COAST-DUNES. 639 



"winds are of greater frequency, duration or strength than the sea- 

 mads, the sands left by the retreating wave will be constantly 

 blown back into the water ; but if the prevailing air-currents are 

 in the opposite direction, the sands will soon be cai-ried out of the 

 reach of the highest waves, and transported continually farther 

 and farther into the interior of the land, unless obstructed by high 

 gi'ounds, vegetation or other obstacles. 



The laws which govern the formation of dunes are substanti- 

 ally these. We have seen that, under certain conditions, sand is 

 accumulated above high-water mark on low sea and lake shores. 

 So long as the sand is kept wet by the spray or by capillary attrac- 

 tion, it is not disturbed by air-currents, but as soon as the waves 

 retire sufficiently to allow it to dry, it becomes the sport of the 

 wind, and is driven up the gently sloping beach until it is arrested 

 by stones, vegetables or other obstructions, and thus an accumu- 

 lation is formed which constitutes the foundation of a dune. 

 However shght the elevation thus created, it serves to stop or re- 

 tard the progress of the sand-grains which are driven against its 



shores, and this law is so universal, that when bluffs are surmounted by them, 

 there is always cause to suspect upheavel, or the removal of a sloping-beach in 

 front of the bluff, after the dunes were formed. Bold shores are usually with- 

 out a suflBcient beach for the accumulation of large deposits ; they are com- 

 monly washed by a sea too deep to bring up sand from its bottom ; their abrupt 

 elevation, even if moderate in amount, would still be too great to allow ordi- 

 nary winds to lift the sand above them ; and their influence in deadening the 

 wind which blows towards them would even more effectually prevent the rais- 

 ing of sand from the beach at their foot. 



Forchhammer, describing the coast of Jutland, says that, in high winds, 

 " one can hardly stand upon the dunes, except when they are near the water 

 line and have been cut down perpendicularly by the waves. Then the wind 

 is little or not at all felt — a fact of experience very common on our coasts, 

 observed on all the steep shore bluffs of 200 feet height, and, in the Faroe 

 Islands, on precipices 2,000 feet high. In heavy gales in those islands, the 

 cattle fly to the very edge of the cliffs for shelter, and frequently fall over. 

 The wind, impinging against the vertical waU, creates an ascending current 

 which shoots somewhat past the crest of the rock, and thus the observer or 

 the animal is protected against the tempest by a barrier of air." — Leonhabd 

 und Bronn, Jahrbuch, 1841, p. 3. 



The calming, or rather diversion, of the wind by cliffs extends to a con- 

 siderable distance in front of them, and no wind would have suflBcient force 

 to raise the sand vertically, parallel to the face of a bluff, even to the height 

 of twenty feet. 



