2016년 1월 20일 수요일

Man and the Glacial Period 5

Man and the Glacial Period 5


[Illustration: Fig. 5.--_c_, _c_, show fissures and seracs where the
glacier moves down the steeper portion of its incline; _s_, _s_, show the
vertical structure produced by pressure on the gentler slopes.]
 
The _fissures_, which, when of large size, are called _crevasses_, are
formed in those portions of a glacier where, from some cause, the ice
is subjected to slight tension. This occurs especially where, through
irregularities in the bottom, the slope of the descent is increased. The
ice, then, instead of moving in a continuous stream at the top, cracks
open along the line of tension, and wedge-shaped fissures are formed
extending from the top down to a greater or less distance, according to
the degree of tension. Usually, however, the ice remains continuous in
the lower strata, and when the slope is diminished the pressure reunites
the faces of the fissure, and the surface becomes again comparatively
smooth. Where there are extensive areas of tension, the surface of the
ice sometimes becomes exceedingly broken, presenting a tangled mass of
towers, domes, and pinnacles of ice called _seracs_.
 
[Illustration: 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.
 
The streams of water arising both from the rainfall and from the melting
of the ice also produce a peculiar effect about the foot of an extensive
glacier. Sometimes these streams cut long, open channels near the end
of the glacier, and sweep into it vast quantities of morainic material,
which is pushed along by the torrential current, and, after being
abraded, rolled, and sorted, is deposited in a delta about its mouth, or
left stranded in long lines between the ice-walls which have determined
its course. At other times the stream has disappeared far back in the
glacier, and plunged into a crevasse (technically called a _moulin_),
whence it flows onwards as a subglacial stream. But in this case the
deposits might closely resemble those of the previous description. In
both cases, when the ice has finally melted away, peculiar ridge-like
deposits of sorted material remain, to mark the temporary line of
drainage. These exist abundantly in most regions which have been covered
with glacial ice, and are referred to in Scotland as _kames_, in Ireland
as _eskers_, and in Sweden as _osars_. In this volume we shall call them
_kames_, and the deltas spread out in front of them will be referred to
as _kame-plains_.
 
With this preliminary description of glacial phenomena, we will proceed
to give, first, a brief enumeration and description of the ice-fields
which are still existing in the world; second, the evidences of the
former existence of far more extensive ice-fields; and, third, the
relation of the Glacial period to some of the vicissitudes which have
attended the life of man in the world.
 
The geological period of which we shall treat is variously designated by
different writers. By some it is simply called the "post-Tertiary," or
"Quaternary"; by others the term "post-Pliocene" is used, to indicate
more sharply its distinction from the latter portion of the Tertiary
period; by others this nicety of distinction is expressed by the term
"Pleistocene." But, since the whole epoch was peculiarly characterised
by the presence of glaciers, which have not even yet wholly disappeared,
we may properly refer to it altogether under the descriptive name of
"Glacial" period.
 
 
 
 
CHAPTER II.
 
EXISTING GLACIERS.
 
 
_In Europe._--Our specific account of existing glaciers naturally begins
with those of the Alps, where Hugi, Charpentier, Agassiz, Forbes, and
Guyot, before the middle of this century, first brought clearly to light
the reality and nature of glacial motion.
 
According to Professor Heim, of Zürich, the total area covered by the
glaciers and ice-fields of the Alps is upwards of three thousand square
kilometres (about eleven hundred square miles). The Swiss Alps alone
contain nearly two-thirds of this area. Professor Heim enumerates 1,155
distinct glaciers in the region. Of these, 144 are in France, 78 in
Italy, 471 in Switzerland, and 462 in Austria.
 
Desor describes fourteen principal glacial districts in the Alps, the
westernmost of which is that of Mont Pelvoux, in Dauphiny, and the
easternmost that in the vicinity of the Gross Glockner, in Carinthia. The
most important of the Alpine systems are those which are grouped around
Mont Blanc, Monte Rosa, and the Finsteraarhorn, the two former peaks
being upwards of fifteen thousand feet in height, and the latter upwards
of fourteen thousand. The area covered by glaciers and snow-fields
in the Bernese Oberland, of which Finsteraarhorn is the culminating
point, is about three hundred and fifty square kilometres (a hundred
square miles), and contains the Aletsch Glacier, which is the longest
in Europe, extending twenty-one kilometres (about fourteen miles) from
the _névé_-field to its foot. The Mer de Glace, which descends from Mont
Blanc to the valley of Chamounix, has a length of about eight miles
below the _névé_-field. In all, there are estimated to be twenty-four
glaciers in the Alps which are upwards of four miles long, and six which
are upwards of eight miles in length. The principal of these are the Mer
de Glace, of Chamounix, on Mont Blanc; the Gorner Glacier, near Zermatt,
on Monte Rosa; the lower glacier of the Aar, in the Bernese Oberland;
and the Aletsch Glacier and Glacier of the Rhône, in Vallais; and the
Pasterzen, in Carinthia.
 
[Illustration: Fig. 7.--Mount Blanc Glacier Region: _m_, Mer de Glace;
_g_, Du Géant; _l_, Leschaux; _t_, Taléfre; _B_, Bionassay; _b_, Bosson.]
 
These glaciers adjust themselves to the width of the valleys down which
they flow, in some places being a mile or more in width, and at others
contracting into much narrower compass. The greatest depth which Agassiz
was able directly to measure in the Aar Glacier was two hundred and
sixty metres (five hundred and twenty-eight feet), but at another point
the depth was estimated by him to be four hundred and sixty metres (or
fifteen hundred and eighty-four feet).
 
The glaciers of the Alps are mostly confined to the northern side and
to the higher portions of the mountain-chain, none of them descending
below the level of four thousand feet, and all of them varying slightly
in extent, from year to year, according as there are changes in the
temperature and in the amount of snow-fall.
 
The Pyrenees, also, still maintain a glacial system, but it is of
insignificant importance. This is partly because the altitude is much
less than that of the Alps, the culminating point being scarcely more
than eleven thousand feet in height. Doubtless, also, it is partly due to
the narrowness of the range, which does not provide gathering-places for
the snow sufficiently extensive to produce large glaciers. The snow-fall
also is less upon the Pyrenees than upon the Alps. As a consequence of
all these conditions, the glaciers of the Pyrenees are scarcely more
than stationary _névé_-fields lingering upon the north side of the range.
The largest of these is near Bagnères de Luchon, and sends down a short,
river-like glacier.
 
In Scandinavia the height of the mountains is also much less than that of
the Alps, but the moister climate and the more northern latitude favours
the growth of glaciers at a much lower level North of the sixty-second
degree of latitude, the plateaus over five thousand feet above the sea
pretty generally are gathering-places for glaciers. From the Justedal a
snow-field, covering five hundred and eighty square miles, in latitude
62°, twenty-four glaciers push outwards towards the German Sea, the
largest of which is five miles long and three-quarters of a mile wide.
The Fondalen snow-field, between latitudes 66° and 67°, covers an
area about equal to that of the Justedal; but, on account of its more
northern position, its glaciers descend through the valleys quite to the
ocean-level. The Folgofon snow-field is still farther south, but, though
occupying an area of only one hundred square miles, it sends down as many
as three glaciers to the sea-level. The total area of the Scandinavian
snow-fields is about five thousand square miles.

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