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.