Popular Science Monthly/Volume 44/April 1894/On New England and the Upper Mississippi Basin in the Glacial Period
|ON NEW ENGLAND AND THE UPPER MISSISSIPPI BASIN IN THE GLACIAL PERIOD.|
By Prof. JAMES D. DANA.
SINCE the publication, in this Journal, of Prof. G. F. Wright's paper on the Unity of the Glacial Epoch, nearly a year since, this subject has been much discussed in the scientific journals of the country, and with some interesting developments besides those within the purpose of the writers.
1. It has been shown that there are good working geologists on each side of the question.
2. It has been made manifest that the advocates of unity are mainly the geologists that have investigated Eastern glacial regions in the country, and especially New England, while the advocates of two Glacial epochs are chiefly those whose glacial studies have been in more western regions.
The writer, who has thus far taken no part in the recent discussion, here states that he has found in his geological explorations, which have been extensive over New England, the State of Maine excepted, no facts that require for their explanation an appeal to two Glacial epochs, and none that has even suggested the idea.
3. The presentation of arguments on the side of unity has been moderate in tone and free from dogmatism. Among geologists on the other side, great confidence in the obvious facts has given occasion to expressions almost of accomplished triumph for the two-epoch theory.
4. Among the prominent glacial investigators, one has been on both sides of the question. Having studied glacial phenomena long and faithfully in New England, Warren Upham explained the facts which he had observed on the theory of one advancing and retreating glacier, and found evidence of its terminal moraine and another halt moraine in the islands south of New England and on part of the adjoining mainland. But after some years of study in Minnesota and the neighboring States and over the region northward through Manitoba, he adopted the theory of two Glacial epochs. Returning again to New England and revising the facts there presented, he was led back to his former opinion, as he has announced in his recent papers. Since no geologist in America is better acquainted with the facts on the two sides, or more faithful and earnest in glacial investigation, these changes in his conclusions have special interest.
5. As the above review of facts makes manifest, the division among geologists on the question, and the differences in intensity of opinion, are to a large extent geographical.
The cause of this sectional divergence in views deserves consideration. The writer has come to the conclusion that the cause is largely meteorological: that the geological differences in opinion are a consequence not only of differences in observed facts in the West as compared with those of the East, but back of these, in meteorological differences in the two regions during the Glacial period.
At the present time the glaciated areas of eastern and central North America differ widely in hygrometric conditions. For New England and three fourths of the State of New York the mean annual precipitation, according to Schott's maps, varies from thirty-eight to forty-two inches—a broad coast region, nearly half the breadth of New England, excepted over which it amounts in some parts to fifty inches; while for Wisconsin it varies from thirty-two to thirty-eight inches, and for the larger part of Minnesota, from twenty to thirty-two inches. North of New England, in British America east of Hudson Bay, the annual precipitation is from thirty-two to twenty inches; but to the west of this region, over Manitoba and beyond, it is twenty to ten inches.
Here is a large present difference between the eastern and western regions, affecting snowfalls as well as rainfalls.
Now, in the Glacial period, this eastern region would not only have had the same great advantage as now of proximity to the Atlantic Ocean, but also that of greater height than now. The evidence appears to be conclusive that along the Atlantic side of the continent from southern New England northward, as well as on the Pacific side, the continent stood much above its present level, and that the elevation was the culmination of that which was in progress during the closing part of the Tertiary era—as urged by Prof. Upham. However much the surface of the great medial valley of the continent was raised, it can not be reasonably questioned that the border mountain regions experienced the greater amount of elevation. Hence, with the mountain condensers on the east so much increased in altitude and extent, the differences between the eastern and interior regions as to precipitation would have been greatly augmented, to the advantage of the eastern region.
Further, the Glacial period was probably a time of greater precipitation than now, as well as of greater cold. Some have said, of greater precipitation, and not of greater cold; but the former of these two statements has general acceptance. If the surface waters of the Atlantic basin were warmer than now—owing to a rise of land along a belt from southeast to northwest through 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.
- From the American Journal of Science, vol. xlvi, November, 1893.
- Vol. xliv, p. 351.