Geomorphology – Topography Associated with Horizontally Layered Rocks

Topography & Horizontal Strata

"Topography": the shape and configuration of the Earth’s surface (landform relief; hills, valleys, plains, etc.)
Focus of lesson: landscapes produced by horizontally layered (stratified) rocks and their associated landforms.
Dominant regional setting in Southern Africa:
- Central (High-veld) Plateau
- Great Escarpment (≈ 5\,000\,\text{km} long, semi-continuous mountain chain)
- Narrow Coastal Plain flanking the Indian & Atlantic Oceans.

Vertical cliff – near-vertical free-face produced where resistant rock is exposed.
Escarpment – a long, steep slope or cliff separating two relatively level areas of differing elevation (often marks plateau edge).
Plateau – broad, high-lying, relatively flat land surface, elevated hundreds–thousands of metres above surroundings.
Strata (sing. stratum) – individual, recognisable layers of sedimentary or volcanic rock; rocks with layers are stratified.
Sediments – solid material (clastic particles, chemical precipitates, bioclasts) moved and deposited in a new location.
Floodplain – flat ground adjoining a river channel that is periodically flooded; prime site for fine sediment deposition.
Cap-rock – hard, resistant layer protecting softer underlying beds; controls shape of mesas/buttes/conical hills.
Knick-point – sudden change in a slope or river gradient indicating differential resistance or active erosion front.
Scarp retreat / back-wasting – near-parallel upslope shift of a free-face due to weathering & erosion with little change in slope angle but loss of horizontal ground.

Composite slope pattern consists of:
- Near-vertical free-face (cliff).
- Debris (talus) slope at 30^\circ – 35^\circ (angle of repose).
- Lower, concave wash slope where fine material accumulates.

Rock resistance dictates gradient:
- Resistant beds ⇒ steep cliffs.
- Weak beds ⇒ gentle, eroded benches.
Climate influences dominant geomorphic processes:
- Humid → sheet flow, chemical weathering, thick fine soils.
- Arid → scarce runoff, mechanical weathering, thin coarse soils.

Origins: prolonged horizontal deposition (marine, floodplain, lava).
Humid regions
- Sheet flow rounds slopes.
- Chemical weathering → deep, fine-grained soils.
- Contour ploughing possible; agriculturally valuable.
Arid regions
- Sparse water; slopes rugged, uneven.
- Dominant mechanical weathering → thin, coarse soils; limited human use.

Formed by thick, horizontal basalt sheets.
Joints/cracks exploited by rivers: down-cutting yields flat high surface with steep cliffs + deep valleys.
Example: Drakensberg portion of Great Escarpment.

Preconditions: tectonic uplift → broad, even plateau.
Over-graded rivers incise vertically along joints ⇒ deep, steep-walled valleys.
Climate contrast:
- Humid: walls round off, canyon eventually widens/degenerates.
- Arid: hard beds keep vertical cliffs; soft beds weather into benches.
Famous examples: Grand Canyon (USA), Fish River & Blyde River Canyons (South Africa).

Evolve from widened canyon terrain under continued scarp retreat.
Sequence of landforms (from large to small):
- Plateau → Mesa → Butte → Pointed butte (spitskop) → Conical hill/koppie.
Process details:
- Protective cap-rock (dolerite, quartzite, basalt) shields softer sandstone/shale.
- Sidewalls erode; plateau narrows, cliffs retreat without losing height.
- Ultimately pediments merge, forming low-relief pediplain.

Mesa
- Flat-topped, steep-sided highland smaller than a plateau but crest width > height.
Butte
- Isolated, flat-topped hill; crest width < height (remnant of a mesa).
Conical hill
- Solitary, smoothly conical residual; final stage of back-wasting sequence.

Weathering (chemical & mechanical) + erosional removal of waste causes cliff to migrate upslope.
Hard strata maintain near-original elevation (minimal vertical lowering).
Produces transformation chain: \text{Plateau} \rightarrow \text{Mesa} \rightarrow \text{Butte} \rightarrow \text{Conical hill}.

Plateaux: can host settlements, transport lines (flat surface), agriculture where soils are thick & water accessible.
Canyon Landscapes:
- Steep slopes & wide valleys hinder farming & infrastructure; prized for tourism/scenery.
Karoo Landscapes:
- Semi-arid; marginal for crops but suitable for extensive sheep farming; astronomy & renewable-energy siting benefit from clear skies & open space.
Hilly Humid Terrains: fertile soils + rounded slopes enable intensive agriculture (maize, tea, forestry) with contour conservation techniques.
Basaltic Plateaux: high waterfalls, hydro-electric potential; jointed basalt provides quarry stone but steep escarpments obstruct road/rail.

On topo maps, look for:
- Closed contours with flat tops = mesa (larger) or butte (smaller).
- Concentric, evenly spaced contours tapering to a point = conical hill.
- Bold cliff symbol / close contours bounding a plateau edge = escarpment.
Example (Table Mountain area): letters A–E on provided map correspond to plateau, mesa, butte, conical hill, canyon.

Debris slope inclination: 30^\circ – 35^\circ (angle of repose of talus).
Great Escarpment length: 5\,000\,\text{km}.
Cap-rock composition in Karoo: often dolerite (intrusive basaltic rock) or quartzite; underlying weaker shale/sandstone.

Differentiate hilly landscape vs basaltic plateau in terms of genesis, dominant rock type, slope form, climatic process.
Explain why mesas & buttes display steep concave slopes yet retain flat tops.
Discuss advantages & obstacles posed by horizontally layered terrain to human transport, settlement, and economic activity (≈ 8 lines).
Describe the role of scarp retreat in the mesa→butte→conical hill sequence.
Match landforms to cap-rock descriptions; analyse photographs for evidence of horizontal bedding.