Rensselaer Union, Volume 7, Number 48, Rensselaer, Jasper County, 19 August 1875 — The Great Ice Sheet. [ARTICLE]

The Great Ice Sheet.

In the heat of summer nothing is more refreshing than a generous quantity of Ice. And it may be refreshing to the mind, at least, to contemplate in these sultry summer days the great sheet of ice that once enveloped all the northern part of our continent, and of the globe as well. All have seen the immense bowlders, called “ lost rock” in some sections, scattered over the northern part of the United States, which have little or no resemblance to any mass of rock in place anywhere in their vicinity, and have perhaps asked the question: Where did they come from? Also the heaps of sand, gravel, and cobblestones of various sires which form many of our ridges, knolls and hills, and which are totally unlike any fixed rock near them. Some of these have coarse and fine pebbles promiscuously mixed together, and others show beds of stratification irf.which the coarse and fine seem to be sorted and arranged by themselves. Some, too, have doubtless seen Ute parallel scratches on the surface of rocks, and noticed, as the Indians did before thenl, that they all have one direction, and that is usually north and south. Now, all these phenomena are attributed to a single cause, and tiJat is the great sheet of ice which nature stqred up ages ago without the necessity of protecting it in an ice-house. The transported bowlders, of all' sizes down to pebbles and even fine sand, and the scratches upon the rocks are clear indications of a movement on a very large scale. And if anyone takes the trouble to search for the plaice w hence these erratic bowlders came, he will invariably find that it is north of the position iff? which they nowrest and varying in distance from a few miles to several hundreds, It will ven frequently be found that-these masses of stone could not have reached their present

resting place without crossing intervening mountains, valleys, lakes and sometimes parts of the wean. They are often perched in very singular and almost inaccessible places. There is a very targe one on the summit of West Rock, New Haven, under which the regicides are said to have taken refuge after the restoration. Some are so nicely poised in their position that they can lie easily rocked by a child, though they weigh tens of togs. One of the most noted of these erratic blocks is called Pierre a Bot, of fine granite, which now rests on the Jura. 800-feet above Lake Neufehatel, near the early home of Pref. Agassiz. This has a solid content of 40'000 cubic feet and was transported from Mont Blanc across Switzerland, a distance of sixty or seventy miles. Through the influence of their world-renowned fellowtownsman, the city authorities of Neufchatel have constructed a promenade to it. To explain the transportation of these wanderers from their homes, various theories have been advanced, as the effects of floods, or of powerful mud currents, gas explosions of great violence, hurlingrocks in all directions, and drifting ice, carrying bowlders on its mass and depositing them wherevefli^thrice melted. But these and many others Qiil to satisfy the observed conditions, ahd ",utterly fall to the ground in presence or the fqcts. To our lamented Agassiz belongs the crejjjmflTfirst attributing the cause of all 'drift phenomena to glacial action. But he was not the first to observe that -bowlders were carried long distances by land ice. More than twenty years before Agassiz announced his conclusions, a chamois-hunter of Valais said toX-Charpentier: “Ourglaciers had formerly a much longer, extent than at present. Our whole valley was occupied by'a vast glacier, extending as far as Martigny, as is proved by the bowlders found in the vicinity of this town, and which are far too large for the water to have carried them thither.” Charpentier adds that he afterward received similar explanations from mountaineers in other parts of Switzerland; once, in 1834, from a woodman, when he had, at the very time, in his pocket, a paper advancing the same theory, which he was then going to Lucerne to submit to a convention of Swiss naturalists. In 1836 Agassiz became acquainted with Charpentier, and was made familiar with his investigations. Soon after he (Agassiz) carried the theory beyond the limits of the Swiss Alps, and made it embrace a sheet of snow and ice extended enough to cover a continent. Having noticed that the markings in the region of the Alps below the glaciers were the same as those found beneath the ice mass, he compared these with similar appearances in Northern Europe and Asia, which were generally attributed to the great flood, and made the bold generalization that all were due to the very same cause, and that one vast sheet of ice must have covered all the northern regions of the globe. While these theories at first met with great opposition, they are now universally accepted. According to Agassiz, the sheet of ice extended in this country as far south as South Carolina or Alabama, and was thick enough to cover all the mountains of the eastern part of North America with the exception of Mount Washington. This peak projected, a lone sentinel on that vast waste of ice, 200 or 300 feet. In the latitude of Northern Massachusetts he conceives the ice to have been between two and three miles thick; and it held its direct southerly course in spite of the mountains and ridges over which it moved, as is indicated by the direction of the parallel grooves it made, and the trains of bowlders which it left. The bowlders were all torn off by the advancing ice sheet from the projecting rocks over which it moved and carried or pushed as “ bottom drift,”scratching and plowing the surface over which they passed, and being scratched and polished themselves in return, till they were finally brought to rest by the melting of the ice. They were not carried as far south as the ice sheet extended, seldom, beyond the parallel of 40 deg. north. The native copper of Lake Superior was drifted 400 or 500 miles south; and the pudding-stones of Roxbury, Mass., were carried as far south as the island of Penikese. The tough, elastic, compact clay, called in this country' a hard-pan” and in Scotland “ till,” is described as the oldest deposit of ice agency. It was formed under the great ice sheet as it ground along on the earth’s surface. This was often plowed and forced forward in a confused mass, was thickest in the valleys and on the lee 'side of crags and other obstruction*, and Ulins out toward, the mountain tops, where it appears only in protected places. During the existence of this ice sheet over the earth’s surface, geologists tell us, there was agreat depression of the crust, which of course resulted in an equally great encroachment of the sea upon the land. Various reasons have been assigned as the cause of this. Adhemar, and later Croll, attribute it to a displacement of the earth’s center of gravity, due to an in-, creased weight of ice at the north pole, while at the same time there is a diminution of ice at die south pole. The latter authority estimates that the melting ot two miles or ice from the antartic regions would raise the ocean level one foot; and

