Man and the Glacial Period 10

Man and the Glacial Period 10

But in August, 1841, the distinguished Swiss investigator had invited

Professor J. D. Forbes, of Edinburgh, to interest himself in solving the

problem of glacial motion. In response to this request, Professor Forbes

spent three weeks with Agassiz upon the Aar Glacier. Stimulated by the

interest of this visit, Forbes returned to Switzerland in 1842 and began

a series of independent investigations upon the Mer de Glace. After a

week's observations with accurate instruments, Forbes wrote to Professor

Jameson, editor of the Edinburgh New Philosophical Journal, that he had

already made it certain that "the central part of the glacier moves

faster than the edges in a very considerable proportion, quite contrary

to the opinion generally maintained." This letter was dated July 4, 1842,

but was not published until the October following, Agassiz's results, so

far as then determined, were, however, published in Comptes Rendus of

the 29th of August, 1842, two months before the publication of Forbes's

letter. But Agassiz's letter was dated twenty-seven days later than that

of Forbes. It becomes certain, therefore, that both Agassiz and Forbes,

independently and about the same time, discovered the fact that the

central portion of a glacier moves more rapidly than the sides.

 

In 1857 Professor Tyndall began his systematic and fruitful observations

upon the Mer de Glace and other Alpine glaciers. Professor Forbes had

already demonstrated that, with an accurate instrument of observation,

the motion of a line of stakes might be observed after the lapse

of a single clay, or even of a few hours. As a result of Tyndall's

observations, it was found that the most rapid daily motion in the Mer de

Glace in 1857 was about thirty-seven inches. This amount of motion was

near the lower end of the glacier On ascending the glacier, the rate was

found in general to be diminished; but the diminution was not uniform

throughout the whole distance, being affected both by the size and by the

contour of the valley. The motion in the tributary glaciers was also much

less than that of the main glacier.

 

This diminution of movement in the tributary glaciers was somewhat

proportionate to their increase in width. For example, the combined

width of the three tributaries uniting to form the Mer de Glace is 2,597

yards; but a short distance below the junction of these tributaries the

total width of the Mer de Glace itself is only 893 yards, or one-third

that of the tributaries combined. Yet, though the depth of the ice is

probably here much greater than in the tributaries, the rapidity of

movement is between two and three times as great as that of any one of

the branches.[AM]

 

[Footnote AM: See Tyndall's Forms of Water, pp. 78-82.]

 

From Tyndall's observations it appears also that the line of most rapid

motion is not exactly in the middle of the channel, but is pushed by its

own momentum from one side to the other of the middle, so as always to be

nearer the concave side; in this respect conforming, as far as its nature

will permit, to the motion of water in a tortuous channel.

 

[Illustration: Fig. 17.]

 

It is easy to account for this differential motion upon the surface

of a glacier, since it is clear that the friction of the sides of the

channel must retard the motion of ice as it does that of water. It is

clear also that the friction of the bottom must retard the motion of ice

even more than it is known to do in the case of water. In the formation

of breakers, when the waves roll in upon a shallowing beach, every one

is familiar with the effect of the bottom upon the moving mass. Here

friction retards the lower strata of water, and the upper strata slide

over the lower, and, where the water is of sufficient depth and the

motion is sufficiently great, the crest breaks down in foam before the

ever-advancing tide. A similar phenomenon occurs when dams give way and

reservoirs suddenly pour their contents into the restricted channels

below. At such times the advancing water rolls onwards like the surf with

a perpendicular front, varying in height according to the extent of the

flood.

 

Seasoning from these phenomena connected with moving water, it was

naturally suggested to Professor Tyndall that an analogous movement must

take place in a glacier. Choosing, therefore, a favourable place for

observation on the Mer de Glace where the ice emerged from a gorge, he

found a perpendicular side about one hundred and fifty feet in height

from bottom to top. In this face he drove stakes in a perpendicular line

from top to bottom. Upon subsequently observing them, Tyndall found, as

he expected, that there was a differential motion among them as in the

stakes upon the surface. The retarding effect of friction upon the bottom

was evident. The stake near the top moved forwards about three times as

fast as the one which was only four feet from the bottom.

 

[Illustration: Fig. 18.]

 

The most rapid motion (thirty-seven inches per day) observed by Professor

Tyndall upon the Alpine glaciers occurred in midsummer. In winter

the rate was only about one-half as great; but in the year 1875 the

Norwegian geologist, Helland, reported a movement of twenty metres (about

sixty-five feet) per day in the Jakobshavn Glacier which enters Disco

Bay, Greenland, about latitude 70°. For some time there was a disposition

on the part of many scientific men to doubt the correctness of Holland's

calculations. Subsequent observations have shown, however, that from the

comparatively insignificant glaciers of the Alps they were not justified

in drawing inferences with respect to the motion of the vastly larger

masses which come down to the sea through the fiords of Greenland.

The Jakobshavn Glacier was about two and a half miles in width and its

depth very likely more than a thousand feet, making a cross-section of

more than 1,400,000 square yards, whereas the cross-section of the Mer

de Glace at Montanvert is estimated to be but 190,000 square yards or

only about one-seventh the above estimate for the Greenland glacier. As

the friction of the sides would be no greater upon a large stream than

upon a small one, while upon the bottom it would be only in proportion

to the area, it is evident that we cannot tell beforehand how rapidly

an increase in the volume of the ice might augment the velocity of the

glacier.

