side
of the southernmost one must meet in the centre of the combined
glaciers. Such are the so-called medial moraines formed by the junction
of two lateral ones. Sometimes a glacier may have a great number of
tributaries, and in that case we may see several such moraines running
in straight lines along its surface, all of which are called medial
moraines in consequence of their origin midway between two combining
glaciers. The glacier of the Aar represented in the wood-cut below
affords a striking example of a large medial moraine. It is formed by
the junction of the glaciers of the Lauter-Aar, on the right-hand side
of the wood-cut, and the Finster-Aar, on the left; and the union of
their inner lateral moraines, in the centre of the diagram, forms the
stony wall down the centre of the larger glacier, called its medial
moraine. This moraine at some points is not less than sixty feet high.
We have here an effect similar to that of the glacier-tables and the
sand-pyramids. The wall protects the ice beneath it, and prevents it
from sinking at the same rate as the surrounding surface, while its
heated surface increases the melting of the adjacent surfaces of ice,
thus forming longitudinal depressions along the medial moraines, in
which the largest rivulets and the most conspicuous sand-pyramids, the
deepest wells and the finest waterfalls, are usually met with. As the
medial moraines rest upon that part of the glacier which moves fastest,
they of course advance much more rapidly than the lateral moraines.

[Illustration: Glacier of the Aar.]

The terminal moraines consist of all the _debris_ brought down by the
glacier to its lower extremity. In consequence of the more rapid
movement of the centre of the glacier, it always terminates in a
semicircle at its lower end, where these materials collect, and the
terminal moraines, of course, follow the outline of the glacier. The
wood-cut below represents the terminal moraine of the glacier of Viesch.

[Illustration]

Sometimes, when a number of cold summers have succeeded each other,
preventing the glacier from melting in proportion to its advance, the
accumulation 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 valley from side to side, through which the
stream issuing from the glacier may be se