dozen or more. Those on the sides of the
ice have evidently been derived from the precipices which they have
passed. Those in the middle have arisen from the union of the moraines
formed in two or more tributary valleys.

[Illustration: Fig. 12.--Map of glaciers and moraines near Mont Blanc.]

Where the avalanches fall most plentifully, the stones lie buried with
the snow, and only melt out when the stream attains the region where
the annual waste of its surface exceeds the snowfall. In this section
we can see how the progressive melting gradually brings the rocky
_debris_ into plain view. Here and there we will find a boulder
perched on a pedestal of ice, which indicates a recent down-wearing of
the field. A frequent sound in these regions arises from the tumble of
the stones from their pedestals or the slipping of the masses from the
sharp ridge which is formed by the protection given to the ice through
the thick coating of detritus on its surface. These movements of the
moraines often distribute their waste over the glacier, so that in its
lower part we can no longer trace the contributions from the several
valleys, the whole area being covered by the _debris_. At the end of
the ice stream, where its forward motion is finally overcome by the
warmth which it encounters, it leaves in a rude heap, extending often
like a wall across the valley, all the coarse fragments which it
conveys. This accumulation, composed of all the lateral moraines which
have gathered on the ice by the fall of avalanches, is called the
terminal moraine. As the ice stream itself shrinks, a portion of the
detritus next the boundary wall is apt to be left clinging against
those slopes. It is from the presence of these heaps in valleys now
abandoned by glaciers that we obtain some information as to the former
greater extent of glacial action.

The next most noticeable feature is the crevasse. These fractures
often exist in very great numbers, and constitute a formidable barrier
in the explorer's way. The greater part of these ruptures below the
_serac_ zone run from the sides of the stream toward the centre
without attaining that region. These are commonly pointed up stream;
their formation is due to the fact that, owing to the swifter motion
in the central parts of the stream, the ice in that section draws away
from the material which is moving more slowly next the shore. As
before noted, these ice fractures when drawn out naturally form
fissur