By permission of the American Museum of Natural History
Mt. Tom, West 83d St., New York
Glaciers are small to-day compared with what they were long ago, in Europe and in America. The climate became warmer, and the ice-cap retreated. Old moraines show that the ice rivers of the Alps once came much farther down the valleys than they do now. Smooth, deeply scored domes of rock, the one in Central Park and the bald head of Mount Tom, are just like those that lie in Alpine valleys from which the glaciers have long ago retreated. There are old moraines far up the sides of valleys, showing that once the glaciers were far deeper than now. No other power could have brought rocks from strata higher up the mountains, and lodged them thus.
Nearer home, Mt. Shasta and Mt. Rainier still have glaciers that have dwindled in size, until they bear little comparison to the gigantic ice-streams that once filled the smooth beds their puny successors flow into. Remnants of glaciers lie in the hollows of the Sierras. We must go north to find the snow-fields of Alaska and glaciers worthy to be compared with those ancient ice rivers whose work is plainly to be seen, though they are gone.
THE GREAT ICE-SHEET
Greenland is green only along its southern edge, and only in summer, so its name is misleading. It is a frozen continent lying under a great ice-cap, which covers 500,000 square miles and is several thousand feet in thickness. The top of this icy table-land rises from five thousand to ten thousand feet above the sea-level. The long, cold winters are marked by great snowfall, and the drifts do not have time to melt during the short summer; and so they keep getting deeper and deeper. Streams of ice flow down the steeps into the sea, and break off by their weight when they are pushed out into the water. These are the icebergs which float off into the North Atlantic, and are often seen by passengers on transatlantic steamers.
Long ago Greenland better deserved its name. Explorers who have climbed the mountain steeps that guard the unknown ice-fields of the interior have discovered, a thousand feet above the sea-level, an ancient beach, strewn with shells of molluscs like those which now inhabit salt water, and skeletons of fishes lie buried in the sand. It is impossible to think that the ocean has subsided. The only explanation that accounts for the ancient beach, high and dry on the side of Greenland's icy mountain is that the continent has been lifted a thousand feet above its former level. This is an accepted fact.
We know that climate changes with changed altitude as well as latitude. Going up the side of a mountain, even in tropical regions, we may reach the snow-line in the middle of summer. Magnolia trees and tree ferns once grew luxuriantly in Greenland forests. Their fossil remains have been found in the rocks. This was long before the continent was lifted into the altitude of ice and snow. And it is believed that the climate of northern latitudes has become more severe than formerly from other causes. It is possible that the earth's orbit has gradually changed in form and position.
If Greenland should ever subside until the ancient beach rests again at sea-level, the secrets of that unknown land would be revealed by the melting of the glacial sheet that overspreads it. Possibly it would turn out to be a mere flock of islands. We can only guess. North America had, not so long ago, two-thirds of its area covered with an ice-sheet like that of Greenland, and a climate as cold as Greenland's. At this time the land was lifted two to three thousand feet higher than its present level. All of the rain fell as snow, and the ice accumulated and became thicker year by year. Instead of glaciers filling the gorges, a great ice flood covered all the land, and pushed southward as far as the Ohio River on the east and Yellowstone Park in the west. The Rocky Mountains and some parts of the Appalachian system accumulated snow and formed local glaciers, separated from the vast ice-sheet.