810 EXTERNAL APPEARANCE OF GLACIERS. 
As the large materials are pressed onward wiih 
the finer ones, that is, with the sand, gravel, and 
mud accumulated at the bottom of the glacier, 
the component parts of this underlying bed of 
debris will be mixed together without any refer- 
ence to their size or weight. The softest mud 
and finest sand may be in immediate contact 
with the bottom of the valley, while larger rocks 
and pebbles may bo held in the ice above ; or 
their position may be reversed, and the coarser 
materials may rest below, while the finer ones 
are pressed between them or overlying them. 
In short, the whole accumulation of loose debris 
under the glacier, resulting from the trituration 
of all kinds of angular fragments reaching the 
lower surface of the ice, presents a sort of paste, 
in which coarser and lighter materials are im- 
pacted without reference to bulk or weight. Those 
fragments which are most polished, rounded, 
grooved, or scratched, have travelled longest 
under the glacier, and are derived from the hard- 
est rocks, which have resisted the general crush- 
ing and pounding for a longer time. The masses 
of rock on the upper surface of the glacier, on 
the contrary, are carried along on its back with- 
out undergoing any such friction. Lying side by 
side, or one above another, without being subject 
to pressure from the ice, they retain, both in the 
lateral and medial moraines, and even in the 
