1885.] SASB-PLAISS OF BELGIUM. 443 



attaining to mountain heights by the sea ever raising new ranges of 

 dunes behind them, by -which the continuous supply of material is 

 arrested. 



The unending variety seen in the form and contour of sand- 

 dunes is attributable to the operation of numerous subsidiary 

 influences. First to be mentioned is the incessant change of wind 

 modifying the dispositions which are being made, and variations in 

 its velocity or force ; and next to be mentioned are existing bodies 

 on or adjacent to the sand-field, which modify the operation of the 

 wind in diverse ways. 



The effect of a compact wall opposed to the wind, is to create a 

 deep furrow to windward, but at the same time a parallel ridge of 

 sand at some little distance to windward. A beautiful illustration 

 of this, cited by Wessely, is one supplied by the sand-drift around 

 the church of Alt. Pillau, on the Baltic. The village some thirty 

 years ago surrounded the church, but in consequence of the strong 

 pressure of the sand, it was about that time rebuilt some distance 

 farther to the east. The church, however, had to be left where it 

 stood. Eound about it there grew up a sand-ridge from 12 feet to 

 2 feet in lieight, but at no point did this extend to the wall of the 

 church. This remained to its very foundation clear of sand ; and 

 though the worshippers had to climb over the sand-ridge, with its 

 precipitous wind-slope, never did they find the church door 

 blocked up. Ultimately the sand-ridge was fixed, and a passage 

 was cut through to the church. The great height of the church, in 

 comparison with that of the sand-field around, accounts for all. 

 But on the open drift, the ridge to windward attaining a height 

 equal to that of the wall, shelters it. There is no longer an eddy 

 to keep the furrow clear, and ultimately the wall is covered with 

 the drift-sand, and in a short time the ridge at its end becomes 

 joined to the wall. But before it is joined by the ridge throughout 

 its extent, a portion of this falls over and forms a similar though 

 much lower wall. And it is remarkable that the original ridge 

 prolongs itself at its extremities, so that later on the intermediate 

 wall is considerably shorter than is the first-formed ridge. 



Very low walls, a few times as high, let the greater part of the 

 sand spring over them ; the low ridges to windward of such are the 

 steeper in their inclination the stronger the winds are. 



When the wall is not continuous, but so broken as to allow the 

 sand to pass through, there is little to be seen of the deep furrow ; 

 but though the wind passes through the breaches, bearing sand with 

 it, its force is so reduced that it lets this fall both in front of and 

 behind the barrier, and the parallel repetitions of the broken wall 

 will bear some comparison with plots of grass and herbage. The 



