560 COAST DUNES. 
low beaches of seas, and sometimes of fresh-water lakes. On 
most coasts, the supply of sand for the formation of dunes is 
derived from tidal waves. The flow of the tide is more rapid, 
and consequently its transporting power greater, than that of 
the ebb; the momentum, acquired by the heavy particles in 
rolling in with the water, tends to carry them even beyond the 
flow of the waves; and at the turn of the tide, the water is ina 
state of repose long enough to allow it to let fall much of the 
solid matter it holds in suspension. Hence, on all low, tide- 
washed coasts of seas with sandy bottoms, there exist several 
conditions favorable to the formation of sand deposits along 
high-water mark.* If the land-winds are of greater frequency, 
duration, or strength than the sea-winds, the sands left by the 
retreating wave will be constantly blown back into the water ; 
bat if the prevailing air-currents are in the opposite dimocuae 
* There are various reasons why the formation of dunes is confined to low 
shores, and this law is so universal, that when bluffs are surmounted by them, 
there is always cause to suspect upheaval, or the removal of a sloping beach in 
front of the bluff, after the dunes were formed. Bold shores are usually with- 
out a sufficient 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 wall, 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.”—LEONTARD 
und Bronn, Jahrbuch, 1841, p. 3. 
The calming, or rather diversion, of the wind by cliffs extends to a considera- 
ble distance in front of them, and no wind would have sufficient force to raise 
the sand vertically, parallel to the face of a bluff, even to the height of twenty 
feet. 
