Write explanatory notes on the formation of any two of the following: (a) Waterfalls (b) Deltas (c) Inselbergs

Answer Details

Formation of any two features

(a) Waterfalls

A waterfall is a sudden vertical or near-vertical fall of a river as it flows over a steep step in its bed. It is formed mainly where a band of hard, resistant rock lies across the river course above a band of softer rock. The river erodes the soft rock more quickly than the hard rock, so a step develops. Water plunging over the step erodes a deep plunge pool at the base and undercuts the soft rock beneath the hard cap. In time the overhanging hard rock breaks off and the waterfall retreats slowly upstream, leaving a steep-sided gorge below it. Waterfalls also form where a river crosses a fault scarp or plunges over the edge of a plateau (e.g. relief and knick-point falls).

(b) Deltas

A delta is a low, flat, often triangular tract of alluvium formed at the mouth of a river where it enters a sea or lake. As the river reaches its mouth, its velocity is suddenly checked by the standing water, so it loses energy and can no longer carry its load. It therefore deposits its sediment (sand, silt and mud) in layers. The river splits into several branches called distributaries as it flows over the deposited material. Continued deposition, faster than the sea can remove it, builds up the land seaward, forming the delta. Deltas form best where the river carries a large load, the sea is shallow and tides/currents are weak (e.g. the Niger Delta).

(c) Inselbergs

An inselberg is an isolated, steep-sided residual hill of resistant rock (often granite) rising abruptly from a level plain, common in the savanna and semi-arid tropics. It is formed by long-continued weathering and erosion of the surrounding land. Chemical weathering, aided by wet-and-dry conditions, attacks and rots the less-resistant rock at depth, and running water and wind remove the loosened material (the weathered mantle), lowering the general land surface. The more resistant masses of rock survive and are gradually exposed as the plain around them is worn down, remaining as isolated upstanding hills (inselbergs), for example Olumo Rock and Zuma Rock.