602 INLAND DUNES. 
north-west, except towards the centre of the desert, where, for a 
distance of one or two hundred paces, they gradually opened 
to the west, and then again gradually resumed the former 
position. 
Tschudi observed, in the same desert, two species of dunes, 
fixed and movable, and he ascribes a falciform shape to the 
movable, a conical to the fixed dunes, or medanos. “The me- 
danos,” he observes, “are hillock-like elevations of sand, some 
having a firm, others a loose base. The former [latter], which 
are always crescent-shaped, are from ten to twenty feet high, 
and have an acute crest. The inner side is perpendicular, and 
the outer or bow side forms an angle with a steep inclination 
downwards.* When driven by violent winds, the medanos pass 
rapidly over the plains. The 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. . . . <A plain often appears to be 
covered with a row of medanos, and some days afterwards it is 
again restored to its level and uniform aspect. . . . 
“The medanos with immovable bases are formed on the 
blocks of rocks which are scattered about the plain. The sand 
is driven against them by the wind, and as soon as it reaches 
the top point, it descends on the other side until that is likewise 
covered ; thus gradually arises a conical-formed hill.t Entire 
hillock chains with acute crests are formed in a similar manner. 
. . . On their southern declivities are found vast masses 
of sand, drifted thither by the mid-day gales. The northern 
declivity, though not steeper than the southern, is only spar- 
* The dunes of the plains between Bokhara and the Oxus are all horse-shoe 
shaped, convex towards the north, from which the prevailing wind blows. On 
this side they are sloping, inside precipitous, and from fifteen to twenty feet 
high.—Burnes, Journal in Bokhara, ii., pp. 1, 2. 
+ The sand-hills observed by Desor in the Algerian desert were fixed, chang- 
ing their form only on the surface as sand was blown to and from them,— 
Sahara und Atlas, 1865, p. 21. 
