The Sea Around Us (16 page)

The Shape of Ancient Seas

Till the slow Sea rise and the sheer cliff crumble,
Till terrace and meadow the deep gulfs drink.

SWINBURNE

WE LIVE IN AN
age of rising seas. Along all the coasts of the United States a continuing rise of sea level has been perceptible on the tide gauges of the Coast and Geodetic Survey since 1930. For the thousand-mile stretch from Massachusetts to Florida, and on the coast of the Gulf of Mexico, the rise amounted to about a third of a foot between 1930 and 1948. The water is also rising (but more slowly) along the Pacific shores. These records of the tide gauges do not include the transient advances and retreats of the water caused by winds and storms, but signify a steady, continuing advance of the sea upon the land.

This evidence of a rising sea is an interesting and even an exciting thing because it is rare that, in the short span of human life, we can actually observe and measure the progress of one of the great earth rhythms. What is happening is nothing new. Over the long span of geologic time, the ocean waters have come in over North America many times and have again retreated into their basins. For the boundary between sea and land is the most fleeting and transitory feature of the earth, and the sea is forever repeating its encroachments upon the continents. It rises and falls like a great tide, sometimes engulfing half a continent in its flood, reluctant in its ebb, moving in a rhythm mysterious and infinitely deliberate.

Now once again the ocean is overfull. It is spilling over the rims of its basins. It fills the shallow seas that border the continents, like the Barents, Bering, and China seas. Here and there it has advanced into the interior and lies in such inland seas as Hudson Bay, the St. Lawrence embayment, the Baltic, and the Sunda Sea. On the Atlantic coast of the United States the mouths of many rivers, like the Hudson and the Susquehanna, have been drowned by the advancing flood; the old, submerged channels are hidden under bays like the Chesapeake and the Delaware.

The advance noted so clearly on the tide gauges may be part of a long rise that began thousands of years ago—perhaps when the glaciers of the most recent Ice Age began to melt. But it is only within recent decades that there have been instruments to measure it in any part of the world. Even now the gauges are few and scattered, considering the world as a whole. Because of the scarcity of world records, it is not known whether the rise observed in the United States since 1930 is being duplicated on all other continents.

Where and when the ocean will halt its present advance and begin again its slow retreat into its basin, no one can say. If the rise over the continent of North America should amount to a hundred feet (and there is more than enough water now frozen in land ice to provide such a rise) most of the Atlantic seaboard, with its cities and towns, would be submerged. The surf would break against the foothills of the Appalachians. The coastal plain of the Gulf of Mexico would lie under water; the lower part of the Mississippi Valley would be submerged.

If, however, the rise should be as much as 600 feet, large areas in the eastern half of the continent would disappear under the waters. The Appalachians would become a chain of mountainous islands. The Gulf of Mexico would creep north, finally meeting in mid-continent with the flood that had entered from the Atlantic into the Great Lakes, through the valley of the St. Lawrence. Much of northern Canada would be covered by water from the Arctic Ocean and Hudson Bay.

All of this would seem to us extraordinary and catastrophic, but the truth is that North America and most other continents have known even more extensive invasions by the sea than the one we have just imagined. Probably the greatest submergence in the history of the earth took place in the Cretaceous period, about 100 million years ago. Then the ocean waters advanced upon North America from the north, south, and east, finally forming an inland sea about 1000 miles wide that extended from the Arctic to the Gulf of Mexico, and then spread eastward to cover the coastal plain from the Gulf to New Jersey. At the height of the Cretaceous flood about half of North America was submerged. All over the world the seas rose. They covered most of the British Isles, except for scattered outcroppings of ancient rocks. In southern Europe only the old, rocky highlands stood above the sea, which intruded in long bays and gulfs even into the central highlands of the continent. The ocean moved into Africa and laid down deposits of sandstones; later weathering of these rocks provided the desert sands of the Sahara. From a drowned Sweden, an inland sea flowed across Russia, covered the Caspian Sea, and extended to the Himalayas. Parts of India were submerged, and of Australia, Japan, and Siberia. On the South American continent, the area where later the Andes were to rise was covered by sea.

With variations of extent and detail, these events have been repeated again and again. The very ancient Ordovician seas, some 400 million years ago, submerged more than half of North America, leaving only a few large islands marking the borderlands of the continent, and a scattering of smaller ones rising out of the inland sea. The marine transgressions of Devonian and Silurian time were almost as extensive. But each time the pattern of invasion was a little different, and it is doubtful that there is any part of the continent that at some time has not lain at the bottom of one of these shallow seas.

