outonnees_, especially the straight
scratches and grooves on the side up which the ice ascended, have led to
a mistaken view of the mode in which large boulders are transported by
ice. It has been supposed, by those who, while they accepted the glacial
theory, were not wholly conversant with the mode of action of glaciers,
that, in passing through the bottom of a valley, for instance, the
glacier would take up large boulders, and, carrying them along with it,
would push them up such a slope and deposit them on its summit. It is
true that large boulders may sometimes be found in front of glaciers
among the materials of their terminal moraines, and may, upon any
advance of the glacier, be pushed forward by it. But I know of no
example of erratic boulders being carried to considerable distances and
raised from lower to higher levels by this means. All the angular
boulders perched upon prominent rocks must have fallen upon the surface
of the glacier in the upper part of its course, where rocky ledges rise
above its surface and send down their broken fragments. The surface of
any boulder carried under the ice, or pushed along for any distance at
its terminus, would show the friction and pressure to which it had been
subjected. In this connection it should be remembered that in the case
of large glaciers low hills form no obstacle to their onward progress,
especially when the glacier is thick enough to cover them completely,
and even to rise far above them. The _roches moutonnees_ about the
Grimsel show that hills many hundred feet high have been passed over by
the great glacier of the Aar, when it descended as far as Meyringen,
without having seemingly influenced its onward progress.

But in enumerating the evidences of glacier-action, we have to remember
not only the effects produced upon the surface of the ground by the ice
itself, but also the deposits it has left behind it. The loose materials
scattered over the face of the earth may point as distinctly to the
source of their distribution as does the character of the rocky surfaces
on which they rest indicate the different causes of abrasion. In
characteristic localities the loose materials deposited by glaciers may
readily be recognized at first sight, and distinguished from water-worn
pebbles; nor is it difficult to distinguish both from loose materials
resulting from the decomposition of rocks on the spot,--the latter
always agreeing with the rocks on which they rest,