Man and the Glacial Period 8

Man and the Glacial Period 8

This manner of the production of icebergs differs from that which has

been ordinarily represented in the text-books, but it conforms to the law

of glacial motion, which we will describe a little later, namely, that

the upper strata of ice move faster than the lower. Hence the tendency

is constantly to push the upper strata forwards, so as to produce

a perpendicular or even projecting front, after the analogy of the

formation of breakers on the shelving shore of a large body of water.

 

Evidently, however, these masses of ice which break off from above the

water do not reach the whole distance to the bottom of the glacier below

the water; so that a projecting foot of ice remains extending to an

indefinite distance underneath the surface. But at occasional intervals,

as the superincumbent masses of ice above the surface fall off and

relieve the strata below of their weight, these submerged masses suddenly

rise, often shooting up considerably higher than they ultimately remain

when coming to rest. The bergs formed by this latter process often bear

much earthy material upon them, which is carried away with the floating

ice, to be deposited finally wherever the melting chances to take place.

 

Numerous opportunities are furnished about the front and foot of

this vast glacier to observe the manner of the formation of _kames_,

kettle-holes, and various other irregular forms into which glacial

_débris_ is accustomed to accumulate. Over portions of the decaying

foot of the glacier, which was deeply covered with morainic _débris_,

the supporting ice is being gradually removed through the influence of

subglacial streams or of abandoned tunnels, which permit the air to exert

its melting power underneath. In some places where old _moulins_ had

existed, the supporting ice is melting away, so that the superincumbent

mass of sand, gravel, and boulders is slowly sliding into a common

centre, like grain in a hopper. This must produce a conical hill, to

remain, after the ice has all melted away, a mute witness of the

impressive and complicated forces which have been so long in operation

for its production.

 

In other places I have witnessed the formation of a long ridge of gravel

by the gradual falling in of the roof of a tunnel which had been occupied

by a subglacial stream, and over which there was deposited a great amount

of morainic material. As the roof gave way, this was constantly falling

to the bottom, where, being exempt from further erosive agencies, it must

remain as a gravel ridge or kame.

 

In other places, still, there were vast masses of ice covering many

acres, and buried beneath a great depth of morainic material which had

been swept down upon it while joined to the main glacier. In the retreat

of the ice, however, these masses had become isolated, and the sand,

gravel, and boulders were sliding down the wasting sides and forming long

ridges of _débris_ along the bottom, which, upon the final melting of

the ice, will be left as a complicated network of ridges and knolls of

gravel, enclosing an equally complicated nest of kettle-holes.

 

Beyond Cross Sound the Pacific coast is bounded for several hundred

miles by the magnificent semicircle of mountains known as the St. Elias

Alps, with Mount Crillon at the south, having an elevation of nearly

sixteen thousand feet, and St. Elias in the centre, rising to a greater

height. Everywhere along this coast, as far as the Alaskan Peninsula,

vast glaciers come down from the mountain-sides, and in many cases their

precipitous fronts form the shore-line for many miles at a time. Icy

Bay, just to the south of Mount St, Elias, is fitly named, on account of

the extent of the glaciers emptying into it and the number of icebergs

cumbering its waters.

 

In the summer of 1890 a party, under the lead of Mr. I. C. Russell, of

the United States Geological Survey, made an unsuccessful attempt to

scale the heights of Mount St. Elias; but the information brought back

by them concerning the glaciers of the region amply repaid them for their

toil and expense, and consoled them for the failure of their immediate

object.

 

[Illustration: Fig. 14.--By the courtesy of the National Geographical

Society.]

 

Leaving Yakutat Bay, and following the route indicated upon the

accompanying map, they travelled on glacial ice almost the entire

distance to the foot of Mount St. Elias. The numerous glaciers coming

down from the summit of the mountain-ridge become confluent nearer the

shore, and spread out over an area of about a thousand square miles. This

is fitly named the Malaspina Glacier, after the Spanish explorer who

discovered it in 1792.

 

It is not necessary to add further particulars concerning the results

of this expedition, since they are so similar to those already detailed

in connection with the Muir Glacier. A feature, however, of special

interest, pertains to the glacial lakes which are held in place by the

glacial ice at an elevation of thousands of feet above the sea. One of

considerable size is indicated upon the map just south of what was called

Blossom Island, which, however, is not an island, but simply a _nunatak_,

the ice here surrounding a considerable area of fertile land, which is

covered with dense forests and beautified by a brilliant assemblage of

flowering plants. In other places considerable vegetation was found upon

the surface of moraines, which were probably still in motion with the

underlying ice.

