than in Norway, Great Britain, or Switzerland. In short, the ice of the great glacial period in America moved over the continent as one continuous sheet, overriding nearly all the inequalities of the surface. Thus the peculiar physical character of the country gives a new aspect to the study of glacial evidences here. The polished surfaces stretch continuously over hundreds and hundreds of miles; the rectilinear scratches, grooves, and furrows are unbroken for great distances; the drift spreads in one vast sheet over the whole land, consisting of an indiscriminate medley of clays, sands, gravels, pebbles, boulders of all dimensions, and so uniformly mixed together that it presents hardly any difference in its composition, whether we examine it in New England, New York, Pennsylvania, Ohio, Michigan, Indiana, Illinois, Wisconsin, in Iowa beyond the Mississippi, in the more northern Territories, or in Canada.
In Europe, boulders of large dimensions do not often occur within the drift, but are usually resting above it with their sharp angles and rough surfaces unchanged, having travelled evidently upon the glacier and not under it. But such large boulders, polished and scratched like the smaller pebbles, are to be found everywhere imbedded in American drift, while the angular fragments of rock resting above these triturated masses are comparatively rare.[1] It is evident from this that the ice overtopped the rocky inequalities of the land, and that the detached fragments remaining beneath the icy covering underwent the same action from friction and pressure to which the whole mass of drift was subjected. The distribution of the few angular boulders scattered over the country no doubt began when some of the higher portions of the land had emerged from the mass of snow and ice; and they are most frequent in New England, where the mountain-elevation is greatest.
The mineralogical character of the loose materials forming the American drift leaves no doubt that the whole movement, with the exception of a few local modifications easily accounted for by the lay of the land, was from north to south, all the fragments not belonging to the localities where they occur being readily traced to rocks in situ to the north of their present resting-places. The farther one journeys from their origin, the more extraordinary does the presence of these boulders become. It strikes one strangely to find even in New England fragments of rock from the shores of Lake Superior; but it is still more impressive to meet with masses of northern rock on the prairies of Illinois or Iowa. One may follow these boulders to the fortieth degree of latitude, beyond which they become more and more rare, while the finer drift alone extends farther south.
It is not only, however, by tracking the boulders back to their origin in the North that we ascertain the starting-point of the whole mass; we have another kind of evidence to this effect, already alluded to in the description of the roches
- ↑ The greater proportion of large, rounded boulders in the American drift, as compared with the European, is a singular fact not fully met by the above explanation; since, while the number of mountain-peaks rising above the ice in Europe would account for the frequency of large angular fragments transported upon its surface, there would seem to be no reason why the drift, carried along by a mass of ice having the same thickness in both continents, should not contain as many rounded masses in one as in the other. The facts, however, are as I have stated them, and the difference may be due partly to the broken character of the ground over which the drift must have passed in Europe, subjecting it to a more violent process of friction and grinding than in America, and partly to the use that has been made of the drift-boulders during so many centuries for building-purposes in the Old World, the drift-boulders being naturally taken first, because they are more easily reached, while the angular ones are frequently perched on almost inaccessible spots. Indeed, the stone fences in both countries tell us the use to which many of the rounded boulders have been put, and the ground in many parts of the United States has already been cleared to a great extent of its rocky fragments for this and like purposes. In the course of time they will, no doubt, disappear from the surface of this country, as they have done from that of Europe.
