en cutting its way. It is
important to notice that such terminal moraines may actually span the
whole width of a valley, from side to side, and be interrupted only
where watercourses of sufficient power break through them. To suppose
that such transverse walls of loose materials could be thrown across a
valley by a river were to suppose that it could build dams across its
bed while it is flowing. Such transverse or crescent-shaped moraines are
everywhere the work of glaciers.

All these moraines are the land-marks, so to speak, by which we trace
the height and extent, as well as the progress and retreat, of glaciers
in former times. Suppose, for instance, that a glacier were to disappear
entirely. For ages it has been a gigantic ice-raft, receiving all sorts
of materials on its surface as it travelled onward, and bearing them
along with it; while the hard particles of rock set in its lower surface
have been polishing and fashioning the whole surface over which it
extended. As it now melts, it drops its various burdens on the ground;
boulders are the mile-stones marking the different stages of its
journey, the terminal and lateral moraines are the framework which it
erected around itself as it moved forward, and which define its
boundaries centuries after it has vanished, while the scratches and
furrows it has left on the surface below show the direction of its
motion.

All the materials which reach the bottom of the glacier, and are moving
under its weight, so far as they are not firmly set in the ice must be
pressed against one another, as well as against the rocky bottom, and
will be rounded off, polished, and scratched, like the rock itself over
which they pass. The pebbles or stones set fast in the ice will be thus
polished and scratched, however, only over the surface exposed; but, as
they may sometimes move in their socket, like a loosely mounted stone,
the different surfaces may in turn undergo this process, and in the end
all the loose materials under a glacier become more or less polished,
scratched, and grooved. These marks exhibit also the peculiarity so
characteristic of the grooves and scratches on the bed and walls of the
valley: they are rectilinear, trending in the direction in which the
superincumbent mass advances, though, of course, owing to the changes in
the position of the pebbles or boulders, they may cross each other in
every direction on their surface.

As the larger materials are pressed onw