Paper 1 · November 2003 · Fluvial Geomorphology
A drainage density study recorded that a sandstone area receiving 900 mm of annual rainfall had a drainage density of 2.5 km per km2, while a limestone area receiving the same 900 mm of annual rainfall had a much lower drainage density of only 1.5 km per km2. Despite receiving identical rainfall, why does the limestone area have a lower drainage density than the sandstone area?
ALimestone is permeable, so rainwater infiltrates through joints and solution features instead of forming surface streams.
BLimestone weathers more slowly than sandstone, so fewer valleys have had time to form.
CLimestone areas are generally flatter than sandstone areas, reducing the gradient available for streams to form.
DLimestone contains more clay minerals than sandstone, which blocks channel formation at the surface.
Explanation
Limestone is a permeable rock: rainwater infiltrates through joints and solution features rather than running off to form surface channels, which suppresses drainage density even where rainfall matches a less permeable rock such as sandstone. Weathering rate, relief and mineral composition are not the reason for the contrast described here.
Derived from ZIMSEC Geography Paper 1, November 2003, Q4