808 EXTERNAL APPEARANCE OF GLACIERS. 
cumulation of materials at its terminus becomes 
very considerable ; and when, in consequence of 
a succession of warm summers, it gradually melts 
and retreats from the line it has been occupying, 
a large semicircular wall is left, spanning the val- 
ley from side to side, through which the stream 
issuing from the glacier may be seen 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 in- 
terrupted 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 sup- 
pose that it could build dams across its bed while 
it is flowing. Such transverse or crcscent-sliaped 
moraines are everywhere the work of glaciers. 
All these moraines are the landmarks, 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 
lias 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 aro 
