Q. Explain the major processes involved in the development of hillslopes.
A hillslope is the surface linking an interfluve crest to a valley floor, the commonest of landforms and the route by which almost all subaerial denudation is delivered. Its development turns on one accounting relationship: the rate at which weathering supplies debris against the rate at which gravity and water remove it. The processes are best read by family, with basal removal setting the terms for all of them.
Production Against Removal: The Master Control
- Weathering-limited slopes — removal outpaces regolith production, rock is bare, and strength and structure dictate the angle; the Western Ghats escarpment free faces and the Bhander sandstone caprock of the Vindhyan plateau belong here.
- Transport-limited slopes — regolith forms faster than it is carried away, a thick mantle accumulates, and transport rather than rock sets the form; the deeply lateritised Ghat slopes of Karnataka and Kerala are the Indian type case.
- Gilbert (1877), in Report on the Geology of the Henry Mountains, drew the distinction and added the law of divides — near a divide the contributing area is small, so gradient must steepen for waste to move. Carson and Kirkby (1972) later restated it as sediment continuity.
Weathering and Regolith Production
- Mechanical weathering — frost shattering above the Great Himalayan tree line, insolation-driven sheeting on Aravalli quartzites, salt weathering along the Thar margin — yields coarse angular waste standing at high angles of repose.
- Chemical weathering — hydrolysis, oxidation and solution — dominates the humid tropics, the laterite-capped Deccan and Ghat slopes carrying mantles tens of metres thick. Coarse waste holds steep rectilinear slopes; fine cohesive waste moves at low angles.
Slow Mass Movement
- Soil creep — grain-by-grain displacement of a millimetre to a few centimetres a year, driven by wetting and drying, freeze and thaw, and bioturbation by roots, termites and burrowing fauna.
- Creep transport grows with distance from the divide, steepening the profile downslope, and is the accepted cause of summital convexity — set out by Gilbert (1909) in “The convexity of hilltops”.
- Solifluction — flow of waterlogged regolith over a frozen or impermeable substrate, active across the periglacial belt of Ladakh, Lahaul and Spiti; where creep and grazing combine, the surface breaks into terracettes, as on Nilgiri and Shillong grasslands.
Rapid Mass Movement
- The Coulomb frame governs failure: shear strength is cohesion plus effective normal stress times the tangent of the angle of internal friction, and failure comes when shear stress exceeds it — usually because rising pore-water pressure destroys effective stress, not because load is added.
- Falls and topples — detachment from a cliff, or outward rotation of jointed columns, building talus below the free face; routine on Himalayan road cuts and the Kaimur and Bhander scarps.
- Slides — translational along a discontinuity dipping out of the face, or rotational on a curved surface.
- Flows — debris and earth flows moving as viscous fluids, dominant on Nilgiri slopes and at Wayanad, where some 570 mm of rain in two days on 30 July 2024 killed or left missing 359 people. They reset the profile in hours.
Wash Processes: Rainsplash, Sheetwash and Rilling
- Rainsplash — drop impact ejects particles, and on a gradient the downslope component is larger, giving transport without flow.
- Sheetwash — unconcentrated overland flow, Hortonian where rainfall intensity exceeds infiltration capacity, saturation-excess on footslopes; its power rises downslope, which is why wash-dominated slopes are concave.
- The convexo-concave profile of humid slopes is a two-process signature — creep above, wash below, the inflexion where their rates equalise.
- Rilling and gullying follow once flow concentrates, as in the Chambal ravines and the gullied Chotanagpur plateau margins. Meghalaya takes the heaviest rainfall on earth yet keeps its wash-limited concavity under forest, and loses it where cover is stripped.
Subsurface Processes
- Throughflow — lateral movement above a less permeable horizon, feeding the footslope and raising the pore pressures that trigger failure there.
- Piping and suffosion — removal of fines along percolation lines, cutting tunnels that collapse into gullies, as in the dispersive alluvium of the Chambal and Yamuna ravines.
- Seepage erosion and sapping — undermining at a spring line, producing the amphitheatre valley heads of the Vindhyan and Deccan scarps.
Basal Removal: The Control on the Whole Profile
- Everything above is conditional on the slope foot. Where a river, wave or glacier evacuates arriving debris the slope stays steep and retreats; where it cannot, waste accumulates and the slope declines — active basal removal is why the Alaknanda and Teesta corridors fail repeatedly.
- Penck’s point survives: the ratio of basal output to upslope supply, not time, decides whether a slope retreats or flattens.
What the Processes Assemble: Slope-Unit Models
Wood’s Four Units (1942)
- Waxing slope — the creep-dominated convex crest.
- Free face — bare outcrop where falls dominate, present only where rock is strong or undercutting is active.
- Constant or debris slope — the rectilinear talus segment at the angle of repose.
- Waning slope — the basal concavity cut by wash, extending and flattening through time.
The Nine-Unit Land-Surface Model (Dalrymple, Blong and Conacher, 1968)
- Replaces form description with process attribution across interfluve, seepage slope, convex creep slope, fall face, transportational midslope, colluvial footslope, alluvial toeslope, channel wall and channel bed.
- Its advance is to carry the slope through to the channel, treat subsurface water explicitly, and define units that can be instrumented — where slope study became measurable.
Process Dominance and the Classic Evolution Models
- Slope decline (Davis, 1899) follows where creep and wash dominate weak rock under vegetation and basal removal has stopped.
- Slope replacement (Penck, Die morphologische Analyse, 1924) follows where a debris slope built from below consumes the steeper face above, the concavity growing at the expense of the maximum angle.
- Parallel retreat (King, 1953, 1962) follows where a resistant caprock sustains a free face and sheetwash extends a pediment at its foot — King’s Great Escarpment of southern Africa, with the Bhander caprock and Deccan step-topography as Indian analogues.
Conclusion
Hillslope development belongs to no single process but to a budget. Weathering fixes what is available, creep and wash move it slowly, mass movement moves it catastrophically, subsurface flow decides when, and the basal condition decides whether any of it registers in form. The three classical models are therefore not rivals but end-states of different process mixtures. Process monitoring, cosmogenic-nuclide denudation rates and landscape-evolution modelling confirm it: slope form is a process signature, read backwards.