Theory question 303
Wind and Desert LandformsWith the aid of annotated diagrams, explain the characteristics and mode of formation of the following features: (a) gorges; (b) seif dunes.
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(a) Gorge: characteristics: a deep, narrow river valley with very steep (almost vertical) sides; often has rapids; found in the upper course or where a river crosses hard rock or a plateau. Formation: a river cuts down vertically through resistant rock; a waterfall retreats upstream by undercutting and collapse, leaving a gorge below it; or rapid uplift (rejuvenation) makes the river cut down fast; in dry areas, little weathering of the sides keeps them steep. Examples: Kainji gorge on the Niger, Batoka gorge (Victoria Falls). Diagram: retreating waterfall and gorge.
(b) Seif dune: characteristics: long, narrow, sharp-crested sand ridges lying parallel to the prevailing wind; may be many kilometres long and up to 100 m high; separated by sand-free corridors. Formation: formed where the dominant wind blows steadily in one direction with occasional cross winds; eddies between the ridges sweep the corridors clear and pile sand along the ridges; a barchan may be elongated into a seif when one horn grows. Examples: Sahara (Libya, Egypt). Diagram: parallel ridges with wind arrows.
Annotate both diagrams.