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Gentle winds carry quartz grains up onto the dune.
Stronger winds carry
both light and heavy grains, but when the winds diminish
the heavy grains drop out first and make a dark band.
Very strong winds mix
all the sand grains so there are no bands of colour.
The
slope of the bands tells us the prevailing wind direction.
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If
you dig down into a dune, you find bands of different coloured
sands. Wind and waves separate sand grains into these bands.
The black magnetite and red garnet grains are heavier than
the pale quartz grains, so it takes a stronger wind to carry
them. 
