving angles are only a few degrees.
- Peneplain with monadnocks — the residual Aravalli quartzite hills and the erosion surfaces of Bundelkhand and Chota Nagpur are the standard Indian illustrations of the end-form.
Why the Two Cannot Be Prised Apart
- A statement about time, not mechanics — Davis specified no transport law and measured no rate. Flattening is deduced from the waning energy of the cycle; withdraw the cycle and nothing generates decline.
- The weathering-limited to transport-limited shift is itself cyclic — a continuous regolith accumulates only once incision slows, and only a mantled slope can decline by creep.
- Base level is the anchor — rejuvenation restores steep angles and returns the profile to youth, a move intelligible only within the cycle. Himalayan knickpoints and valley-in-valley forms record that reset.
The Contrast That Proves the Dependence
- Walther Penck (Die morphologische Analyse, 1924; English translation 1953) tied form to the ratio of Aufbereitung, the preparation of waste by weathering, to Abtragung, its removal, and to that of uplift to denudation. His profile is replaced from below: a Haldenhang (waste slope) extends upslope at the expense of the Böschung, so “flattening always takes place from below upwards”. His Primärrumpf, Endrumpf and Piedmonttreppen (piedmont staircase) need no stillstand at all — the Black Forest and the Andes gave him ground still rising.
- L.C. King (1953, 1962) made parallel scarp retreat with coalescing pediments the near-universal mechanism and treated semi-arid, not humid temperate, conditions as normal. The Great Escarpment of southern Africa is his type ground and the pediplain his end-form; retreat is indifferent to stage.
- Alan Wood (1942) resolved the developed profile into four units — waxing slope, free face, constant or debris slope and waning slope. Davis’s cycle is Wood’s anatomy with the free face and constant slope eliminated early; King’s the same anatomy with both preserved. The quarrel is over the history assumed, not the anatomy.
Where the Davisian Scheme Fails
- Arid and semi-arid slopes — sparse vegetation and thin regolith keep slopes weathering-limited, so they retreat at the angle of repose. The stepped topography of the Colorado Plateau, the buttes of Monument Valley and Nilgiri and Bundelkhand inselbergs record retreat, not decline.
- Resistant caprock — a sandstone cap over shale sustains the free face indefinitely. On the Bhander plateau near Maihar, capped scarps retreat parallel while hills that have already lost their cap, such as Sharda Pole hill, decline by downwasting: both models run inside one landscape. Deccan trap step topography and the retreating Western Ghats escarpment make the same point.
- Tectonically active belts — Himalayan and Meghalaya slopes are undercut faster than any decline could proceed, and landslide-prone profiles reset to threshold angles rather than drifting toward the peneplain.
- Measurement — A.N. Strahler found maximum slope angles within a locality clustered tightly about a mean irrespective of the slopes’ ages, the signature of retreat at an equilibrium angle rather than of decline. R.A.G. Savigear in Carmarthen Bay found decline where the slope foot is protected and parallel retreat where waves remove debris — basal conditions, not stage, are the control.
- Process geomorphology — J.T. Hack (1960), in the Appalachians and the Shenandoah Valley, replaced stage with dynamic equilibrium between resistance and process; Schumm’s thresholds, Carson and Kirkby (1972) and the nine-unit model of Dalrymple, Blong and Conacher (1968) moved attention to measurable rates.
Conclusion
The justification is internal, not incidental. Every load-bearing element of Davis’s slope model — the steepening of youth, the graded waste sheet of maturity, the convexo-concave old-age profile, the free face consumed early — is derived from the stage of the cycle and from nothing else. Outside that frame slope decline is a bare assertion; inside it, a disciplined deduction. Which is exactly why the alternatives had to abandon the cycle before they could abandon decline.