Man and the Glacial Period 7

Man and the Glacial Period 7

This ice-cliff or barrier was followed by Captain Ross as far as 198°

west longitude, and found to preserve very much the same character during

the whole of that distance. On the lithographic view of this great

ice-sheet given in Ross's work it is described as "part of the South

Polar Barrier, one hundred and eighty feet above the sea-level, one

thousand feet thick, and four hundred and fifty miles in length."

 

A similar vertical wall of ice was seen by D'Urville, off the coast of

Adelie Land. He thus describes it: "Its appearance was astonishing. We

perceived a cliff having a uniform elevation of from one hundred to one

hundred and fifty feet, forming a long line extending off to the west....

Thus for more than twelve hours we had followed this wall of ice, and

found its sides everywhere perfectly vertical and its summit horizontal.

Not the smallest irregularity, not the most inconsiderable elevation,

broke its uniformity for the twenty leagues of distance which we followed

it during the day, although we passed it occasionally at a distance of

only two or three miles, so that we could make out with ease its smallest

irregularities. Some large pieces of ice were lying along the side of

this frozen coast; but, on the whole, there was open sea in the offing."

[AH]

 

[Footnote AH: Whitney's Climatic Changes, pp. 315, 316.]

 

[Illustration: Fig. 10.--Iceberg in the Antarctic Ocean.]

 

_North America._--In North America living glaciers begin to appear in

the Sierra Nevada Mountains, in the vicinity of the Yosemite Park, in

central California. Here the conditions necessary for the production

of glaciers are favourable, namely, a high altitude, snow-fields of

considerable extent, and unobstructed exposure to the moisture-laden

currents of air from the Pacific Ocean. Sixteen glaciers of small size

have been noted among the summits to the east of the Yosemite; but none

of them descend much below the eleven-thousand-foot line, and none of

them are over a mile in length. Indeed, they are so small, and their

motion is so slight, that it is a question whether or not they are to be

classed with true glaciers.

 

Owing to the comparatively low elevation of the Sierra Nevada north of

Tuolumne County, California, no other living glaciers are found until

reaching Mount Shasta, in the extreme northern part of the State. This

is a volcanic peak, rising fourteen thousand five hundred feet above the

sea, and having no peaks within forty miles of it as high as ten thousand

feet; yet so abundant is the snow-fall that as many as five glaciers are

found upon its northern side, some of them being as much as three miles

long and extending as low down as the eight-thousand-foot level. Upon the

southern side glaciers are so completely absent that Professor Whitney

ascended the mountain and remained in perfect ignorance of its glacial

system. In 1870 Mr. Clarence King first discovered and described them on

the northern side.

 

North of California glaciers characterise the Cascade Range in increasing

numbers all the way to the Alaskan Peninsula. They are to be found upon

Diamond Peak, the Three Sisters, Mount Jefferson, and Mount Hood, in

Oregon, and appear in still larger proportions upon the flanks of Mount

Rainier (or Tacoma) and Mount Baker, in the State of Washington. The

glacier at the head of the White River Valley is upon the north side of

Rainier, and is the largest one upon that mountain, reaching down to

within five thousand feet of the sea-level, and being ten miles or more

in length. All the streams which descend the valleys upon this mountain

are charged with the milky-coloured water which betrays their glacial

origin.

 

[Illustration: Fig. 11.--Map of Southeastern Alaska. The arrow-points

mark glaciers.]

 

In British Columbia, Glacier Station, upon the Canadian Pacific Railroad,

in the Selkirk Mountains, is within half a mile of the handsome

Illicilliwaet Glacier, while others of larger size are found at no great

distance. The interior farther north is unexplored to so great an extent

that little can be definitely said concerning its glacial phenomena. The

coast of British Columbia is penetrated by numerous fiords, each of which

receives the drainage of a decaying glacier; but none are in sight of the

tourist-steamers which thread their way through the intricate network of

channels characterising this coast, until the Alaskan boundary is crossed

and the mouth of the Stickeen River is passed.

 

A few miles up from the mouth of the Stickeen, however, glaciers of

large size come down to the vicinity of the river, both from the north

and from the south, and the attention of tourists is always attracted

by the abundant glacial sediment borne into the tide-water by the river

itself and discolouring the surface for a long distance beyond the

outlet. Northward from this point the tourist is rarely out of sight of

ice-fields. The Auk and Patterson Glaciers are the first to come into

view, but they do not descend to the water-level. On nearing Holcomb

Bay, however, small icebergs begin to appear, heralding the first of the

glaciers which descend beyond the water's edge. Taku Inlet, a little

farther north, presents glaciers of great size coming down to the

sea-level, while the whole length of Lynn Canal, from Juneau to Chilkat,

a distance of eighty miles, is dotted on both sides by conspicuous

glaciers and ice-fields.

 

The Davidson Glacier, near the head of the canal, is one of the most

interesting for purposes of study. It comes down from an unknown

distance in the western interior, bearing two marked medial moraines upon

its surface. On nearing tide-level, the valley through which it flows is

about three-quarters of a mile in width; but, after emerging from the

confinement of the valley, the ice spreads out over a fan-shaped area

until the width of its front is nearly three miles. The supply of ice

not being sufficient to push the front of the glacier into deep water,

equilibrium between the forces of heat and cold is established near the

water's edge. Here, as from year to year the ice melts and deposits its

burdens of earthy _débris_, it has piled up a terminal moraine which

rises from two hundred to three hundred feet in height, and is now

covered with evergreen trees of considerable size. From Chilkat, at the

head of Lynn Canal, to the sources of the Yukon River, the distance is

only thirty-five miles, but the intervening mountain-chain is several

thousand feet in height and bears numerous glaciers upon its seaward side.

