


Part IT. Sect. i. § 1.) SAND DUNES, 323 
“dunes.” This takes place more especially on windward coasts either 
of the sea or of large inland lakes, where sandy shores are exposed 
to the drying influence of solar heat and wind; but similar effects 
may be seen even in the heart of a continent, as in the sandy deserts 
of the Sahara, Arabia, and in the arid lands of Utah, Arizona, &c. 
The dunes travel in parallel, irregular, and often confluent ridges, 
their general direction being transverse to the prevalent. course of 
the wind. Local wind-eddies cause many irregularities of. form. In 
humid climates rain-water or the drainage of small brooks is some- 
times arrested between the ridges to form pools (étangs of the French 
coasts), where formations of peat occasionally take place. On the coast 
of Gascony the sea for 100 miles is so barred by sand-dunes, that in 
all that distance only two outlets exist for the discharge of the drainage 
of the interior. As fast as one ridge is driven away from a beach 
another forms in its place, so that a series of huge sandy billows, as 
‘it were, is continually on the move from the sea margin towards the 
interior. A stream or river may temporarily arrest their progress, 
but eventually they push the obstacle aside or in front of them. In 
this way the river Adour, on the west coast of France, has had its 
mouth shifted two or three miles. Occasionally, as at the mouths of 
estuaries, the sand is blown across so as gradually to exclude the sea, 
and thus to aid the fluviatile deposits in adding to the breadth of the 
land. In Fig. 82 a stream (e e) is represented as crossing a plain (a) 
aah i+, = 
Z af} iH wil li lif. Yay \ 
unt yt aly ul) 
| wi 7 
x 
My . 
f } \ \S 
Fe 



AN 
\ Ni 
\\ 
. 
No Ay 
ip ie) = | 
A | Bl = 
g ion 
OS 
Oy, 7 
le 
Fie. 82.—SAND-DUNES AFFECTING LAND-DRAINAGE (B.). 
. WE: 
ie yl 
ih = 

—s 

= 
at the margin of the sea or of a large inland sheet of water, bounded 
by a range of sand-dunes (b 6) extending between the two lines of 
cliff (c g). The stream has been turned to its right bank by the 
advance of the dunes driven by a prevalent wind blowing in the 
direction of the arrows. <A brook (f) has been arrested among the 
sandy wastes, whence, after forming a few pools, it finds egress by 
soaking through the sandy barrier. 
Perfect “ ripple-marks ” may often be observed on blown sand. 
The sand grains, pushed along by the wind, travel up the long slopes 
¥Z 
