472 
FORMATION OF DUNES. 
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 accumulation is formed which consti¬ 
tutes the foundation of a dune. However slight the elevation 
thus created, it serves to stop or retard the progress of the sand 
grains which are driven against its shoreward face, and to pro¬ 
tect from the further influence of the wind the particles which 
are borne beyond it, or rolled over its crest, and fall down 
behind it. If the shore above the beach line were perfectly 
level and straight, the grass or bushes upon it of equal height, 
the sand thrown up by the waves uniform in size and weight 
of particles as well as in distribution, and if the action of the 
wind were steady and regular, a continuous bank would be 
formed, everywhere alike in height and cross section. But no 
such constant conditions anywhere exist. The banks are 
curved, broken, unequal in elevation; they are sometimes 
bare, sometimes clothed with vegetables of different structure 
and dimensions ; the sand thrown up is variable in quantity 
and character; and the winds are shifting, gusty, vortical, 
and often blowing in very narrow currents. From all these 
causes, instead of uniform hills, there rise irregular rows of 
sand heaps, and these, as would naturally be expected, are of 
a pyramidal, or rather conical shape, and connected at bottom 
by more or less continuous ridges of the same material. 
On a receding coast, dunes will not attain so great a height 
as on more secure shores, because they are undermined and 
carried off before they have time to reach their greatest di¬ 
mensions. Hence, while at sheltered points in Southwestern 
France, there are dunes three hundred feet or more in height, 
those on the Frisic Islands and the exposed parts of the coast 
of Schleswig-Holstein range only from twenty to one hundred 
feet. On the western shores of Africa, it is said that they 
sometimes attain an elevation of six hundred feet. This us one 
of the very few points known to geographers where desert 
sands are advancing seaward, and here they rise to the great¬ 
est altitude to which sand grains can be carried by the wind. 
The hillocks, once deposited, are held together and kept in 
