146 OlDHAM: SANDHILLS OF CLIFTON NEAR KARACHI. 



In the second stage a form like fig. 7 is attained, these ground plans 

 being in both cases taken from actual sandhills seen near Clifton ; 

 here it will be seen that the generally oval outline is interrupted by a 

 notch in the leeward end. At the centre of this notch, where the line 

 drawn in the direction of the wind covers the highest point of the 



Fig. 7. — Ground plan of sandhill further advanced in growth. 



sandheap, the section resembles that indicated in fig. 4, but on either 

 side of this line the height of the heap grows gradually less, and the 

 section approximates more and more to fig. 3, till finally we find 

 the sand thinning off gradually to leeward as well as to windward. 

 From one point of view the sandhill in this stage may be regarded 

 as an imperfect form of a complete sandheap of oval outline, the 

 imperfection being due to a lack of material ; but this is only an 

 incomplete statement of the case, as the notch, after its first establish- 

 ment, is by no means solely due to a deficiency of material, but in part 

 to a prolongation of the horns on either side of it. 



At its commencement the notch in the leeward end of the sand- 

 heap appears to be due simply to a failure of material, sufficient in 

 amount to fill in the whole of the space under the upcurving wind 

 currents, but, once established, the effect of the eddy set up in it has 

 to be reckoned with. This not only keeps the space within the 

 notch clear, but appears to cause, or at any rate to assist in causing, 

 an upward bend in the wind currents on either side, leading to the 

 deposit of sand on either side in the same manner as the original 

 accumulation, but further down wind. As the sandheap grows in size, 

 and spreads laterally, there is doubtless some scouring out of the notch 

 by the eddy set up in it, but apart from this, the horns continue to grow 

 outwards and down wind, in a direction usually oblique to its direction, 



( 14 ) 



