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.