176 BOTANICAL GAZETTE [MARCH 
as the wind blows continuously from one direction, the mound of 
sand keeps growing and the trail to the leeward becomes mort 
and more conspicuous. Indeed, the growth increment is often 
greater during the later stages of a continuous wind current, 
since the growing mound of sand becomes more and more al 
obstacle to the progress of the laden wind. As a result of this 
action, there appears a topographic form with a steep windward 
side and a gently sloping leeward side. 
When the wind changes, the trails of sand are no longer in 
the lee of the obstacle, but are more or less exposed to the wind, 
and hence are rapidly removed. The contour of the mound is 
changed and there results, just as before, a topographic form, 
steep to the windward but gently sloping to the leeward. It 
will thus be seen that it is always possible in dry weather to 
determine the direction of the last scrong wind by observing the 
position and direction of the leeward trails of sand. A clump 
of grass with a poorly developed leeward trail may be seen , 
the foreground of fig. 4. 
Under ordinary conditions no permanent results follow from 
such wind action as has been described, since one wind destroys 
the products of another. There is, perhaps, a tendency for sand 
to accumulate on the landward side of obstacles, since the ene) 
of the lake winds is likely to be greater than the energy of winds 
from other directions. But no extensive dune formation a 
occur on the beach, unless the obstacle, which compels the wind 
to deposit its load, is itself increasing in size. A mound of sai 
which is being built up by the wind, becomes more and more @ 
formidable obstacle to the progress of the sand-laden currents 
and it might be supposed that the growth of such 4 nom 
would continue indefinitely. Such does not appear to 4 
Case on the beaches studied. The wind blows ovet the bea 
from so many directions and with such resistless ene! 
mounds of sand rarely accumulate without the aid ° 
factors, 
The formation of beach dunes, then, depends up 
more than wind and sand. An obstacle is neede 
ee 
f othef 
on somethin 
d which ” 
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4 
3 
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