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Alongshore

Coastwise—Exploring the World at Land's End

By Tamia Nelson
tamia@paddling.net

January 14, 2003

Beaches get most of the press, but they're just the beginning of the story. The seacoast—the wavering line where sea meets land—is 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 time—and the sea has all the time in the world—small 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 behind—a 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 change—when it comes—will 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 currents—are 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 life.

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 almost indefinitely.

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 blocks—deltas, beaches and sand spits are among the most visible results—and 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 stopped—at 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 time—and 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.

A Gull's-Eye View

There's more to the seacoast than geology, of course. The living world often gets into the act, too. For example, the roots of tropical mangrove trees trap wayward grains of sand, slowly creating new land on the margin of the sea. Elsewhere, away from the steamy mangrove swamps, salt marshes act as settling pools, catching and holding the huge amounts of sediment quarried and transported by storm surges. In time, meadows replace these marshes. And once again, new land is born from the sea.

Nothing lasts forever. The sea giveth and the sea taketh away. Always beautiful and sometimes terrifying, the world at land's end is never, never boring.

Copyright 2002 by Verloren Hoop Productions. All rights reserved.










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