Here, the beach is shaded green. It rests on bedrock (brown) and lies
between a height of land and the sea (blue). The intertidal zone marks
the limits of high and low tides.
And what is a beach made of? Rocks and rock fragments, mostly, from
boulders all the way down to fine sand. But not always. Anything that
water can move and waves can grind can find its way ontoand
intoa beach. Look at any handful of beach sand with a pocket
magnifier or dissecting scope and you'll probably see tiny shards of
ground plastic and other trash. Sometimes you won't even need a
magnifier. Engineer (and beachcomber) Willard Bascom once described a
pocket beach near Fort Bragg, California, that consisted entirely of tin
cans from a nearby ocean dump. Not a great place to take the kids for a
picnic, perhaps, but still a beach.
A pocket beach, by the way, is just a beach contained between two
rocky headlands andin many if not all casesbacked by a sheer
cliff-face. Winter storms often strip away all the sand from these
pockets, leaving only cobbles (or tin cans) behind. And this illustrates
a larger point. Beaches live on the edge. A beach isn't frozen in time.
It's a happening place. Beaches grow as material is deposited, and shrink
when it's stripped away. Change is the only constant. Beaches migrate up
and down a coast. They get washed out to sea. Sometimes they even become
stranded far from water. Only then do they settle down. Beachfront
property owners take note! A beach is not forever.
Geologists like to compare the shoreline to a battlefield. (Few
geologists have seen a battlefield, but the imagery is compelling
nonetheless.) Miles back from any beach, exposed rock is attacked by
wind, water, and ice. Bulldozers and plows tear up the soil. Then rain
washes bits and pieces of the displaced landscape into nearby rivers,
which later empty into lakes and seas. What started out as scenery ends
up as sediment, ranging in size from cobbles to clays. Sooner or later
this sediment settles out. Some is lost to the deeps, but the rest
accumulates along the shifting shoreline. When this first occurs, a beach
is born. And that's only the first chapter of the story. Waves constantly
lay siege to the shore, plucking sand from existing beaches and washing
it out to sea again, or smashing large stones into smaller fragments.
Alongshore currents carry these freshly-liberated sediments to new
locations. The result? One beach shrinks, while another grows.
The beach is dead. Long live the beach! The only thing constant is
Beaches are shaped by this never-ending struggle. Their character
depends on the parent material that gave birth to the sediments that
formed them. It also depends on the forces at work on those sediments.
Tahiti has beaches of black volcanic sand, whereas the shore of Florida's
Gulf coast boasts fine white "sugar," the remains of shattered coral
reefs. The Labrador coast is studded with storm-washed cobbles, while
beaches on the western margin of Lake Champlain contain the
coarse-grained remnants of frost-cracked Adirondack rock. And Fort Bragg
has tin cans.
That's just the beginning, of course.
Remember the hazy phrase gently sloping from the definition of
a beach? Here's a slippery slope, indeed! What's gentle? What's steep? It
depends. If a beach slopes too little, it will simply disappear beneath
the water. On the other hand, if it's too steep, sediment will roll off
without establishing a beachhead. It's all relative. What's steep in one
place, under one set of conditions, would be judged gentle in another.
If beaches can be drowned, they can also be stranded. When continental
land masses riseas they did at the end of the last Ice Ageor
when waters retreat in response to climate change (or other causes),
beaches are sometimes left high and dry. Such "relict" or "fossil"
beaches can be found in the Adirondack foothills of northern New York,
for example, along with the skeletons of long-dead whales and seals.
Today, sand from these fossil beaches is mined for use in aggregate. In
the fullness of time, some of this sand ends up in streams and finds its
way back to local lakes, where it helps to build new beaches. The earth's
been in the recycling business for a long time.
But this isn't a surprise, is it? In the world of rock, wind, and
water, the only constant is change, and one of the best places to get a
feel for this is on the beach. Now there's a thought that's almost
certain to bring a smile to any snow-bound paddler!
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