From the above mentioned facts, it must be at once obvious that the various kinds of loose materials all over the northern hemisphere, have been accumulated, not only under different circumstances, but during long-continued subsequent distinct periods, and that great changes have taken place since their deposition, before the present state of things was fully established.
To the first period,—the ice period, as I have called it,—belong all the phenomena connected with the transportation of erratic boulders, the polishing, scratching, and furrowing of the rocks, and the accumulation of unstratified, scratched, and loamy drift. During that period the mainland seems to have been, to some extent at least, higher above the level of the sea than now; as we observe, on the shores of Great Britain, Norway, and Sweden, as well as on the eastern shores of North America, the polished surfaces dipping under the level of the ocean, which encroaches everywhere upon the erratics proper, effaces the polished surfaces, and remodels the glacial drift. During these periods, large terrestrial animals lived upon both continents, the fossil remains of which are found in the drift of Siberia, as well as of this continent. A fossil elephant, recently discovered in Vermont, adds to the resemblance, already pointed out, between the northern drift of Europe and that of North America; for fossils of that genus are now known to occur upon the northern-most point of the western extremity of North America, in New England, in Northern Europe, as well as all over Siberia.
To the second period we would refer the stratified deposits resting upon drift, which indicate, that during their deposition the northern continent had again extensively subsided under the surface of the ocean.
During this period, animals, identical with those which occur in the northern seas, spread widely over parts of the globe which are now again above the level of the ocean. But, as this last elevation seems to have been gradual, and is even still going on in our day, there is no possibility of tracing more precisely, at least for the present, the limit between that epoch and the present state of things. Their continuity seems almost demonstrated by the identity of fossil-shells found in these stratified deposits, with those now living along the present shores of the same continent, and by the fact, that changes in the relative level between sea and mainland are still going on in our day.
Indications of such relative changes between the level of the waters and the land are also observed about Lake Superior. And here they assume a very peculiar character, as the level of the lake itself, in its relation to its shores, is extensively changed.[48]
[45] Vide Lake Superior, its physical character, vegetation, and animals. By Professor Louis Agassiz. 1850.
[46] A comparison of the maps, shewing the arrangement of the moraines upon the glacier of the Aar, in my Système Glaciaire, with the maps which Professor Guyot is about to publish of the distribution of the erratic boulders in Switzerland, will shew more fully the identity of the two phenomena.
[47] Berlin Academy, 1846.
[48] An interesting account of the natural terraces around Lake Superior is given at p. 413-416 of “Lake Superior.”