Fragments of Earth Lore: Sketches & Addresses Geological and Geographical

by James Geikie

Changes of Climate in North America during the Ice Age.

American geologists are now pretty well agreed that their “interglacial deposits”—the existence of which is not disputed—have precisely the same meaning as the similar deposits which occur in Europe. They tell of great climatic changes. At present, however, there is no certain evidence in the American deposits of more than one interglacial epoch; but the proofs of such an epoch having obtained are overwhelming. The occurrence again and again of fossiliferous beds intercalated between two separate and distinct sheets of boulder-clay and morainic accumulations, leaves us in no doubt that we are dealing with precisely the same phenomena which confront us in Europe. No mere partial recession and re-advance of the mer de glace will account for the facts. We have seen that during the culmination of the Glacial period the American ice-sheet overflowed Ohio, Indiana, and Illinois. Now interglacial deposits occur as far north as the Canadian shores of Lakes Ontario and Superior, so that all the country to the south must have been uncovered by ice before those interglacial deposits were laid down. But the evidence entitles us to say much more than this. The interglacial beds of Ohio, Indiana, Illinois, and other States, afford abundant evidence of a great forest-growth having covered the regions vacated by the ice of the penultimate glacial epoch. The trees of this forest-land included sycamore, beech, hickory, red-cedar, and others; and amongst the plants were grape vines of enormous growth, which, according to Professor Cox, “indicate perhaps the luxuriance of a warmer climate.” At all events, the climate that nourished such a forest-growth could not have been less genial than the present. And such being the case, we may reasonably infer that the vast regions to the north of the lakes were no more inhospitable then than they are now.

To this genial interglacial epoch succeeded the last glacial epoch, when a great ice-sheet once more enveloped a wide area. In the extreme east this latest mer de glace appears to have reached as far south as that of the earlier epoch; but as we follow its terminal moraines westward they lead us further and further away from the southern limits attained by the preceding ice-sheet. These great terminal moraines form an interesting study, and the general results obtained by American observers have been very carefully put together by Professor Chamberlin. I traversed wide regions of those moraines in Indiana, Illinois, Wisconsin, and Minnesota, and, so far as my observations went, I could only confirm the conclusions arrived at by Professor Chamberlin and others. The mounds, banks, cones, and ridges are unquestionably moraines—of enormous dimensions, no doubt, but in all their phenomena strictly analogous to similar gravelly moraines in our own country and the Continent. Many of the American moraines consist almost entirely of water-worn material—sand, gravel, shingle, and boulders, together with large angular and sub-angular erratics. These deposits are generally stratified, and frequently show diagonal or false-bedding. In this and other respects they exactly reproduce—but of course on a much larger scale—our Scottish kames, and the similar accumulations of north Germany and Finland, and the low-grounds of Italy opposite the mouths of the great Alpine lakes. The kames of Wisconsin again and again reminded me of the gravelly moraines that cover the ground for many miles round the lower end of Lake Garda. It is this gravelly and sandy aspect of the American moraines that is most conspicuous, water-assorted materials seeming everywhere to form their upper and outer portions. Now and again, however, a deep cutting discloses underneath and behind such water-worn detritus a mass of confused materials, consisting of clay, sand, gravel, shingle, and boulders, which are angular and sub-angular, often smoothed and striated, and of all shapes and sizes. According to Mr. Chamberlin, this unstratified material “is indistinguishable from true till, and is doubtless to be regarded as till pushed up into corrugations by the mechanical action of the ice.”

This grand series of moraines stretches from the peninsula of Cape Cod across the northern States, and passes in a north-westerly direction into the British Possessions, over which it has been followed for some 400 miles. The disposition of the moraines, forming as they do a series of great loops, shows that the ice-sheet terminated in a number of lobes or gigantic tongue-like processes. Nothing seen by me suggested any marine action; on the contrary, every appearance, as I have said, betokened the morainic origin of the mounds; and Mr. Chamberlin assured me that their peculiar distribution was everywhere suggestive of this origin. No one who has traversed the regions I refer to is at all likely to agree with Sir W. Dawson’s view, that the American mounds, etc., are the shore-accumulations of an ice-laden sea.

The morainic origin of these accumulations having been demonstrated by American geologists, we are now able to draw another parallel between the European and American glacial deposits. We have seen that in Europe the ice-sheet of the latest glacial epoch was by no means so extensive as that of the preceding glacial epoch. The same was the case in North America. Moreover, in America, just as in Europe, the latest occupant of the land was not the sea, but glacier-ice. In Scotland and Scandinavia the gradual disappearance of the latest ice-sheets was marked by a partial submergence, which in the former country did not greatly exceed 100 feet, and in the latter 700 feet. In America, in like manner, we find traces of a similar partial submergence. In Connecticut this did not exceed 40 or 50 feet, but increased to some 500 feet in the St. Lawrence, and to over 1000 feet in the Arctic regions. If there ever was during the Glacial period a greater submergence than this in North America it must have taken place in earlier glacial or interglacial times, but of such a submergence no trace has yet been recognised. In this respect the American record differs somewhat from our own, for in Britain we have evidence of a submergence of over 1000 feet, which supervened in times immediately preceding the latest great extension of continental ice.[P] But nowhere in middle Europe, and nowhere in North America, in the region south and west of the great lakes, is there any trace of a general marine submergence. The “Palæocrystic Sea” is as idle a dream for the northern States of America as it is for any part of Europe.

[P] See footnote, p. 173.

VI.

The Results of Fluvio-glacial Action in North America.

The close analogies which obtain between the glacial and interglacial deposits of Europe and North America are equally characteristic of the fluvio-glacial accumulations of the two continents. As in Europe, so in America we meet with considerable sheets of gravel and shingle, sand, fine clay, and loam, which are evidently of freshwater origin. In the gently-undulating tracts of the northern States those deposits often spread continuously over wide regions; in the hillier districts, however, they are most characteristic of the valleys. They are very well represented, for example, in the Connecticut valley, where they have been carefully studied by Professor Dana. Like the similar deposits of our own Continent, they have been laid down by the torrents and swollen rivers of the Glacial period. The great range of moraines which marks the extreme limits reached by the latest ice-sheet is generally associated with sheets of gravel and sand, which one can see at a glance are of contemporaneous origin, having been spread out by the water escaping from the melting ice. Nor can one doubt that the vast sheets of löss in the Missouri and Mississippi valleys are strictly analogous in origin, as they are in structure and disposition, to the löss of Europe. I have spoken of the probable existence of a glacial lake formed by the damming back of the Rhine and other rivers by the European ice-sheet. Now, in North America we meet with evidence of the same phenomenon. When the last ice-sheet of that continent attained its maximum development, all the water escaping from its margin in the north States necessarily flowed south into the Mississippi and Missouri rivers. But in course of time the ice melted away beyond the drainage-area of those rivers, and disappeared from the valley of the Red River of the north, which, it will be remembered, empties itself northward into Lake Winnipeg. When the ice-front had retired so far it naturally impeded the drainage of the Red River basin, and thus formed a vast glacial lake, the limits of which have been approximately mapped out by Mr. Upham, by whom the ancient lake has been designated Lake Agassiz. The deposits laid down in this lake consist of finely laminated clays, etc., which resemble in every particular the similar unfossiliferous clays so frequently found associated with glacial accumulations in Europe. Had the drainage of the Red River valley been south instead of north, the clays and loams of the far north-west would not have been arrested and spread out where they now are, and Manitoba would have been covered for the most part with loose shingle, gravel, and sand.