What brings the ocean out of its deep basins, where it has been contained for eons of time, to invade the lands? Probably there has always been not one alone, but a combination of causes.
The mobility of the earth’s crust is inseparably linked with the changing relations of sea and land—the warping upward or downward of that surprisingly plastic substance which forms the outer covering of our earth. The crustal movements affect both land and sea bottom but are most marked near the continental margins. They may involve one or both shores of an ocean, one or all coasts of a continent. They proceed in a slow and mysterious cycle, one phase of which may require millions of years for its completion. Each downward movement of the continental crust is accompanied by a slow flooding of the land by the sea, each upward buckling by the retreat of the water.
But the movements of the earth’s crust are not alone responsible for the invading seas. There are other important causes. Certainly one of them is the displacement of ocean water by land sediments. Every grain of sand or silt carried out by the rivers and deposited at sea displaces a corresponding amount of water. Disintegration of the land and the seaward freighting of its substance have gone on without interruption since the beginning of geologic time. It might be thought that the sea level would have been rising continuously, but the matter is not so simple. As they lose substance the continents tend to rise higher, like a ship relieved of part of its cargo. The ocean floor, to which the sediments are transferred, sags under its load. The exact combination of all these conditions that will result in a rising ocean level is a very complex matter, not easily recognized or predicted.
Then there is the growth of the great submarine volcanoes, which build up immense lava cones on the floor of the ocean. Some geologists believe these may have an important effect on the changing level of the sea. The bulk of some of these volcanoes is impressive. Bermuda is one of the smallest, but its volume beneath the surface is about 2500 cubic miles. The Hawaiian chain of volcanic islands extends for nearly 2000 miles across the Pacific and contains several islands of great size; its total displacement of water must be tremendous. Perhaps it is more than coincidence that this chain arose in Cretaceous time, when the greatest flood the world has ever seen advanced upon the continents.
For the past million years, all other causes of marine transgressions have been dwarfed by the dominating role of the glaciers. The Pleistocene period was marked by alternating advances and retreats of a great ice sheet. Four times the ice caps formed and grew deep over the land, pressing southward into the valleys and over the plains. And four times the ice melted and shrank and withdrew from the lands it had covered. We live now in the last stages of this fourth withdrawal. About half the ice formed in the last Pleistocene glaciation remains in the ice caps of Greenland and Antarctica and the scattered glaciers of certain mountains.
Each time the ice sheet thickened and expanded with the un-melted snows of winter after winter, its growth meant a corresponding lowering of the ocean level. For directly or indirectly, the moisture that falls on the earth’s surface as rain or snow has been withdrawn from the reservoir of the sea. Ordinarily, the withdrawal is a temporary one, the water being returned via the normal runoff of rain and melting snow. But in the glacial period the summers were cool, and the snows of any winter did not melt entirely but were carried over to the succeeding winter, when the new snows found and covered them. So little by little the level of the sea dropped as the glaciers robbed it of its water, and at the climax of each of the major glaciations the ocean all over the world stood at a very low level.
Today, if you look in the right places, you will see the evidences of some of these old stands of sea. Of course the strand marks left by the extreme low levels are now deeply covered by water and may be discovered only indirectly by sounding. But where, in past ages, the water level stood higher than it does today you can find its traces. In Samoa, at the foot of a cliff wall now 15 feet above the present level of the sea, you can find benches cut in the rocks by waves. You will find the same thing on other Pacific islands, and on St. Helena in the South Atlantic, on islands of the Indian Ocean, in the West Indies, and around the Cape of Good Hope.
Sea caves in cliffs now high above the battering assault and the flung spray of the waves that cut them are eloquent of the changed relation of sea and land. You will find such caves widely scattered over the world. On the west coast of Norway there is a remarkable, wave-cut tunnel. Out of the hard granite of the island of Torghattan, the pounding surf of a flooding interglacial sea cut a passageway through the island, a distance of about 530 feet, and in so doing removed nearly 5 million cubic feet of rock. The tunnel now stands 400 feet above the sea. Its elevation is due in part to the elastic, upward rebound of the crust after the melting of the ice.
During the other half of the cycle, when the seas sank lower and lower as the glaciers grew in thickness, the world’s shorelines were undergoing changes even more far-reaching and dramatic. Every river felt the effect of the lowering sea; its waters were speeded in their course to the ocean and given new strength for the deepening and cutting of its channel. Following the downward-moving shorelines, the rivers extended their courses over the drying sands and muds of what only recently had been the sloping sea bottom. Here the rushing torrents—swollen with melting glacier water— picked up great quantities of loose mud and sand and rolled into the sea as a turgid flood.
During one or more of the Pleistocene lowerings of sea level, the floor of the North Sea was drained of its water and for a time became dry land. The rivers of northern Europe and of the British Isles followed the retreating waters seaward. Eventually the Rhine captured the whole drainage system of the Thames. The Elbe and the Weser became one river. The Seine rolled through what is now the English Channel and cut itself a trough out across the continental shelf—perhaps the same drowned channel now discernible by soundings beyond Lands End.
