SAND DUNES AND BLOWING SAND. 
141 
smaller and lighter ones move quickly forward, before the 
larger ; but the latter soon overtake and crush them, whilst 
they are themselves shivered by the collision. These medanos 
assume all sorts of extraordinary figures, and sometimes move 
along the plain in rows forming most intricate labyrinths.” * 
Even more remarkable than the medanos are those tall 
revolving columns of sand which are sometimes seen in the 
deserts of Africa and Australia. The sand is taken up by the 
whirling wind, and is carried along like a waterspout. Several 
such columns are often seen at the same time ; they travel at a 
great speed, the lower ends touching the ground. Sometimes 
obscuring the sun, sometimes glowing in its rays, they seem to 
be veritable pillars of cloud and fire. It was a sand-storm with 
columns of sand such as these which played terrible havoc with 
the army of Cambyses in the Libyan desert. 
Baddeley, in describing the dust whirlwinds of northern 
India, says : — “ A broad wall of dust is observed advancing 
rapidly, apparently composed of a number of large vertical 
-columns, each preserving its respective position in the moving 
mass, and each column having a whirling motion of its own.” 
The continuous action of blowing sand wears the rocks in a 
peculiar manner. Mr. Bauerman thus describes the sands of 
part of Arabia Petrsea : — “ Sand-scored stones are abundant 
everywhere As a rule, the hardest rocks are the best 
polished — this being more especially the case with quartz, 
jasper, carnelian, and similar siliceous substances ; while the 
limestones, in addition to being polished, are furrowed and 
scored in every direction, and their surfaces studded with num- 
berless small reticulating grooves, resembling the hill-shading 
on a topographical map.” 
A similar polishing action is apparent on the harder rocks of 
•our own coasts, where these rocks are exposed to the wear of 
sand. Professor Ramsay has suggested that the mushroom- 
shaped and undercut rocks, which are so characteristic of many 
sandstone areas, may have been worn into their present shape 
by the action of wind, blowing the loose sand against the under 
surfaces of the rocks. No one who has visited such rocks 
during high winds will doubt that the wind and sand are 
capable of so cutting out rocks. 
The erosive action of sand, when let fall in a continuous 
stream, or propelled by means of a blast, has been turned to 
account during the last few years in etching or deeply graving 
glass and metal ; the effect is singularly sharp and beautiful. 
* Tschudi’s i: Travels in Peru,” cli. ix. (quoted by Marsh, p. 602). The 
inner side of the hillock can only be (i perpendicular ” when the medanos are 
in motion. 
