external afpearance of glaciers. 309 
the milestones 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 boun- 
daries 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, pol- 
ished, 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 char- 
acteristic of the grooves and scratches on the 
bed and walls of the valley ; they are rectilinear, 
trending in the direction in which the superin- 
cumbent mass advances, though, of course, owing 
to the changes in the position of the pebbles or 
boulders, they may cross each other in evc.-ry 
direction on their surface. 