the simultaneous abstraction of heat from the arctic regions would add a mile to the thickness of the ice at the north pole; and that these results would so change the earth’s center of gravity as to cause a submergence of nearly 500 feet in the northern polar regions, anda gradual .diminution of this amount toward the equator. But, unfortunately for this theory, the facts show that the amount of submersion does not diminish w-ith uniformity as we recede from the pole, but with much greater rapidity than the theory requires. Another theory, advanced by Prof. Shaler, attributes this depression to the weight of ice accumulated on the continents during the glacial period. This theory assumes that a cap of ice a mile or more in thickness would, by its gravity, depress the earth's crust at least 500 feet from the great lakes to the Arctic Ocean. That this weight would tend to produce a depression cannot be denied; but it is hardly probable that it would be sufficient to produce so great an effect upon the wirth’s crust, which is probably not less than 100 miles thick and so shdped as to offer a resistance to pressure very similar to that of a monstrous arched bridge. It would seem more natural and reasonable to attribute this depression to the commonly accepted cause of all the elevations and depressions on "the earth's surface: the contraction of its crust due to constant cooling. After the disappearance of the ice sheet there were local glaciers which conformed to the larger valleys, like those now seen in the Alps. They carried down bowlders torn from the rocks along their sides and deposited them, with other debris, as terminal and laternal moraines, which often dammed up rivers and formed many of the lakes which now beautify our landscapes. Other lakes were formed in the solid rock by the gouging action of the bowlders frozen into the ice. .At perhaps about the same time there was again an upheaval or elevation of the land, or, more strictly, successions of elevations. These caused a rapid drainage from the land of the water previously making up the sheet of ice by which the drifts were stratified and its pebbles assorted and rounded; hills and ridges were formed by a deposition of material suspended in the water, just as drifts of various shapes and sizes are formed by the snow suspended in the air; and the beautiful terraces which ornament so richly our river valleys were formed. These elevations were also due to shrinkage of the earth’s crust, for a depression in one place must necessarily make an elevation in another. — Scientific American.