THE NILE GRINDING AWAY ITS ROCKY CHANNEL.

Wanganui Herald, Volume XXXVII, Issue 10972, 11 June 1903, Page 6

THE NILE GRINDING AWAY ITS ROCKY CHANNEL.

The erosive action of the Nile at one point in its long career through the desert ha 3 recently formed the subject of an exceedingly interesting paper by Dr John Ball, Ph.D., F.G.S. The author chose for investigation the Semna Cataract 'or rapid, which is situated sonic 43 miles from Wady Haifa. At this point ;> ridge of hard gneiss forms a kind oi natural barrier across the river, which is 1300 ft wide immediately above and below the ridge. At high Nile the river flows over the barrier without any very considerable diminution of its width, r but owing to the shallowness of the water on the barrier the velocity of the stream is much increased, and violent eddies are often set up. At low Nile the gneiss bank entirely bars the stream, except for a narrow channel about 130 ft in wid/th. Observing that this comparatively narrow opening was sufficiently large to -allow the low Nile to pass through at a. rate of two miles and ahalf per hour, Dr Ball surmised that it must be very deer>. Upon making soundings, he found that it was about 65feet deep in the centre. The question, then, was how this gulch came to be formed. Dr Ball has found almost conclusive evidence that it has been carved out of the solid rock by the Nile itself. All the odaervations made by Dr Ball appear to support the view that S6ft of solid gneiss has been removed by erosion since B.C. 2300. That portion of the barrier which is laid bare at low Nile is honey-combed with large potholes; frequently a number of potholes < cut into one another, the re?ult being that large fragments fall into the holes to continue the grinding. It is estimated that the gneiss has been worn away at the rate of 500 tons per annum. Tins represents a lowering of the bed at the rate of two millimetres per annum. Thus Dr Ball estimates that the yearly discharge of the Nile past Semna is very nearly 100,000 million tons of water, which flows at a mc-an velocity of 4 V killometres per hour at high Nile and at 2\ kilometres per hour nt low Nile.