The contributions that are supplied to moraines by glaciers themselves, from the abrasion of the rocks over which their ice passes, are minute compared with the accumulations which are furnished from other sources. These great rubbish-heaps are formed—one may say almost entirely—from débris which falls or is washed down the flanks of mountains, or from cliffs bordering glaciers; and are composed, to a very limited extent only, of matter that is ground, rasped or filed off by the friction of the ice.
If the contrary view were to be adopted, if it could be maintained that “glaciers, by their motion, break off masses of rock from the sides and bottoms of their valley-courses, and crowd along everything that is movable, so as to form large accumulations of débris in front and along their sides,”[[29]] the conclusion could not be resisted, the greater the glacier the greater should be the moraine.
[29] Atlas of Physical Geography, by Augustus Petermann and the Rev. T. Milner. The italics are not in the original.
This doctrine does not find much favor with those who have personal knowledge of what glaciers do at the present time. From De Saussure[[30]] downward it has been pointed out, time after time, that moraines are chiefly formed from débris coming from rocks or soil above the ice, not from the bed over which it passes. But amongst the writings of modern speculators upon glaciers and glacier-action in bygone times it is not uncommon to find the notions entertained that moraines represent the amount of excavation (such is the term employed) performed by glaciers, or at least are comprised of matter which has been excavated by glaciers; that vast moraines have necessarily been produced by vast glaciers; and that a great extension of glaciers—a glacial period—necessarily causes the production of vast moraines. It is needless to cite more than one or two examples to show that such generalizations cannot be sustained. Innumerable illustrations might be quoted.
[30] “The stones that are found upon the upper extremities of glaciers are of the same nature as the mountains which rise above; but, as the ice carries them down into the valleys, they arrive between rocks of a totally different nature from their own.”—De Saussure, § 536.
In the chain of Mont Blanc one may compare the moraines of the Miage with those of the Glacier d’Argentiere. The latter glacier drains a basin equal to or exceeding that of the former, but its moraines are small compared with those of the former. More notable still is the disparity of the moraines of the Corner glacier (that which receives so many branches from the neighborhood of Monte Rosa) and of the Z’Muttgletscher. The area drained by the Corner greatly exceeds the basin of the Z’Mutt, yet the moraines of the Z’Mutt are incomparably larger than those of the Corner. No one is likely to say that the Z’Mutt and Miage glaciers have existed for a far greater length of time than the other pair: an explanation must be sought amongst the causes to which reference has been made.
More striking still is it to see the great interior Mer de Glace of Greenland almost without moraines. This vast ice-plateau, although smaller than it was in former times, is still so extensive that the whole of the glaciers of the Alps might be merged into it without its bulk being perceptibly increased. If the size of moraines bore any sort of relation to the size of glaciers, the moraines of Greenland should be far greater than those of the Alps.
This interior ice-reservoir of Greenland, enormous as it is, must be considered as but the remnant of a mass which was incalculably greater, and which is unparalleled at the present time outside the Antarctic Circle. With the exception of localities where the rocks are easy of disintegration, and the traces of glacier-action have been to a great extent destroyed, the whole country bears the marks of the grinding and polishing of ice; and, judging by the flatness of the curves of the roches moutonnées, and by the perfection of the polish which still remains upon the rocks after they have sustained (through many centuries) extreme variations of temperature, the period during which such effects were produced must have widely exceeded in duration the “glacial period” of Europe. If moraines were built from matter excavated by glaciers, the moraines of Greenland should be the greatest in the world!
The absence of moraines upon and at the termination of this great Mer de Glace is due to the want of rocks rising above the ice.[[31]] On two occasions in 1867 I saw, at a glance, at least six hundred square miles of it from the summits of small mountains on its outskirts. Not a single peak or ridge was to be seen rising above, nor a single rock reposing upon, the ice. The country was completely covered up by glacier: all was ice as far as the eye could see.[[32]]
[31] I refer to those portions of it which I have seen in the neighbourhood of Disco Bay. There are moraines in this district, but they were formed when the great Mer de Glace stretched nearer to the sea,—when it sent arms down through the valleys in the belt of land which now intervenes between sea and glacier.