The Dunes:
Barrier for a Barrier Island
Story and photographs by Paul Roberts
The Sand
If asked how the sand dunes along our beaches
got there, most people would answer that they
obviously come from the sea. Clearly sand is
required to build a sand dune and it does come
from the ocean. However, the ocean does not
produce the sand; it only sorts it by size and
pushes it up on to the beaches. Consequently,
you could say that the process of building dunes
actually begins onshore. The sand is brought to
the oceanfront by fast-running streams from
the mountains and hills, where larger rocks are
broken down by freezing and thawing, by the
grinding actions of glaciers, and by the steady
action of running water. Rivers disgorge this load
of sediment into the sea where offshore banks of
sand are deposited underwater and moved grain by
grain to the beachfront by wave movement, where
it is eventually exposed by tidal action to drying in
the sun and the wind.
The Wind
The smaller grains are then blown by the wind
across the surface of the beach, bouncing and
rolling until they fall into a place where they are
protected—where the wind is no longer strong
enough to move them, and there they sit, behind
a log, a bush, a bottle, a small rise, or a piece of
beach flotsam until something changes. Some
very common windbreaks on our beaches are
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bundles of dead spartina grass (referred to locally
as “rack”) sloughed off the banks of tidal creeks by
undercutting currents and carried into the ocean
by the receding flow—eventually washed up on
the beach by the tide and waves. Rack serves as a
common point of beginning for the formation of a
dune. It provides that little windbreak that disrupts
the wind and catches more and more grains of
sand in the quiet spot in the lee of the wind. As
the wind continues to blow, more and more grains
are pushed up the dune and fall off the backside
expanding the base, and the dune gets higher
and higher, creating an even larger windbreak.
Eventually, the wind stops blowing, or changes
direction, and the dune may shrink or change shape
to conform to the new conditions, changing daily.
The Valleys
Up and down the beach, different conditions exist
locally. Where the beaches are wider and prevailing
winds stronger, higher dunes can be built. Where
there is beachfront erosion, new dune formation is
impeded. Seasonal storms may reshape the dune
field dramatically. Rising water may push through
the low spots between dunes into the valleys
behind the beach ridge (or foredune) with its load
of sand, plant debris, and fine sediment, building
up the land and creating conditions favoring the
growth of larger plants and bushes.