144 OLDHAM: SANDHiLLS OF CLIFTON NEAR KARACHI. 



Central Asian sandhills by v. Middendorff, 1 who first brought the 

 term burkhan into geological literature ; his figures are reproduced 

 in Plate II, and it will be seen that he distinctly gives a rounded out- 

 line to the extremities of the cusps and shows them as dying out with 

 a gentle slope in every direction. 



The distinction is not merely an unessential refinement of detail, 

 but has an important bearing on the theory of the cause of the 

 crescentic form adopted by this type of sandhill. This is usually 

 said to be due to the wings travelling faster than the central portion 

 of the dune, on account of their lesser height and the consequently 

 smaller amount of sand which has to be transported, but the explanation 

 is only partial, and leaves much unexplained, especially the primary 

 question of why one part of the dune should be higher than another. 



The growth of the sandhills from their earliest beginnings to the 

 typical barchane can be studied at Clifton. In the first instance a 

 broad low patch of sand forms on the stony surface, six to ten feet 

 in its longer diameter, which lies in the direction of the wind, and not 

 much more than as many inches in thickness at the centre, from which 

 the sand thins out on all sides and finally gives way to the ordinary 

 stony surface without any well-defined margin. These patches of sand 

 do not appear to owe their origin to any local obstacle, they lie to 

 leeward, but not necessarily directly to leeward, of the fully formed 

 sandhills and evidently owe their origin to an upward bend of the air 

 currents, as indicated in fig. 3, by which a space of comparative calm 

 is formed. In this space the sand, drifting along the surface by the 



Fig. 3. — Longitudinal section of an incipient sandhill, 

 wind, comes to rest; in the earlier stages the supply of sand is 

 sufficient to completely fill the space and its surface, following the 

 stream lines of the air currents, has a smooth, gently-rounded outline 

 from one end to the other. 



The heap of sand, once formed, helps to accentuate the upward 



1 A. v. Middendorff : Mem. Acad. Imp. Sri. St, Petersbourg, XXIX, 29(1881). 

 ( .12 ) 