The greatest of all Pleistocene glaciations came rather late in the period—probably only about 200 thousand years ago, and well within the time of man. The tremendous lowering of sea level must have affected the life of Paleolithic man. Certainly he was able, at more than one period, to walk across a wide bridge at Bering Strait, which became dry land when the level of the ocean dropped below this shallow shelf. There were other land bridges, created in the same way. As the ocean receded from the coast of India, a long submarine bank became a shoal, then finally emerged, and primitive man walked across ‘Adam’s Bridge’ to the island of Ceylon.
Many of the settlements of ancient man must have been located on the seacoast or near the great deltas of the rivers, and relics of his civilization may lie in caves long since covered by the rising ocean. Our meager knowledge of Paleolithic man might be increased by searching along these old drowned shorelines. One archaeologist has recommended searching shallow portions of the Adriatic Sea, with ‘submarine boats casting strong electric lights’ or even with glass-bottomed boats and artificial light in the hope of discovering the outlines of shell heaps—the kitchen middens of the early men who once lived here. Professor R. A. Daly has pointed out:
The last Glacial stage was the Reindeer Age of French history. Men then lived in the famous caves overlooking the channels of the French rivers, and hunted the reindeer which throve on the cool plains of France south of the ice border. The Late-Glacial rise of general sealevel was necessarily accompanied by a rise of the river waters downstream. Hence the lowest caves are likely to have been partly or wholly drowned … There the search for more relics of Paleolithic man should be pursued.*
Some of our Stone Age ancestors must have known the rigors of life near the glaciers. While men as well as plants and animals moved southward before the ice, some must have remained within sight and sound of the great frozen wall. To these the world was a place of storm and blizzard, with bitter winds roaring down out of the blue mountain of ice that dominated the horizon and reached upward into gray skies, all filled with the roaring tumult of the
advancing glacier, and with the thunder of moving tons of ice breaking away and plunging into the sea.
But those who lived half the earth away, on some sunny coast of the Indian Ocean, walked and hunted on dry land over which the sea, only recently, had rolled deeply. These men knew nothing of the distant glaciers, nor did they understand that they walked and hunted where they did because quantities of ocean water were frozen as ice and snow in a distant land.
In any imaginative reconstruction of the world of the Ice Age, we are plagued by one tantalizing uncertainty: how low did the ocean level fall during the period of greatest spread of the glaciers, when unknown quantities of water were frozen in the ice? Was it only a moderate fall of 200 or 300 feet—a change paralleled many times in geologic history in the ebb and flow of the epicontinental seas? Or was it a dramatic drawing down of the ocean by 2,000, even 3000 feet?
Each of these various levels has been suggested as an actual possibility by one or more geologists. Perhaps it is not surprising that there should be such radical disagreement. It has been only about a century since Louis Agassiz gave the world its first understanding of the moving mountains of ice and their dominating effect on the Pleistocene world. Since then, men in all parts of the earth have been patiently accumulating the facts and reconstructing the events of those four successive advances and retreats of the ice. Only the present generation of scientists, led by such daring thinkers as Daly, have understood that each thickening of the ice sheets meant a corresponding lowering of the ocean, and that with each retreat of the melting ice a returning flood of water raised the sea level.
Of this ‘alternate robbery and restitution’ most geologists have taken a conservative view and said that the greatest lowering of the sea level could not have amounted to more than 400 feet, possibly only half as much. Most of those who argue that the drawing down was much greater base their reasoning upon the submarine canyons, those deep gorges cut in the continental slopes. The deeper canyons lie a mile or more below the present level of the sea. Geologists who maintain that at least the upper parts of the canyons were stream-cut say that the sea level must have fallen enough to permit this during the Pleistocene glaciation.
This question of the farthest retreat of the sea into its basins must await further searchings into the mysteries of the ocean. We seem on the verge of exciting new discoveries. Now oceanographers and geologists have better instruments than ever before to probe the depths of the sea, to sample its rocks and deeply layered sediments, and to read with greater clarity the dim pages of past history.
Meanwhile, the sea ebbs and flows in these grander tides of earth, whose stages are measurable not in hours but in millennia— tides so vast they are invisible and uncomprehended by the senses of man. Their ultimate cause, should it ever be discovered, may be found to be deep within the fiery center of the earth, or it may lie somewhere in the dark spaces of the universe.
*From The Changing World of the Ice Age, 1934 edition, Yale University Press, p. 210.
II
The Restless Sea
Wind and Water
The Wind’s feet shine along the Sea.
SWINBURNE
AS THE WAVES ROLL in toward Lands End on the westernmost tip of England they bring the feel of the distant places of the Atlantic. Moving shoreward above the steeply rising floor of the deep sea, from dark blue water into troubled green, they pass the edge of ‘soundings’ and roll up over the continental shelf in confused ripplings and turbulence. Over the shoaling bottom they sweep landward, breaking on the Seven Stones of the channel between the Scilly Isles and Lands End, coming in over the sunken ledges and the rocks that roll out their glistening backs at low water. As they approach the rocky tip of Lands End, they pass over a strange instrument lying on the sea bottom. By the fluctuating pressure of their rise and fall they tell this instrument many things of the distant Atlantic waters from which they have come, and their messages are translated by its mechanisms into symbols understandable to the human mind.
