The conception of immense ice-sheets, like those of Greenland and Antarctica, covering large parts of North America and Europe at comparatively recent geological dates, is one that at first seems to be incredible, and made its way very slowly in the face of determined opposition. Originally suggested by Agassiz about 1846, it required nearly thirty years to gain the general acceptance of geologists. This change of view was brought about by evidence too strong to be resisted; not that all difficulties have been removed and all problems solved, but no other hypothesis can rival the glacial in satisfactorily explaining so many and such varied classes of phenomena. It will be well to summarize this evidence briefly. The work of erosion, transportation, and deposition accomplished by existing glaciers was described in Chapters VI and VIII, but many of the illustrations and descriptions of those chapters were drawn from the areas of Pleistocene glaciation, partly for the convenience of dealing with things near home, and partly in order to bring the past and present into immediate juxtaposition.

The characteristics of glacial erosion, the rounded and flowing outlines, the smoothed and polished rocks, the parallel striae cut by the stones and boulders held in the bottom of the ice, are all to be found abundantly in the glaciated area wherever the rocks are hard enough to receive and retain the marks and have been protected from the weather since the withdrawal of the ice; in many instances, even prolonged exposure to weathering has not sufficed to destroy the markings. The striae, which are parallel in small areas, when examined on a large scale, are found to be arranged in definite systems, which show the outward movement of the ice from the centres of dispersion. The roches moutonnees and hummocks of rock, gently sloping and smoothed on the side against 3d which the ice impinged (stoss side) abrupt and often rough on the sheltered side (lee side) characterize the areas of Pleistocene glaciation, just as they do the rocky beds recently abandoned by retreating glaciers. In short, all the characteristic features of glacial erosion, which can be reproduced by no other known agency, occur where, by the theory, they should be found.

When the contact of the drift with the smoothed and striated ice-floor can be observed (see Fig. 69, p. 160), the change is sudden and abrupt, the drift resting upon the hard, clean, unaltered rock, not at all like the gradual transition of soil, subsoil, and rotten rock (of. Fig. 34, p. 106) which occurs when the soil arises from the decomposition of rock in place. There are some exceptions to this rule, where the erosive action of the ice was feeble and insufficient to remove all the old soil and rotten rock, but such exceptions offer no difficulty of explanation.

The drift itself is as convincing in its testimony of glacial deposition as the ice-floors are in their evidence of glacial erosion. The unstratified drift, made by the ice alone, in the form of moraines, chiefly terminal, but also lateral around projecting lobes of the ice, are highly characteristic of glaciers, as are the huge erratic and perched blocks, which often have travelled hundreds of miles. The ground moraine, or till, made up of finely ground rock-flour, in which are embedded boulders large and small, many of them faceted, smoothed, and striated, as only ice-worn boulders can be, and spread out in sheets of very variable thickness, its large boulders often deposited high above the points whence they were taken, testifies eloquently that it was accumulated by moving ice, which alone can deposit the finest and the coarsest materials together and can move to a large extent independently of the topographical features. The materials of the till are mostly of local origin and have travelled but a few miles, but there is almost always a greater or less admixture of stones from a long distance, and generally these are smaller in proportion to the distance travelled, because of the wear to which they have been subjected.

The stratified drift is no less indicative of glacial action. Whether advancing or retreating, the ice margin was melting, and the drift left by retreating ice-sheets was more or less worked over by water. Subglacial streams discharging in valleys made valley trains (see p. 234), and water descending in broad, shallow sheets, where the ice ended on a plain, made overwash plains, both connected with morainic deposits at their head. Eskers are gravel-filled subglacial tunnels, but drumlins and kames offer much difficulty of interpretation and have not yet been explained in a way that commands general assent. The ice barriers frequently formed lakes large and small, and in these lakes water-made and ice-made deposits were intimately associated. The glacial theory "distinctly affirms that rivers, lakes, the sea, icebergs, and pan-ice must have cooperated with glacier ice in the production of the drift, each in its appropriate way and measure." (Chamberlin and Salisbury).

Finally, the evidence of the fossils, marine and terrestrial, animal and plant, strongly supports the glacial theory, by demonstrating a general refrigeration of the climate, when Arctic molluscs lived on the coasts of New England and northern Europe, and Arctic vegetation covered the lands in low latitudes, and Arctic mammals, like the reindeer and the musk-ox, descended to the south of France and to Arkansas. The testimony is thus all harmonious as to the great expansion of the Pleistocene glaciers and ice-sheets.

No one would pretend that there are no difficulties, still unexplained, in the way of accepting the theory. Some of these have been alluded to above; another is the enormous thickness of the ice-sheets required by the evidence, several thousand feet, for the glacial marks sweep over the tops of the highest mountains in New England and New York. On the other hand, it is held by physicists that 1600 feet is the maximum possible thickness of ice, as a greater amount would cause the bottom to melt from pressure, and in confirmation of this it is pointed out that the Antarctic icecap does not exceed the theoretical maximum, and that " at the present day no ice more than 1600 feet (thick) has been recorded." (Ferrar.) This conflict of evidence it must be left to future investigations to reconcile, but the probable solution is to be found in the temperature relations of the ice. The theoretical maximum depends upon the assumption that the bottom of the ice is at or near 320 F., but if it were considerably below this, the thickness might be greatly increased.