£02 EXTERNAL APPEARANCE OF GLACIERS. 
these materials, more or less firmly set in the 
ice, form the grating surface with which, in its 
onward movement down the Alpine valleys, it 
leaves everywhere unmistakable traces of its pas- 
sage. 
We come now to the moraines, those walls of 
loose materials built by the glaciers themselves 
along their road. They have been divided into 
three classes, namely, lateral, medial, and ter- 
minal moraines. Let us look first at the lateral 
ones ; and to understand them we must examine 
the conformation of the glacier below the nevt, 
where it assumes the character of pure compact 
ice. We have seen that the fields of snow, where 
the glaciers have their origin, are level, and that 
lower down, where these masses of snow begin 
to descend toward the narrower valley, they fol- 
low its trough-like shape, sinking toward the 
centre and sloping upward against the sides, so 
that the surface of the glacier, about the region 
of the neve is slightly concave. But lower down 
in the glacier proper, where it is completely 
transformed into ice, its surface becomes convex, 
for the following reason. The rocky walls of 
the valley, as they approach the plain, partake 
of its high temperature. They become heated 
by the sun during the day in summer, so that 
the margins of the glacier melt rapidly in contact 
with them. In consequence of this, there is 
