he valley, as they
approach the plain, partake of its higher 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 always in the lower part of the glacier a broad depression between
the ice and the rocky walls, while, as this effect is not felt in the
centre of the glacier, it there retains a higher level. The natural
result of this is a convex surface, arching upward toward the middle,
sinking toward the sides. It is in these broad, marginal depressions
that the lateral moraines accumulate; masses of rock, stones, pebbles,
dust, all the fragments, in short, which become loosened from the rocky
walls above, fall into them, and it is a part of the materials so
accumulated which gradually work their way downward between the ice and
the walls, till the whole side of the glacier becomes studded with them.
It is evident, that, when the glacier runs in a northerly or southerly
direction, both the walls will be affected by the sun, one in the
morning, the other in the afternoon, and in such a case the sides will
be uniform, or nearly so. But when the trend of the valley is from east
to west, or from west to east, the northern side only will feel the full
force of the sun; and in such a case, only one side of the glacier will
be convex in outline, while the other will remain nearly on a level with
the middle. The large masses of loose materials which accumulate between
the glacier and its rocky walls and upon its margins form the lateral
moraines. These move most slowly, as the marginal portions of the
glacier advance at a much slower rate than its centre.

The medial moraines arise in a different way, though they are directly
connected with the lateral moraines. It often happens that two smaller
glaciers unite, running into each other to form a larger one. Suppose
two glaciers to be moving along two adjoining valleys, converging toward
each other, and running in an easterly or westerly direction; at a
certain point these two valleys open into a single valley, and here, of
course, the two glaciers must meet, like two rivers rushing into a
common bed. But as glaciers consist of a solid, and not a fluid, there
will be no indiscriminate mingling of the two, and they will hold their
course side by side. This being the case, the lateral moraine on the
southern side of the northernmost glacier and that on the northern