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Page:Popular Science Monthly Volume 44.djvu/836

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THE POPULAR SCIENCE MONTHLY.

Iceland as part of the general rise on the American and European sides—this would account for greater precipitation on the borders of the ocean, and especially over its western border, the American.

But leaving this source of increased precipitation out of consideration, it is plain that in the Glacial period the difference in amount of precipitation over the high eastern border made into a lofty ice plateau by the accumulation of snow and ice, and over the broad medial belt from Wisconsin and Minnesota north-westward, should have been much greater than it is now. Moreover, this central valley of North America would have had something of the existing disadvantage of a relatively warm summer temperature. At the present time, in July, a mean temperature of 70º Fahr. extends beyond the latitude of Lake Winnipeg even to 56º north, and this is 10º in latitude, or nearly seven hundred miles, farther north than the position of the same heat line over New England.

The advantages for ice-making of eastern over central North America were, therefore, very great, both as regards temperature and precipitation. When the conditions over the interior were sufficient to produce a small annual gain of ice, those over New England would have been making a very large annual gain. A small gain continued for many scores of centuries would make finally a great thickness of ice. But with the conditions over the interior near the critical point, a small unfavorable meteorological change, if long continued, might carry off the ice for scores or hundreds of miles from a southern limit, with proportionate floods from the melting, while the eastern border was all the time gaining in ice, or was making only a short retreat.

The actual facts correspond with these views. The distance in the upper Mississippi basin between the farthest southern limit of the ice and the line of the great moraine, or that of the so called "second Glacial epoch," is over five hundred miles; but to the eastward it narrows through Indiana, Ohio, Pennsylvania, and New Jersey; and in New England a corresponding moraine interval can not be certainly made out, and nothing exists that could not be better explained by reference to short retreats in a single glacier.

I leave the subjects here for the consideration of geologists of the East and West. The cause appealed to explains at least why the geologists of the East and West are divided on the subject; and also why the grand display of terminal and retreat moraines characterizing the West produces there the stronger opinions and the stronger expressions of opinions; and why also a complete survey of the facts will probably lead to a general agreement in favor of a single Glacial epoch only.