22 
GLACIAL PERIOD. 
glacier while uniformly retreating forms no 
high walls of loose materials around its edges 
and at its lower extremity; as it melts away, 
it only drops the burden of angular rocky frag¬ 
ments carried upon its back over the loose 
fragments above which it moves, and which it 
grinds to powder, or to sand, or to rounded 
pebbles, in its progress. It is only where the 
glacier remains stationary for a longer or 
shorter period that large terminal moraines 
can accumulate ; and they are generally found 
in such places in the valleys of the Alps as 
would naturally determine the lower limit of a 
glacier for the .time being. We cannot escape 
the conclusion that the ancient glaciers must 
have begun that series of oscillations to which 
the accumulation of the moraines is to be as¬ 
cribed, at a time when ice-fields already occu¬ 
pied the whole area which they have covered 
during their greatest extension. After we 
shall have seen how many centres of dispersion 
of erratic boulders, similar to that of the Alps, 
existed in the northern hemisphere, we may 
perhaps be able to form some idea of the man¬ 
ner in which these ice-fields originated and 
gradually vanished. 
