The loose materials which produced, in their onward movement under the pressure of ice, such polishing and grooving, consisted of various-sized boulders, pebbles, and gravels, down to the most minute sand and loamy powder. Accumulations of such materials are found everywhere upon these smooth surfaces, and in their arrangement they present everywhere the most striking contrast when compared with deposits accumulated under the agency of water. Indeed, we nowhere find this glacial drift regularly stratified, being every where irregular accumulations of loose materials, scattered at random without selection, the coarsest and most minute particles being piled irregularly in larger or smaller heaps, the greatest boulders standing sometimes uppermost, or in the centre, or in any position among smaller pebbles and impalpable powder.
And these materials themselves are scratched, polished and furrowed, and the scratches and furrows are rectilinear as upon the rocks in situ underneath, not bruised simply, as the loose materials carried onward by currents or driven against the shores by the tides, but regularly scratched, as fragments of hard materials would be if they had been fastened during their friction against each other, just as we observe them upon the lower surface of glaciers where all the loose materials are set in ice, as stones in their setting are pressed and rubbed against underlying rocks. But the setting here being simply ice, these loose materials, fast at one time and moveable another, and fixed and loosened again, have rubbed against the rock below in all possible positions; and hence, not only their rounded form, but also their rectilinear grooving. How such grooves could be produced under the action of currents, I leave to the advocates of such a theory to shew; as soon as they shall be prepared for it.
I should not omit here to mention a fact which, in my opinion, has a great theoretical importance, namely, that in the northern erratics, even the largest boulders, as far as I know, are rounded, and scratched and polished; at least, all those which are found beyond the immediate vicinity of the higher mountain ranges, shewing that the accumulations of ice which moved the northern erratics covered the whole country; and this view is sustained by another set of facts equally important, namely, that the highest ridges, the highest rugged mountains, at least, in this continent and north of the Alps in Europe, are as completely polished and smoothed as the lower lands, and only a very few peaks seem to have risen above the sheet of ice; whilst, in the Alps, the summits of the mountains stand generally above these accumulations of ice, and have supplied the surface of the glaciers with large numbers of angular boulders, which have been carried upon the back of glaciers to the lower valleys and adjacent plains without losing their angular forms.
With respect to the irregular accumulation of drift-materials in the north, I may add, that there is not only no indication of stratification among them, such, unquestionably, as water would have left, but that the very nature of these materials shews plainly that they are of terrestrial origin; for the mud which sticks between them adheres to all the little roughnesses of the pebbles, fills them out, and has the peculiar adhesive character of the mud ground under the glaciers, and differing entirely in that respect from the gravels, and pebbles, and sands washed by water-currents, which leave each pebble clean, and never form adhering masses, unless penetrated by an infiltration of limestone.
Another important fact respecting this glacial draft consists in the universal absence of marine, as well as fresh-water fossils in its interior—a fact which strengthens the view that they have been accumulated by the agency of strictly terrestrial glaciers; such is, at least, the case everywhere far from the sea-shore. But we may conclude that these ancient glaciers reached, upon various points, the sea-shore at the time of their greatest extension, just as they do at present in Spitzbergen and other arctic shores; and that therefore, in such proximity, phenomena of contact should be observed, indicating the onward movement of glacial material into the ocean, such as the accumulation within these materials of marine fossil remains, and also the influence of the tidal movements upon them. And now such is really the case. Nearer the sea-shores we observe distinctly, in some accumulations of the drift, faint indications of the action of the tide, reaching the lower surface of glaciers, and the remodelling to some extent of the materials which these poured into the sea. A beautiful example of the kind may be observed near Cambridge, along Charles River, not far from Mount Auburn, where the unstratified glacial drift (a) presents in its upper masses strictly the characters of true terrestrial glacial accumulation, but shews underneath faint indications (b) of the action of tides. Above, regular tidal strata (c) are observed, formed probably after the masses below had subsided. The surface of this accumulation is covered with soil (d).
The period at which these phenomena took place cannot be fully determined, nor is it easy to ascertain whether all glacial drift is contemporaneous. It would seem, however, as if the extensive accumulation of drift all around the northern pole in Europe, Asia, and America was of the same age as the erratic of the Alps. The climatic circumstances capable of accumulating such large masses of ice around the north pole, having no doubt extended their influence over the temperate zone, and probably produced, in high mountain chains, as the Alps, the Pyrenees, the Black Forest, and the Vosges, such accumulations of snow and ice as may have produced the erratic phenomena of those districts. But extensive changes must have taken place in the appearance of the continents over which we trace erratic phenomena, since we observe in the Old World, as well as in North America, extensive stratified deposits containing fossils which rest upon the erratics; and as we have all possible good reasons and satisfactory evidence for admitting that the erratics were transported by the agency of terrestrial glaciers, and that, therefore, the tracts of land over which they occur stood at that time above the level of the sea, we are led to the conclusion that these continents have subsided since that period below the level of the sea, and that over their inundated portions, animal life has spread, remains of organized beings have been accumulated, which are now found in a fossil state in the deposits formed under those sheets of water.
Such deposits occur at various levels in different parts of North America. They have been noticed about Montreal, on the shores of Lake Champlain, in Maine, and also in Sweden and Russia; and what is most important, they are not everywhere at the same absolute level above the surface of the ocean, shewing that both the subsidence and the subsequent upheaval which has again brought them above the level of the sea, have been unequal; and that we should therefore be very cautious in our inferences respecting both the continental circumstances under which the ancient glaciers were formed, and also the extent of the sea afterward, as compared with its present limits.
The contrast between the unstratified drift and the subsequently stratified deposits is so great, that they rest everywhere unconformably upon each other, shewing distinctly the difference of the agency under which they were accumulated. This unconformable superposition of marine drift upon glacial drift is so beautifully shewn at the above-mentioned locality near Cambridge (see diagram, p. 114.) In this case the action of tides in the accumulation of the stratified materials is plainly seen.
The various heights at which these stratified deposits occur, above the level of the sea, shew plainly, that since their accumulation the main land has been lifted above the ocean at different rates in different parts of the country; and it would be a most important investigation to have their absolute level, in order more fully to ascertain the last changes which our continents have undergone.