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.

The unstable crust of the earth began to sink at length, and gradually the ice-sheet's progress southward was checked, and it began to recede by melting. All along the borders of this great fan-shaped ice-field water accumulated from the melting, and flooded the streams which drained it to the Atlantic and the Gulf. Icebergs broken off of the edge of the retiring ice-sheet floated in a great inland sea. The land sank lower and lower until the general level was five hundred to one thousand feet lower than it now is. The climate became correspondingly warm, and the icebergs melted away. Then the land rose again, and in time the inland sea was drained away into the ocean, except for the waters that remained in thousands of lakes great and small that now occupy the region covered by the ice.

Ancient sea beaches mark the level of high water at the time that the flood followed the melting ice. On the shores of Lake Champlain, but nearly five hundred feet higher than the present level of the lake, curious geologists have found many kinds of marine shells on a well-marked old sea beach. The members of one exploring party in the same region were surprised and delighted to come by digging upon the skeleton of a whale that had drifted ashore in the ancient days when the inland sea joined the Atlantic.

Lake Ontario's ancient beach is five hundred feet above the present water-level; Lake Erie's is two hundred fifty feet above it; Lake Superior's three hundred thirty feet higher than the present beach. No doubt when the water stood at the highest level, the Great Lakes formed one single sheet of water which settled to a lower level as the rivers flowing south cut their channels deep enough to draw off the water toward the Gulf. Lake Winnipeg is now the small remnant of a vast lake the shores of which have been traced. The Minnesota River finally made its way into the Mississippi and drained this great area the stranded beaches of which still remain. The name of Agassiz has been given to the ancient lake formed by the glacial flood and drained away thousands of years ago but not until it had built the terraced beach which locates it on the geological map of the region.

When the ice-sheet came down from the north it dragged along all of the soil and loose rock material that lay in its path. With the boulders frozen into its lower surface it scratched and grooved the firm bedrock over which it slid, and rounded it to a smooth and billowy surface. The progress of the ice-sheet was southward, but it spread like a fan so that its widening border turned to east and west.

When it reached its southernmost limit and began to melt, it laid down a great ridge of unsorted rock material, remnants of which remain to this day,—the terminal moraine of the ancient ice-sheet. The line of this ancient deposit starts on Long Island, crosses New Jersey and Pennsylvania, then dips southward, following the general course of the Ohio River to its mouth, forming bluffs in southern Ohio, Indiana, and Illinois. The line bends upward as it crosses central Missouri, a corner of Kansas, and eastern Nebraska, parallel with the course of the Missouri.

As the ice-sheet melted, boulders were dropped all over the Northern States and Canada. These were both angular and rounded. In some places they are scattered thickly over the surface and are so numerous as to be a great hindrance to agriculture. In many places great boulders of thousands of tons weight are perched on very slight foundations, just where they lodged when the ice went off and left them, after carrying them hundreds of miles. Around them are scattered quantities of loose rock material, not scored or ground as are those which were carried on the under-surface of the glacial ice. These unscarred fragments rode on the top of the ice. They were a part of the top moraine of the glacial sheet.

The finest material deposited is rock meal, ground by the great glacial mill, and called "boulder clay." It is a stiff, dense, stony paste in which boulders of all sizes, gravel, pebbles, and cobblestones are cemented.

The "drift" of the ice-sheet is the rubbish, coarse and fine, it left behind as it retreated. Below the Ohio River there is a deep soil produced by the decay of rocks that lie under it. North of Ohio is spread that peculiar mixture of earth and rock fragments which was transported from the north and spread over the land which the ice-sheet swept bare and ground smooth and polished.

The drift has been washed away in places by the floods that followed the ice. Granite domes are thus exposed, the grooves and scratches of which tell in what direction the ice flood was travelling. Miles away from that scored granite, but in the same direction as the scratches, scattered fragments of the same foundation rock cover fields and meadows. Thus, much of the drift material can be traced to its original home, and the course of the ice-sheet can be determined. Many immense boulders the home of which was in the northern highlands of Canada rode southward, frozen into icebergs that floated in the great inland sea. Great quantities of dÉbris were added to the original glacial drift through the agency of these floating ice masses, which melted by slow degrees.