444 SAND-PLAINS OF BELGIUM. [April 



spaces beweeii the wall and before and behind it soon get covered 

 with sand, and all the sooner the nearer to the wall. 



A round obstacle like a boulder gives such a direction to the 

 wind that there is formed'in front of it a wall of sand disproportion- 

 ately higher and without any furrow. 



The same effect is produced by a tree ; but the wall of sand is 

 lower, and gradually diminishes till it is lost beliind the trunk. 



An isolated bush or stemless herb-like grass becomes filled up so 

 far with the sand, and there is formed a hillock which slopes away 

 gradually behind. This sanding-up increases in magnitude with 

 the size of the bush, and it may become the nucleus of a sandhill, 

 on which the bush will maintain its stand, until the sand loses its 

 hold of the roots by being blown away from under them. 



Isolated plants of Arundb arenaria often promote this blowing 

 away of the sand by the breakage or sidelong movements of their 

 long stiff side leaves shaking free the sand, the effect of which may be 

 seen in circular furrows produced by the sweep of the broken leaves. 



While isolated plants, as a rule, are thus blown out, it is other- 

 wise when numbers grow in close proximity to each other. They 

 so divert the wind as to prevent its reaching the sand, excepting on 

 the windward margin. 



From these observations it becomes manifest that a sheltering 

 wall and a covering of vegetation are both of them appropriate 

 appliances to arrest a sand-drift. 



But the rain-formed gutters have also a considerable influence on 

 the forms of the dunes. As these cannot carry off all the rain 

 which falls, what is retained has an influence on the superficial 

 sand-layer. But this operates in a new way from that in which 

 the wind does. The modifications produced thus are in many cases 

 great. Deep fissures and extensive subsidences are thus produced, 

 and summits of sand ridges are cut up with these, presenting 

 walls a fathom and more in height ; and afterwards, as these become 

 dry, the sand crumbles down and forms precipices and conical 

 heaps, which go on gaining height till they reach the level of the 

 compact sand wall, and form with it a continuous slope on a 

 succession of similar slopes. 



The operation of rain-formed gutters shows itself also in the 

 formation of trough-like hollows descending from the higher range 

 of the sand-layer to the lowest points, carrying with them the larger 

 sand-grains left in the hollows, robbing many spots of their covering, 

 and often disturbing the self-adjustment and settlement of the hollows. 



Also somewhat grotesque and abnormal effects on the forms of 

 sand-fields are produced by wliirlwinds, seen in what are called 

 Kcssellocher, kettle-holes, and Windschluchten, wind-formed ravines. 



