in the glacier proper, where it is completely transformed into ice, its surface becomes convex, for the following reason:  The rocky walls of the 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 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