You do not have to travel to find the sea, for the traces of its ancient stands are everywhere about. Though you may be a thousand miles inland, you can easily find reminders that will reconstruct for the eye and ear of the mind the processions of its ghostly waves and the roar of its surf, far back in time. So, on a mountain top in Pennsylvania, I have sat on rocks of whitened limestone, fashioned of the shells of billions upon billions of minute sea creatures. Once they had lived and died in an arm of the ocean that overlay this place, and their limy remains had settled to the bottom. There, after eons of time, they had become compacted into rock and the sea had receded; after yet more eons the rock had been uplifted by bucklings of the earth's crust and now it formed the backbone of a long mountain range.

Far in the interior of the Florida Everglades I have wondered at the feeling of the sea that came to me—wondered until I realized that here were the same flatness, the same immense spaces, the same dominance of the sky and its moving, changing clouds; wondered until I remembered that the hard rocky floor on which I stood, its flatness interrupted by upthrust masses of jagged coral rock, had been only recently constructed by the busy architects of the coral reefs under a warm sea. Now the rock is thinly covered with grass and water; but everywhere is the feeling that the land has formed only the thinnest veneer over the underlying platform of the sea, that at any moment the process might be reversed and the sea reclaim its own.

So in all lands we may sense the former presence of the sea. There are outcroppings of marine limestone in the Himalayas, now at an elevation of 20,000 feet. These rocks are reminders of a warm, clear sea that lay over southern Europe and northern Africa and extended into southwestern Asia. This was some 50 million years ago. Immense numbers of a large protozoan known as nummulites swarmed in this sea and each, in death, contributed to the building of a thick layer of nummulitic limestone. Eons later, the ancient Egyptians were to carve their Sphinx from a mass of this rock; other deposits of the same stone they quarried to obtain material to build their pyramids.

The famous white cliffs of Dover are composed of chalk deposited by the seas of the Cretaceous period, during that great inundation we have spoken of. The chalk extends from Ireland through Denmark and Germany, and forms its thickest beds in south Russia. It consists of shells of those minute sea creatures called foraminifera, the shells being cemented together with a fine-textured deposit of calcium carbonate. In contrast to the foraminiferal ooze that covers large areas of ocean bottom at moderate depths, the chalk seems to be a shallow-water deposit, but it is so pure in texture that the surrounding lands must have been low deserts, from which little material was carried seaward. Grains of wind-borne quartz sand, which frequently occur in the chalk, support this view. At certain levels the chalk contains nodules of flint. Stone Age men mined the flint for weapons and tools and also used this relic of the Cretaceous sea to light their fires.

Many of the natural wonders of the earth owe their existence to the fact that once the sea crept over the land, laid down its deposits of sediments, and then withdrew. There is Mammoth Cave in Kentucky, for example, where one may wander through miles of underground passages and enter rooms with ceilings 250 feet overhead. Caves and passageways have been dissolved by ground water out of an immense thickness of limestone, deposited by a Paleozoic sea. In the same way, the story of Niagara Falls goes back to Silurian time, when a vast embayment of the Arctic Sea crept southward over the continent. Its waters were clear, for the borderlands were low and little sediment or silt was carried into the inland sea. It deposited large beds of the hard rock called dolomite, and in time they formed a long escarpment near the present border between Canada and the United States. Millions of years later, floods of water released from melting glaciers poured over the cliff, cutting away the soft shales that underlay the dolomite, and causing mass after mass of the undercut rock to break away. In this fashion Niagara Falls and its gorge were created.

Some of these inland seas were immense and important features of their world, although all of them were shallow compared with the central basin where, since earliest time, the bulk of the ocean waters have resided. Some may have been as much as 600 feet deep, about the same as the depths over the outer edge of the continental shelf. No one knows the pattern of their currents, but often they must have carried the warmth of the tropics into far northern lands. During the Cretaceous period, for example, bread-fruit, cinnamon, laurel, and fig trees grew in Greenland. When the continents were reduced to groups of islands there must have been few places that possessed a continental type of climate with its harsh extremes of heat and cold; mild oceanic climates must rather have been the rule.

Geologists say that each of the grander divisions of earth history consists of three phases: in the first the continents are high, erosion is active, and the seas are largely confined to their basins; in the second the continents are lowest and the seas have invaded them broadly; in the third the continents have begun once more to rise. According to the late Charles Schuchert, who devoted much of his distinguished career as a geologist to mapping the ancient seas and lands: ‘Today we are living in the beginning of a new cycle, when the continents are largest, highest, and scenically grandest. The oceans, however, have begun another invasion upon North America.'

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.