_ON MORAINES._
Now, in order clearly to understand the formation of moraines, I must
first say a little more about the movement of glaciers and the _débris_
which they bring down.
I have sometimes heard unthinking persons remark that the snowfall
of each winter must tend to increase the height of snow-peaks. This
observation shows that such people entirely overlook the four great
factors in the maintenance of a uniform height on mountain summits,
namely, melting, evaporation (which, in the dry air of the heights, is
a very powerful factor in causing the disappearance of snow), glaciers,
and avalanches. It is to the two latter of these that we must look for
the construction of moraines, in which work they are very largely aided
by two other factors, frost and rain. The glacier, starting in its
infant purity from some white, unsullied peak, loses, before many years
have past, its spotless character. The wintry frosts, gathering into
iron bonds the streams which trickle down the mountain-sides, expand
the water in freezing, and shatter the rocks with a force that the most
solid cliffs cannot resist. Broken and weathered fragments are washed
down from the slopes on every fall of rain, and dropping on to the once
unspotted bosom of the glacier, swell the burden which is gradually
laid upon it with advancing years. Spring after spring, furious
avalanches rush down, laden with earth and stones, which they fling
recklessly upon the now begrimed edges of the icy stream. The winds
and storms, too, contribute their share of dust and sand, and as the
glacier still flows on, shrunken in size and laden with heaps of earth
and rocks, at length it lays itself to rest, a mass of dirty ice and
stones, in the valley towards which it has been ceaselessly progressing.
The glacier of the Alps which comes farthest down into the lower
regions is the Grindelwald glacier, which descended to 1080 metres
above sea-level in 1870, while that which presents the largest surface
is the Aar glacier, and the longest is the Aletsch. Heim gives an
estimate, in his valuable work on glaciers, of the number of glaciers
existing in Europe, dividing them into those of the first and second
order.
The list is as follows:--
1st Order. 2nd Order. Total.
Switzerland 138 333 471
Austria 71 391 462
France 25 119 144
Italy 15 63 78
--- --- -----
249 906 1,155
Glaciers have regular periods during which they advance or retreat.
Many persons who visited the Mer de Glace some twenty or more years
ago remember that it then came down nearly to the level of the valley
of Chamonix, while the Rhone glacier reached almost to where the lower
hotel now stands. In old days, too, the two arms of the Fee glacier
united below the Gletscher alp, so that the cows had to pass across the
ice in order to reach their summer pastures. A period of advance is
always preceded for some years by a noticeable swelling of the upper
portions of glaciers; this, of course, is quite what one would expect.
A succession of cold, rainy summers and exceptionally snowy winters
eventually causes an increase in the glaciers, and the reverse has
naturally the contrary effect.
You have learnt that a moraine is a mixture of earth and stones which
is borne down by a glacier, and you know how all this _débris_ has
accumulated on the ice, chiefly by means of the shattering power of
frost on the rocks. Now let us notice the position which moraines
assume on a glacier like, say, the Morteratsch. As I have said, persons
unaccustomed to the mountain world, and thus unable to estimate the
relative sizes of objects seen at a distance, have been known to
inquire, when ascending Piz Languard, if the dark streak down the
centre of the Morteratsch glacier is a path. They are astonished to
learn that it is about fifty feet or more broad, and perhaps twenty
feet high in the centre. It is, in fact, neither more nor less than
a moraine, and belongs to that class known as medial moraines. Each
glacier has a moraine on either side of it, and when two glaciers
unite, their lateral moraines join and form a medial moraine. The
moraine at the end of a glacier (terminal moraine) is almost entirely
formed of the earth and stones which fall off the end of a glacier, and
not, as used to be supposed, by any pushing or scooping of the base of
the glacier.
In fact, the erosive power of a glacier is infinitesimal as compared
with that of water. Dr. Heim cites various examples to show that a
glacier leaves undisturbed much of what it finds in its way, and he
says that the Forno glacier, which some years ago greatly retreated
and left blocks of itself covered with _débris_ behind, rapidly
advanced once more in 1884 over the old accumulations at its base, but
did not disturb them in any way. Many of our readers will have noticed
the many glacier-worn rocks in the Engadine valley; they are especially
abundant near Maloja.
Now it will be seen, on near examination, that these rocks have been
gently polished by the ice constantly slipping over them, and that they
have not those deep smooth hollows which are formed by rushing, eddying
water.
The great glaciers which, in the glacier period, flowed down from
Mont Blanc to the Jura have left ample proof of their origin in the
huge blocks of granite which were transported by the ice, and now lie
stranded on the hill-sides at a distance of sixty miles and more from
the rocks out of which they were quarried. The size of some of these
erratic blocks is very remarkable. The biggest boulder in the Alps is
in Val Masino (one of the Italian valleys near the Bernina district).
Its dimensions, according to the late Mr. Ball, are--length, 250 feet;
breadth, 120 feet; height, 140 feet; in fact, as Mr. Douglas Freshfield
remarks, “as tall as an average church tower, and large enough to fill
up many a London square.” Many of my readers will remember the great
serpentine boulder in front of the little inn at Maltmark, which was no
doubt brought down by the glacier which must have originally filled the
basin of the lake. The ancient moraines near Aosta are also remarkable
evidences of the glacial epoch.
One word here as to the shape of a moraine. It rises, as you know, to
a ridge in the centre, and slopes down like the roof of a house at the
sides. This is because the heaping together of the earth and stones
in the middle has protected the ice from melting as rapidly there as
towards the sides; in fact, the same cause brings about the shape of
moraines as applies in that of sand cones.
I will close this chapter by a brief explanation of an appearance
which many of my readers who have visited Montanvert may have noticed,
especially on cloudy, dull days and after sunset. I refer to _dirt
bands_, which are especially noticeable on the Mer de Glace. I observed
them under peculiarly favourable circumstances from the summit of the
Grandes Jorasses, when a cloudy sky showed them up most distinctly.
They are often seen from the Montanvert hotel, however, and take the
form of dark bands across the glacier, the convex side of the curve
of each being in the direction of the motion of the ice. These dirt
bands are very simple in their origin, which is as follows:--At the
foot of an ice-fall the tottering blocks reunite and freeze together,
presenting a tolerably smooth surface with gentle undulations. The
glacier streams sweep dust and small _débris_ into the depressions,
which gradually form themselves across the glacier. This dust finally
freezes into the ice, and lower down presents the appearance of the
famed dirt bands.
Sometimes a photograph will give dirt bands with great distinctness;
they are very clearly seen in a view of the Mer de Glace from the
Aiguilles Rouge, taken by the late Mr. W. F. Donkin.