 

At any rate, all reasonable grounds for distrusting the accuracy of

Helland's estimates seem to have been removed by later investigations.

According to my own observations in the summer of 1886 upon the Muir

Glacier, Alaska, the central portions, a mile back from the front of

that vast ice-current, were moving from sixty-five to seventy feet per

day. These observations were taken with a sextant upon pinnacles of ice

recognizable from a baseline established upon the shore. It is fair

to add, however, that during the summer of 1890 Professor H. F. Reid

attempted to measure the motion of the same glacier by methods promising

greater accuracy than could be obtained by mine. He endeavoured to plant,

after the method of Tyndall, a line of stakes across the ice-current. But

with his utmost efforts, working inwards from both sides, he was unable

to accomplish his purpose, and so left unmeasured a quarter of a mile

or more of the most rapidly-moving portion of the glacier. His results,

therefore, of ten feet per day in the most rapidly-moving portion

observed cannot discredit my own observations on a portion of the stream

inaccessible by his method. A quarter of a mile in width near the centre

of so vast a glacier gives ample opportunity for a much greater rate of

motion than that observed by Professor Reid. Especially may this be true

in view of Tyndall's suggestion that the contour of the bottom over which

the ice flows may greatly affect the rate in certain places. A sudden

deepening of the channel may affect the motion of ice in a glacier as

much as it does that of water in a river.

 

Other observations also amply sustain the conclusions of Helland. As

already stated, the Danish surveying party under Steenstrup, after

several years' work upon the southwestern coast of Greenland, have

ascertained that the numerous glaciers coming down to the sea in that

region and furnishing the icebergs incessantly floating down Baffin's

Bay, move at a rate of from thirty to fifty feet per day, while

Lieutenants Ryder and Bloch, of the Danish Navy, who spent the year 1887

in exploring the coast in the vicinity of Upernavik, about latitude 73°,

found that the great glacier entering the fiord east of the village had a

velocity of ninety-nine feet per day during the month of August.[AN]

 

[Footnote AN: Nature, December 29, 1887.]

 

It is easier to establish the fact of glacial motion than to explain how

the motion takes place, for ice seems to be as brittle as glass. This,

however, is true of it only when compelled suddenly to change its form.

When subjected to slow and long-continued pressure it gradually yet

readily yields, and takes on new forms. From this capacity of ice, it has

come to be regarded by some as a really viscous substance, like tar or

cooling lava, and upon that theory Professor Forbes endeavours to explain

all glacial movement.

 

The theory, however, seems to be contradicted by familiar facts; for

the iceman, after sawing a shallow groove across a piece of ice, can

then split it as easily as he would a piece of sandstone or wood. On the

glaciers themselves, likewise, the existence of innumerable crevasses

would seem to contradict the plastic theory of glacier motion; for,

wherever the slope of the glacier's bed increases, crevasses are formed

by the increased strain to which the ice is subjected. Crevasses are also

formed in rapidly-moving glaciers by the slight strain occasioned by the

more rapid motion of the middle portion. Still, in the words of Tyndall,

"it is undoubted that the glacier moves like a viscous body. The centre

flows past the sides, the top flows over the bottom, and the motion

through a curved valley corresponds to fluid motion."[AO]

 

[Footnote AO: Forms of Water, p. 163.]

 

To explain this combination of the seemingly contradictory qualities of

brittleness and viscosity in ice, physicists have directed attention

to the remarkable transformations which take place in water at the

freezing-point. Faraday discovered in 1850 that "when two pieces of

thawing ice are placed together they freeze together at the point of

contact.[AP]

 

[Footnote AP: Ibid., p. 164.]

 

"Place a number of fragments of ice in a basin of water and cause them

to touch each other; they freeze together where they touch. You can form

a chain of such fragments; and then, by taking hold of one end of the

chain, you can draw the whole series after it. Chains of icebergs are

sometimes formed in this way in the arctic seas."[AQ]

 

[Footnote AQ: Ibid., pp. 164, 165.]

 

This is really what takes place when a hard snow-ball is made by pressure

in the hand. So, by subjecting fragments of ice to pressure it is first

crumbled to powder, and then, as the particles are pressed together in

close contact, it resumes the nature of ice again, though in a different

form, taking now the shape of the mould in which it has been pressed.

 

Thus it is supposed that, when the temperature of ice is near the

melting-point, the pressure of the superincumbent mass may produce at

certain points insensible disintegration, while, upon the removal of

the pressure by change of position, regulation instantly takes place,

and thus the phenomena which simulate plasticity are produced. As the

freezing-point of water is, within a narrow range, determined by the

amount of pressure to which it is subjected, it is not difficult to see

how these changes may occur. Pressure slightly lowers the freezing-point,

and so would liquefy the portions of ice subjected to greatest pressure,

wherever that might be in the mass of the glacier, and thus permit

a momentary movement of the particles, until they should recongeal

in adjusting themselves to spaces of less pressure.[AR] This is the

theory by which Professor James Thompson would account for the apparent plasticity of glacial ice.