 

About forty miles west of Lynn Canal, and separated from it by a range

of mountains of moderate height, is Glacier Bay, at the head of one of

whose inlets is the Muir Glacier, which forms the chief attraction for

the great number of tourists that now visit the coast of southeastern

Alaska during the summer season. This glacier meets tide-water in

latitude 58° 50', and longitude 136° 40' west of Greenwich. It received

its name from Mr. John Muir, who, in company with Rev. Mr. Young, made

a tour of the bay and discovered the glacier in 1879. It was soon found

that the bay could be safely navigated by vessels of large size, and from

that time on tourists in increasing number have been attracted to the

region. Commodious steamers now regularly run close up to the ice-front,

and lie-to for several hours, so that the passengers may witness the

"calving" of icebergs, and may climb upon the sides of the icy stream

and look into its deep crevasses and out upon its corrugated and broken

surface.

 

[Illustration: Fig. 12.--Map of Glacier Bay. Alaska, and its

surroundings. Arrow-points indicate glaciated area.]

 

The first persons who found it in their way to pay more than a tourist's

visit to this interesting object were Rev. J. L. Patton, Mr. Prentiss

Baldwin, and myself, who spent the entire month of August, 1886, encamped

at the foot of the glacier, conducting such observations upon it as

weather and equipment permitted. From that time till the summer of 1890

no one else stopped off from the tourist steamers to bestow any special

study upon it. But during this latter season Mr. Muir returned to the

scene of his discovered wonder, and spent some weeks in exploring the

interior of the great ice-field. During the same season, also, Professors

H. F. Reid and H. Cushing, with a well-equipped party of young men,

spent two months or more in the same field, conducting observations and

experiments, of various kinds, relating to the extent, the motion, and

the general behaviour of the vast mass of moving ice.

 

[Illustration: Fig. 13.--Shows central part of the front of Muir Glacier

one half mile distant. Near the lower left hand corner the ice is seen

one mile distant resting for about one half mile on gravel which it had

overrun. The ice is now retreating in the channel. (From photograph.)]

 

The main body of the Muir Glacier occupies a vast amphitheatre, with

diameters ranging from thirty to forty miles, and covers an area of about

one thousand square miles. From one of the low mountains near its mouth I

could count twenty-six tributary glaciers which came together and became

confluent in the main stream of ice. Nine medial moraines marked the

continued course of as many main branches, which becoming united formed

the grand trunk of the glacier. Numerous rocky eminences also projected

above the surface of the ice, like islands in the sea, corresponding to

what are called "_nunataks_" in Greenland. The force of the ice against

the upper side of these rocky prominences is such as to push it in great

masses above the surrounding level, after the analogy of waves which

dash themselves into foam against similar obstructions. In front of the

_nunataks_ there is uniformly a depression, like the eddies which appear

in the current below obstacles in running water.

 

Over some portions of the surface of the glacier there is a miniature

river system, consisting of a main stream, with numerous tributaries,

but all flowing in channels of deep blue ice. Before reaching the front

of the glacier, however, each one of these plunges down into a crevasse,

or _moulin_, to swell the larger current, which may be heard rushing

along in an impetuous course hundreds of feet beneath, and far out of

sight. The portion of the glacier in which there is the most rapid

motion is characterised by innumerable crags and domes and pinnacles of

ice, projecting above the general level, whose bases are separated by

fissures, extending in many cases more than a hundred feet below the

general level. These irregularities result from the combined effect

of the differential motion (as illustrated in the diagram on page 4),

and the influence of sunshine and warm air in irregularly melting the

unprotected masses. The description given in our introductory chapter of

medial moraines and ice-pillars is amply illustrated everywhere upon the

surface of the Muir Glacier. I measured one block of stone which was

twenty feet square and about the same height, standing on a pedestal of

ice three or four feet high.

 

The mountains forming the periphery of this amphitheatre rise to a height

of several thousand feet; Mount Fairweather, upon the northwest, from

whose flanks probably a portion of the ice comes, being, in fact, more

than fifteen thousand feet high. The mouth of the amphitheatre is three

miles wide, in a line extending from shoulder to shoulder of the low

mountains which guard it. The actual water-front where the ice meets

tide-water is one mile and a half.[AI] Here the depth of the inlet is so

great that the front of the ice breaks off in icebergs of large size,

which float away to be dissolved at their leisure. At the water's edge

the ice presents a perpendicular front of from two hundred and fifty to

four hundred feet in height, and the depth of the water in the middle of

the inlet immediately in front of the ice is upwards of seven hundred

feet; thus giving a total height to the precipitous front of a thousand

feet.

 

[Footnote AI: These are the measurements of Professor Reid. In my former

volume I have given the dimensions as somewhat smaller.]

 

The formation of icebergs can here be studied to admirable advantage.

During the month in which we encamped in the vicinity the process

was going on continuously. There was scarcely an interval of fifteen

minutes during the whole time in which the air was not rent with the

significant boom connected with the "calving" of a berg. Sometimes this

was occasioned by the separation of a comparatively small mass of ice

from near the top of the precipitous wall, which would fall into the

water below with a loud splash. At other times I have seen a column of

ice from top to bottom of the precipice split off and fall over into the

water, giving rise to great waves, which would lash the shore with foam two miles below.