Page:The Atlantic Monthly, Volume 14.djvu/100

90 the surface are characteristic of the rocks in those regions, wherever they are not disintegrating under the influence of the present atmospheric agents.

Upon these surfaces, through the whole expanse of the country, rests the drift, having everywhere the characteristic composition of glacier-drift, and nowhere that of an aqueous stratified deposit, except when afterwards remodelled by the action of water. But of this stratified drift I shall have occasion to speak more in detail hereafter. There is, however, one circumstance, of frequent occurrence along our New-England shores, requiring special explanation, because it is generally misunderstood. Along our sea-shore, and even within the harbor of Boston, at the base of the harbor-islands, as well as at the outlet of our larger Atlantic streams, numbers of boulders are found of considerable size; and this fact is often adduced as showing the power of water to transport massive fragments of rock to great distances, the mineralogical character of these boulders being frequently such as to show that they cannot have originated in the neighborhood of their present resting-places. But a careful examination of the surrounding country, and a comparison of the nature and level of the drift on the mainland with those of the same deposits on the harbor-islands, (a series of evidence to be given with more detail in a future article,) suggest a different explanation of these phenomena. The sheet of drift was once more continuous and extensive than it is now, and the localities in which we find these crops of boulders are spots where the tide has eaten into the drift, wearing away the finer materials, or the paste in which the larger fragments were imbedded, and allowing them to fall to the bottom, or where the same result has been produced by the action of rivers cutting their way through the drift, and thus finding an outlet to the sea. In short, instead of showing the power of currents to carry along heavy fragments, these stranded boulders prove, on the contrary, the inability of water to produce any such effect, since it is evident that the tides washing against the shore, or the rivers rushing down to the sea, were equally incapable of bearing off the weightier materials, and allowed them to drop to the bottom, while they readily swept away the lighter ones. Such localities compare with the surrounding drift much as the bottom of a gravel-pit which has been partially worked compares with its banks. Look into any gravel-pit, a portion of which has already been carted away. At its bottom a number of larger stones and boulders are usually lying, too heavy for the cart, and therefore left upon the spot. Fragments of the same size and character, and equally numerous, will be seen protruding at various heights from the sides, where they are imbedded in the general mass of the drift. As soon as the work progresses a little farther, and the finer materials are removed, these boulders will also drop out, and lie as thickly scattered over the surface of the ground, as they now do in that portion of the bottom where the pit has been completely opened and the gravel removed. We shall see hereafter how these boulders, derived from the land-drift and scattered along the coast, may be distinguished from those cast ashore by icebergs.

Notwithstanding the number of facts thus far collected respecting glacial phenomena in America, certainly forming in their combination a very strong chain of evidence, the scientific world has, nevertheless, been slow to admit the possibility of the former existence of glaciers over such a wide, unbroken expanse of level land. This backwardness is, no doubt, partly due to the fact, that, as glaciers have hitherto been studied in mountainous countries, their presence has been supposed to imply the presence of mountains, this impression being strengthened by the downward and onward movement of existing glaciers, so long supposed to be exclusively due to the slopes along which all modern glaciers advance. Were it true that glaciers move solely or mainly on account of the sloping bottom on which they rest, and that they can