It required a person who was perfectly familiar with the behavior of Alpine glaciers, and knew exactly what marks they left behind in their passage, to point out the proofs of their former presence in Northern Europe and America, where it seems almost impossible to believe they existed. Such a man was Louis Agassiz, the eminent naturalist. Born and educated in Switzerland, he spent nine years in researches among the glaciers of the mountains of his native country. He proved the former wide extension of the glaciers of Switzerland. With these results before them, geologists were not long in showing that there had once been glacial ice over a large part of Europe and North America.
The proofs in this case are almost exactly the same as those used to show that the ancient glaciers of Switzerland were once larger than now. But as the great glaciers of the glacial age were many times larger than any thing we know of at the present day, there were of course different results produced.
For instance, the water circulating under Alpine glaciers is enabled to wash out and carry away the mass of pulverized rock and dirt ground along underneath the ice. But when the glaciers covered such an enormous extent of country as they did in the Glacial Age, the water could not sweep away this detritus, and so great beds of gravel, sand, and clay would be formed over a large extent of country. But to go over the entire ground would require volumes; it is sufficient to give the results.
The interior of Greenland to-day is covered by one vast sea of ice. Explorers have traversed its surface for many miles; not a plant, or stone, or patch of earth is to be seen. In the Winter it is a snow-swept waste. In the Summer streams of ice-cold water flow over its surface, penetrating here and there by crevasses to unknown depths. This great glacier is some twelve hundred miles long, by four hundred in width.3 Vast as it is, it is utterly insignificant as compared with the great continental glacier that geologists assure us once held in its grasp the larger portion of North America.
The conclusions of some of our best scholars on this subject are so opposed to all that we would think possible, according to the present climate and surroundings, that they seem at first incredible, and yet they have been worked out with such care that there is no doubt of the substantial truth of the results.
The terminal moraine of the great glacier has been carefully traced through several States. We now know that one vast sea of ice covered the eastern part of North America, down to about the thirty-ninth parallel of latitude. We have every reason to think that the great glacier, extending many miles out in the Atlantic, terminated in a great sea of ice, rising several hundred feet perpendicularly above the surface of the water. Long Island marks the southern extension of this glacier. From there its temporal moraine has been traced west, across New Jersey and Pennsylvania, diagonally across Ohio, crossing the river near Cincinnati, and thence west across Indiana and Illinois. West of the Mississippi it bears off to the north-west, and finally passes into British America.4
All of North America, to the north and northeast of this line, must have been covered by one vast sea of ice.5 Doubtless, as in Greenland to-day, there was no hill or patch of earth to be seen, simply one great field of ice. The ice was thick enough to cover from sight Mt. Washington, in New Hampshire, and must have been at least a mile thick over a large portion of this area,6 and even at its southern border it must in places have been from two hundred to two thousand feet thick.7 This, as we have seen, is a picture very similar to what must have been presented by Europe at this time.8
The Northern Atlantic Ocean must have presented a dreary aspect. Its shores were walls of ice, from which ever and anon great masses sailed away as icebergs. These are startling conclusions. Yet, in the Southern Hemisphere to-day is to be seen nearly the same state of things. It is well-known that all the lands around the South Pole are covered by a layer of ice of enormous thickness. Sir J. A. Ross, in attempting to reach high southern latitudes, while yet one thousand four hundred miles from the pole, found his further progress impeded by a perpendicular wall of ice one hundred and eighty feet thick. He sailed along that barrier four hundred and fifty miles, and then gave up the attempt. Only at one point in all that distance did the ice wall sink low enough to allow of its upper surface being seen from the mast-head. He describes the upper surface as an immense plain shining like frosted silver, and stretching away as far as eye could reach into the illimitable distance.9