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.
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.