CoastwiseExploring the World at Land's End
By Tamia Nelson
January 14, 2003
Beaches get most of the press, but they're
just the beginning of the story. The seacoastthe wavering line
where sea meets landis a place of infinite variety and endless
challenge. It's also a place where past and present come together.
Whether it's a gentle paradise of palm trees and pink sand or a sheer
wall of ancient ice, geology shapes the coastal environment as surely as
a skeleton defines the structure of a human body.
"Infinite variety." That sounds a little over the top, doesn't it? But
is it? Not really. Consider
. The mountains of Oregon's Coastal
Range thrust their feet deeply into the ocean, exposed to the full fury
of the stormy north Pacific. On the other side of the continent, however,
North Carolina's coastal plain meets the sea in the sheltered waters of
Pamlico Sound, shielded from the open Atlantic by long barrier islands.
To the south, on America's Gulf Coast, the Mississippi discharges its
burden of mud and toxic waste though an enormous fan-shaped delta. At the
same time, more than 2,000 miles closer to the Pole, rivers of ice send
huge bergs tumbling into narrow Alaskan fjords.
Variety aplenty! Yet these, too, are only pieces of the picture, and
the picture's always changing. Beaches
are born and die. River mouths silt up. Deltas grow and then wither.
The hungry sea eats away at the land in one place and builds barriers in
another. Coastal marshes become meadows. New islands thrust up toward the
light. Old islands sink beneath the waves, never to be seen again.
And two great engines drive these changes: erosion and accretion.
But what fuels the engines? Wind, for one thing. Wind makes waves, and
as anyone who's ever spent any time in the surf knows, waves are powerful
hammers. Even the hardest, most impervious rock will yield to their
relentless assault. Hour after hour, day after day, year after year,
waves smash against sea cliffs, searching out the smallest cracks,
widening and extending them. Seawater mounts a chemical assault on rock
minerals, too, accelerating the process. In timeand the sea has all
the time in the worldsmall cracks grow large, and the rocky
bulwarks of the land flake and crumble away. Sometimes an entire cliff
face, undermined by years of pounding, suddenly tumbles into the ocean,
leaving only a pile of broken rock behinda talus field.
Wherever they're found, such talus fields are evidence of earlier
catastrophes. They're also agents of change in their own right. The talus
at the foot of cliffs is exposed to the full force of the sea. Great
waves heave even the largest boulders about. These massive grindstones
break smaller rocks into still smaller fragments. The resulting shards
of stone are picked up by the waves, where they become cutting tools in
the lathe of the sea, grinding away yet more rock. Year after year the
relentless engine turns, knifing into the land and driving it back.
The sea advances. The land retreats. This has practical implications
even for confirmed landlubbers. Build a house near the edge of a sea
cliff and you're sure to have a fine view. But the picture that you see
won't be static. From one year to the next, your view of the sea will
become more spectacular, until someday you'll be able to look straight
down into the spume thrown up by breaking waves. And then? Unless you
want a one-way ticket on a trip of a lifetime, it's time to move to
someplace where the view's a little less vertiginous.
How long will it take for a cliff-top house to become part of the
scenery? That's hard to say. Statistical measures are often invoked in
making a guess, but there's a pretty good chance that many coastal
processes are chaotic. If this proves to be the case, statistics have
little or no predictive power, and catastrophic changewhen it
comeswill come quickly and without warning. A single "perfect"
storm can alter a coastline beyond all recognition in a matter of hours.
And not all waves are driven by wind. Wind force at least can be
measured, and storms can be tracked. But tsunamis, the great
seismic sea waves that are often tagged with the misleading name "tidal
waves"misleading because they have nothing to do with tides or
tidal currentsare born of earthquakes, volcanic eruptions, or
undersea landslides. With wavelengths measuring hundreds of miles, they
can pass unnoticed beneath ships at sea, yet strike with devastating
force when they enter the shallow waters of the continental shelf. Here
they often rise to heights of many tens of feet and inundate whole
swathes of coastline. And while Pacific tsunamis get most attention,
almost all the world's coasts are vulnerable. A seismic wave can
devastate communities thousands of miles from the earthquake that gave it
So much for the hammer. What happens to the bits of coastline that the
repeated blows of waves knock loose? They have to go somewhere, and they
do. Some serve as cutting tools, of course, but others settle out to
build new landscapes. These bits are called sediments, and they range in
size from microscopic particles of clay to rocks as big as houses. Waves
smash rocks and pluck sand from beaches. Then other waves, assisted by
tidal currents, carry these trophies away, taking them out to sea or
transporting them along the shore.
Every continent and island is rimmed with an undersea apron of
sediment, derived from the whole of the parent landmass. Those sediments
were carried there by streams and rivers, wind and wave, even icebergs
and glaciers. And the sediments don't stay put. Boulders and cobbles roll
downslope in subaqueous landslides. Pebbles and sand are picked up and
hauled great distances by moving water, often forming new beaches in the
process. Very light particles like silts and clays can remain suspended
Geologists (most of whom delight in calling a spade a
manually-operated earth-moving implement) call land-building "accretion."
Erosion and accretion are complementary processes in a cycle that will
continue until the earth itself ceases to exist. Erosion quarries raw
materials from the landscape. Next, accretion fashions new landscapes
from these building blocksdeltas, beaches and sand spits are among
the most visible resultsand they are then eroded in their turn.
Nothing lasts forever. Every grain of sand is a rolling stone with no
permanent address. A wave smashes into a sea cliff and breaks off a flake
of rock. This flake of rock grinds against other flakes in a talus apron,
giving rise in time to grains of sand. One of those grains is carried for
miles by a longshore current, till the current slows and deposits it on a
submarine bar. Other grains of sand join it. The bar becomes a barrier
beach. Much later, a storm surge washes the sand grain out to sea and
redeposits it farther down the beach. This happens again and again. With
what result? The beach is on the move. It creeps down the coast, one
grain of sand at a time.
This relentless advance can be slowed, but not stoppedat least
not until the beach encounters a wide river mouth or bay. Then the beach
becomes a spit. Even here the movement continues, however, and in time
the spit may bridge the gap, at least in the case of a bay. (Rivers eat
away at spits and bars, carrying their sediments far offshore.)
Notwithstanding this, coastal communities everywhere pour billions of
dollars into the sea in hopes of stopping "beach erosion." The resulting
feats of engineering make wonderful newspaper copy and reassure
beachfront homeowners, but the best engineering in the world can only buy
timeand often only a little time, at that. The engineers have no
more chance of stopping the cycle of erosion and accretion than King
Canute did in commanding the tide to rise no further than his feet.
Still, all this movement of material creates a wonderfully intricate
seascape, sculpted by waves and tidal currents. Bars and barrier islands
close off bays from the sea, forming sheltered lagoons. Tombolos
join islands to the mainland. The resulting labyrinth is a delightful
playground for knowledgeable paddlers in seaworthy craft, a playground
that changes with every turn of the tide.