If you visited this place and talked to the meteorologist in charge, he could tell you the life histories of the waves that are rolling in, minute by minute and hour after hour, bringing their messages of far-off places. He could tell you where the waves were created by the action of wind on water, the strength of the winds that produced them, how fast the storm is moving, and how soon, if at all, it will become necessary to raise storm warnings along the coast of England. Most of the waves that roll over the recorder at Lands End, he would tell you, are born in the stormy North Atlantic eastward from Newfoundland and the south of Greenland. Some can be traced to tropical storms on the opposite side of the Atlantic, moving through the West Indies and along the coast of Florida. A few have rolled up from the southernmost part of the world, taking a great-circle course all the way from Cape Horn to Lands End, a journey of 6000 miles.
On the coast of California wave recorders have detected swell from as great a distance, for some of the surf that breaks on that coast in summer is born in the west-wind belt of the Southern Hemisphere. The Cornwall recorders and those in California, as well as a few on the east coast of America, have been in use since the end of the Second World War. These experiments have several objects, among them the development of a new kind of weather forecasting. In the countries bordering the North Atlantic there is no practical need to turn to the waves for weather information because meteorological stations are numerous and strategically placed. The areas in which the wave recorders are presently used have served rather as a testing laboratory to develop the method. It will soon be ready for use in other parts of the world, for which there are no meteorological data except those the waves bring. Especially in the Southern Hemisphere, many coasts are washed by waves that have come from lonely, unvisited parts of the ocean, seldom crossed by vessels, off the normal routes of the air lines. Storms may develop in these remote places, unobserved, and sweep down suddenly on mid-ocean islands or exposed coasts. Over the millions of years the waves, running ahead of the storms, have been crying a warning, but only now are we learning to read their language. Or only now, at least, are we learning to do so scientifically. There is a basis in folklore for these modern achievements in wave research. To generations of Pacific Island natives, a certain kind of swell has signaled the approach of a typhoon. And centuries ago, when peasants on the lonely shores of Ireland saw the long swells that herald a storm rolling in upon their coasts, they shuddered and talked of death waves.
Now our study of waves has come of age, and on all sides we can find evidence that modern man is turning to the waves of the sea for practical purposes. Off the Fishing Pier at Long Branch, New Jersey, at the end of a quarter-mile pipeline on the bed of the ocean, a wave-recording instrument silently and continuously takes note of the arrival of waves from the open Atlantic. By electric impulses transmitted through the pipeline, the height of each wave and the interval between succeeding crests are transmitted to a shore station and automatically recorded as a graph. These records are carefully studied by the Beach Erosion Board of the Army Corps of Engineers, which is concerned about the rate of erosion along the New Jersey coast.
Off the coast of Africa, high-flying planes recently took a series of overlapping photographs of the surf and the areas immediately offshore. From these photographs, trained men determined the speed of the waves moving in toward the shore. Then they applied a mathematical formula that relates the behavior of waves advancing into shallow water to the depths beneath them. All this information provided the British government with usable surveys of the depths off the coast of an almost inaccessible part of its empire, which could have been sounded in the ordinary way only at great expense and with endless difficulty. Like much of our new knowledge of waves, this practical method was born of wartime necessity.
Forecasts of the state of the sea and particularly the height of the surf became regular preliminaries to invasion in the Second World War, especially on the exposed beaches of Europe and Africa. But app
lication of theory to practical conditions was at first difficult; so was the interpretation of the actual effect of any predicted height of surf or roughness of sea surface on the transfer of men and supplies between boats or from boats to beaches. This first attempt at practical military oceanography was, as one naval officer put it, a ‘most frightening lesson’ concerning the ‘almost desperate lack of basic information on the fundamentals of the nature of the sea.’
As long as there has been an earth, the moving masses of air that we call winds have swept back and forth across its surface. And as long as there has been an ocean, its waters have stirred to the passage of the winds. Most waves are the result of the action of wind on water. There are exceptions, such as the tidal waves sometimes produced by earthquakes under the sea. But the waves most of us know best are wind waves.
It is a confused pattern that the waves make in the open sea—a mixture of countless different wave trains, intermingling, overtaking, passing, or sometimes engulfing one another; each group differing from the others in the place and manner of its origin, in its speed, its direction of movement; some doomed never to reach any shore, others destined to roll across half an ocean before they dissolve in thunder on a distant beach.
Out of such seemingly hopeless confusion the patient study of many men over many years has brought a surprising amount of order. While there is still much to be learned about waves, and much to be done to apply what is known to man’s advantage, there is a solid basis of fact on which to reconstruct the life history of a wave, predict its behavior under all the changing circumstances of its life, and foretell its effect on human affairs.
The Sea Around Us Page 13