Fig. 6.—Section across Glacial Valley, showing old Lateral Moraines.
Like running water, moving ice is a powerful agent in transporting rocks and earthy débris of all grades of fineness; but, owing to the different consistencies of ice and water, there are great differences in the mode and result of transportation by them. While water can hold in suspension only the very finest material, ice can bear upon its surface rocks of the greatest magnitude, and can roll or shove along under it boulders and pebbles which would be Unaffected except by torrential currents of water. We find, therefore, a great amount of earthy material of all sizes upon the top of a glacier, which has reached it very much as débris reaches the bed of a river, namely, by falling down upon it from overhanging cliffs, or by land-slides of greater or less extent. Such material coming into a river would either disappear beneath its surface, or would form a line of débris along the banks; in both cases awaiting the gradual erosion and transportation which running water is able to effect. But, in case of a glacier, the material rests upon the surface of the ice, and at once begins to partake of its motion, while successive accessions of material keep up the supply at any one point, so as to form a train of boulders and other débris, extending below the point as far as the glacial motion continues.
Such a line of débris is called a moraine. When it forms along the edge of the ice, it is called a lateral moraine. It is easy to see that, where glaciers come out from two valleys which are tributary to a larger valley, their inner sides must coalesce below the separating promontory, and the two lateral moraines will become united and will move onward in the middle of the surface of the glacier. Such lines of débris are called medial moraines. These are characteristic of all extensive glaciers formed by the union of tributaries. There is no limit to the number of medial moraines, except in the number of tributaries.
A medial moraine, when of sufficient thickness, protects the ice underneath it from melting; so that the moraine will often appear to be much larger than it really is: what seems to be a ridge of earthy material being in reality a long ridge of ice, thinly covered with earthy débris, sliding down the slanting sides as the ice slowly wastes away Large blocks of stone in the same manner protect the ice from melting underneath, and are found standing on pedestals of ice, often several feet in height. An interesting feature of these blocks is that, when the pedestal fails, the block uniformly falls towards the sun, since that is the side on which the melting has proceeded most rapidly.
If the meteorological forces are so balanced that the foot of a glacier remains at the same place for any great length of time, there must be a great accumulation of earthy débris at the stationary point, since the motion of the ice is constantly bearing its lines of lateral and medial moraine downwards to be deposited, year by year, at the melting line along the front.
Such accumulations are called terminal moraines, and the process of their formation may be seen at the foot of almost any large glacier. The pile of material thus confusedly heaped up in front of some of the larger glaciers of the world is enormous.
The melting away of the lower part of a glacier gives rise also to several other characteristic phenomena. Where the foot of a glacier chances to be on comparatively level land, the terminal moraine often covers a great extent of ice, and protects it from melting for an indefinite period of time. When the ice finally melts away and removes the support from the overlying morainic débris, this settles down in a very irregular manner, leaving enclosed depressions to which there is no natural outlet. These depressions, from their resemblance to a familiar domestic utensil, are technically known as kettle-holes. The terminal moraines of ancient glaciers may often be traced by the relative abundance of these kettle-holes.