Wlarvels of the Universe 731 
gust. In sand there must be a steady blow to produce ripples. Water, after being rippled, will 
in time again attain a level. Sand will remain rippled. 
As sea-waves are merely big ripples, so sand-waves are but magnified sand-ripples. The big is 
the magnification of the small in both cases, and the formation of ripple-marked sand is the first 
stage towards that of sand-waves, and these may rise in the form of sand-hills to a height of two 
hundred and fifty feet. Some have occasionally reached even five hundred feet. 
The whole of the Western Sahara is more or less smothered in sand. Vast areas of sand move 
with the trade-winds from east to west. The rocky surface is hidden below, and the sand forms 
wave after wave, shifting gradually with every wind more and more to the west, until the sand 
becomes blown out to sea to form sand-banks along the shores of Western Africa. 
Each particle of sand is on the move. Immediately a depression in the sand has been formed by 
the blowing-out of the sand-grains, the tendency is for the next supply of sand to pile up on the 
windward side of the depression until the ripple-crest reaches a height of instability before the 
wind, when the further supplies will be carried over the crest and drop on the lee side of the ripple. 
Any slight irregularity in the surface will give rise to a bank, or if the solid rocks are too far below 
the surface to be of any effect, a bank or wave may be given rise to by intermittent winds acting at 
various angles or with different degrees of intensity. The sand will thus be arranged in more or less 
parallel waves running at right angles to the source of the material, and when they reach the coast 
will give rise to what are known as coast-dunes. 
Looking away across a sand-desert where the banks of sand rise and fall with great regularity 
as far as the eye can reach, the appearance presented resembles nothing so much as an ocean whose 
waves have suddenly been solidified in a position of rest. The sand of the desert has been sorted 
Pee ences 
Se OS 
— i a 
“gaa sapcmpsonenenonncoet 
SAND-WAVES. 
Both sand-waves and sea-waves are formed by the action of wind, but while a series of water-ripples may be produced by a 
single gust, in sand there must be a steady current to cause the same effect. The photograph shows a sand-sea on the Algerian 
Frontier. 
