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 seen cutting its way.  It is important to notice that such terminal moraines may actually span the whole width of a valley, from side to side, and be interrupted only where watercourses of sufficient power break through them.  To suppose that such transverse walls of loose materials could be thrown across a valley by a river were to suppose that it could build dams across its bed while it is flowing.  Such transverse or crescent-shaped moraines are everywhere the work of glaciers.

All these moraines are the land-marks, so to speak, by which we trace the height and extent, as well as the progress and retreat, of glaciers in former times.  Suppose, for instance, that a glacier were to disappear entirely.  For ages it has been a gigantic ice-raft, receiving all sorts of materials on its surface as it travelled onward, and bearing them along with it; while the hard particles of rock set in its lower surface