 

_Greenland._--The continental proportions of Greenland, and the extent

to which its area is covered by glacial ice, make it by far the most

important accessible field for glacial observations. The total area of

Greenland can not be less than five hundred thousand square miles--equal

in extent to the portion of the United States east of the Mississippi

and north of the Ohio. It is now pretty evident that the whole of this

area, except a narrow border about the southern end, is covered by one

continuous sheet of moving ice, pressing outward on every side towards

the open water of the surrounding seas.

 

For a long time it was the belief of many that a large region in the

interior of Greenland was free from ice, and was perhaps inhabited.

It was in part to solve this problem that Baron Nordenskiöld set out

upon his expedition of 1883. Ascending the ice-sheet from Disco Bay, in

latitude 69°, he proceeded eastward for eighteen days across a continuous

ice-field. Rivers were flowing in channels upon the surface like those

cut on land in horizontal strata of shale or sandstone, only that the

pure deep blue of the ice-walls was, by comparison, infinitely more

beautiful. These rivers were not, however, perfectly continuous. After

flowing for a distance in channels on the surface, they, one and all,

plunged with deafening roar into some yawning crevasse, to find their way

to the sea through subglacial channels. Numerous lakes with shores of ice

were also encountered.

 

[Illustration: Fig. 15.--Map of Greenland. The arrow-points mark the

margin of the ice-field.]

 

"On bending down the ear to the ice," says this explorer, "we could hear

on every side a peculiar subterranean hum, proceeding from rivers flowing

within the ice; and occasionally a loud, single report, like that of a

cannon, gave notice of the formation of a new glacier-cleft.... In the

afternoon we saw at some distance from us a well-defined pillar of mist,

which, when we approached it, appeared to rise from a bottomless abyss,

into which a mighty glacier-river fell. The vast, roaring water-mass had

bored for itself a vertical hole, probably down to the rock, certainly

more than two thousand feet beneath, on which the glacier rested."[AJ]

 

[Footnote AJ: Geological Magazine, vol. ix, pp. 393, 399.]

 

At the end of the eighteen days Nordenskiöld found himself about a

hundred and fifty miles from his starting-point, and about five thousand

feet above the sea. Here the party rested, and sent two Eskimos forward

on _skidor_--a kind of long wooden skate, with which they could move

rapidly over the ice, notwithstanding the numerous small, circular holes

which everywhere pitted the surface. These Eskimos were gone fifty-seven

hours, having slept only four hours of the period. It is estimated that

they made about a hundred and fifty miles, and attained an altitude of

six thousand feet. The ice is reported as rising in distinct terraces,

and as seemingly boundless beyond. If this is the case, two hundred miles

from Disco Bay, there would seem little hope of finding in Greenland

an interior freed from ice. So we may pretty confidently speak of that

continental body of land as still enveloped in an ice-sheet. Up to about

latitude 75°, however, the continent is fringed by a border of islands,

over which there is no continuous covering of ice. In south Greenland

the continuous ice-sheet is reached about thirty miles back from the

shore.

 

A summary of the results of Greenland exploration was given by Dr. Kink

in 1886, from which it appears that since 1876 one thousand miles of

the coast-line have been carefully explored by entering every fiord and

attempting to reach the inland ice. According to this authority--

 

We are now able to demonstrate that a movement of ice from the central

regions of Greenland to the coast continually goes on, and must be

supposed to act upon the ground over which it is pushed so as to detach

and transport fragments of it for such a distance.... The plainest idea

of the ice-formation here in question is given by comparing it with an

inundation.... Only the marginal parts show irregularity; towards the

interior the surface grows more and more level and passes into a plain

very slightly rising in the same direction. It has been proved that,

ascending its extreme verge, where it has spread like a lava-stream over

the lower ground in front of it, the irregularities are chiefly met with

up to a height of 2,000 feet, but the distance from the margin in which

the height is reached varies much. While under 68-1/2° north latitude it

took twenty-four miles before this elevation was attained, in 72-1/2° the

same height was arrived at in half the distance....

 

A general movement of the whole mass from the central regions towards the

sea is still continued, but it concentrates its force to comparatively

few points in the most extraordinary degree. These points are represented

by the ice-fiords, through which the annual surplus ice is carried off

in the shape of bergs.... In Danish Greenland are found five of the

first, four of the second, and eight of the third (or least productive)

class, besides a number of inlets which only receive insignificant

fragments. Direct measurements of the velocity have now been applied on

three first-rate and one second-rate fiords, all situated between 69°

and 71° north latitude. The measurements have been repeated during the

coldest and the warmest season, and connected with surveying and other

investigations of the inlets and their environs. It is now proved that

the glacier branches which produce the bergs proceed incessantly at a

rate of thirty to fifty feet per diem, this movement being not at all

influenced by the seasons. . . .