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1899] DUNE FLORAS OF LAKE MICHIGAN 197 
side currents and eddies at many points, but with the main current 
in one direction. 
It has already been stated that the windward slope of an 
advancing dune is very gentle, averaging perhaps about ma 
That portion of the windward slope up which the main wind 
currents pass is also trough-shaped and may be called a wind- 
sweep. fig. 7 shows a small trough-shaped wind-sweep at Beaver 
island; the direction of advance is from the foreground to the 
background. In’ the path of the wind there may be seen dead 
branches, the remnants of a vegetation that has been swept 
away. In the background are small dunes which have been 
formed by sand carried along the trough by the wind. A wind- 
‘Weep more characteristic of the dune-complex is shown in jig. 
he Here, likewise, the prevailing wind direction is up the 
geatle slope away from the foreground. At this particular place 
tre: is.in reality much more of a trough than is shown in the 
Capa since there is a conspicuous rise both at the right 
is ast : a Just beyond the pines in the background there 
steep pitch downward, the advancing lee slope. 
« Most. remarkable wind-sweep at Dune Park reaches 
almost to the water level, appearing like a cafion, by reason 
Sweep a sides from ten to twenty meters in height. This 
ing ft a; : € most of the troughs, is curved so that a wind enter- 
West wind northwest wind becomes a west and finally a south- 
toward the sy wepally contributes to the advance of a dune 
in Sa 2 . will be discussed more Hany at another place 
Which these te fig. To. The concentration of the wind se 
able. At ey wind-sweeps permit is something remark- 
the vegetation, fei, is the destructive power of the wind fie 
T Wind-sw eit more keenly than along the sides of these 
Part of one of ee The foreground of fig. 22 shows the pps 
Wind’s destruct: ese troughs, and gives a vivid impression of the 
Ctiveness 
down 
of its 
dee pe 
ave oe vabcing lee slopes, as has been previously mentioned, 
Sradient in the neighborhood of 30°. The slope 1s 
xactly . 
Y that at which sand, whose grains have the size and cohe- 
