CHAPTER VI.

GLACIERS.

We have already noted the fact that the water in the clouds is very

commonly in the frozen state; a large part of that fluid which is

evaporated from the sea attains the solid form before it returns to

the earth. Nevertheless, in descending, at least nine tenths of the

precipitation returns to the fluid state, and does the kind of work

which we have noted in our account of water. Where, however, the water

arrives on the earth in the frozen condition, it enters on a rôle

totally different from that followed by the fluid material.

Beginning its descent to the earth in a snowflake, the little mass

falls slowly, so that when it comes against the earth the blow which

it strikes is so slight that it does no effective work. In the state

of snow, even in the separate flakes, the frozen water contains a

relatively large amount of air. It is this air indeed, which, by

dividing the ice into many flakes that reflect the light, gives it the

white colour. This important point can be demonstrated by breaking

transparent ice into small bits, when we perceive that it has the hue

of snow. Much the same effect is given where glass is powdered, and

for the same reason.

As the snowflakes accumulate layer on layer they imbed air between

them, so that when the material falls in a feathery shape--say to the

depth of a foot--more than nine tenths of the mass is taken up by the

air-containing spaces. As these cells are very small, the circulation

in them is slight, and so the layer becomes an admirable

non-conductor, having this quality for the same reason that feathers

have it--i.e., because the cells are small enough to prevent the

circulation of the air, so that the heat which passes has to go by

conduction, and all gases are very poor conductors. The result is that

a snow coating is in effect an admirable blanket. When the sun shines

upon it, much of the heat is reflected, and as the temperature does

not penetrate it to any depth, only the superficial part is melted.

This molten water takes up in the process of melting a great deal of

heat, so that when it trickles down into the mass it readily

refreezes. On the other hand, the heat going out from the earth, the

store accumulated in its superficial parts in the last warm season,

together with the small share which flows out from the earth's

interior, is held in by this blanket, which it melts but slowly. Thus

it comes about that in regions of long-enduring snowfall the ground,

though frozen to the depth of a foot or more at the time when the

accumulation took place, may be thawed out and so far warmed that the

vegetation begins to grow before the protecting envelope of snow has

melted away. Certain of the early flowers of high latitudes, indeed,

begin to blossom beneath the mantle of finely divided ice.

In those parts of the earth which for the most part receive only a

temporary coating of snow the effect of this covering is

inconsiderable. The snow water is yielded to the earth, from which it

has helped to withdraw the frost, so that in the springtime, the

growing season of plants, the ground contains an ample store of

moisture for their development. Where the snowfall accumulates to a

great thickness, especially where it lodges in forests, the influence

of the icy covering is somewhat to protract the winter and thus to

abbreviate the growing season.

Where snow rests upon a steep slope, and gathers to the depth of

several feet, it begins to creep slowly down the declivity in a manner

which we may often note on house roofs. This motion is favoured by the

gradual though incomplete melting of the flakes as the heat

penetrates the mass. Making a section through a mass of snow which has

accumulated in many successive falls, we note that the top may still

have the flaky character, but that as we go down the flakes are

replaced by adherent shotlike bodies, which have arisen from the

partial melting and gathering to their centres of the original

expanded crystalline bits. In this process of change the mass can move

particle by particle in the direction in which gravity impels it. The

energy of its motion, however, is slight, yet it can urge loose stones

and forest waste down hill. Sometimes, as in the cemetery at Augusta,

Me., where stone monuments or other structures, such as iron railings,

are entangled in the moving mass, it may break them off and convey

them a little distance down the slope.

So long as the summer sun melts the winter's snow, even if the ground

be bare but for a day, the rôle of action accomplished by the snowfall

is of little geological consequence. When it happens that a portion of

the deposit holds through the summer, the region enters on the glacial

state, and its conditions undergo a great revolution, the consequences

of which are so momentous that we shall have to trace them in some

detail. Fortunately, the considerations which are necessary are not

recondite, and all the facts are of an extremely picturesque nature.

Taking such a region as New England, where all the earth is

life-bearing in the summer season, and where the glacial period of the

winter continues but for a short time, we find that here and there on

the high mountains the snow endures throughout most of the summer, but

that all parts of the surface have a season when life springs into

activity. On the top of Mount Washington, in the White Mountains of

New Hampshire, in a cleft known as Tuckerman's Ravine, where the

deposit accumulates to a great depth, the snow-ice remains until

midsummer. It is, indeed, evident that a very slight change in the

climatal conditions of this locality would establish a permanent

accumulation of frozen water upon the summit of the mountain. If the

crest were lifted a thousand feet higher, without any general change

in the heat or rainfall of the district, this effect would be

produced. If with the same amount of rainfall as now comes to the

earth in that region more of it fell as snow, a like condition would

be established. Furthermore, with an increase of rainfall to something

like double that which now descends the snow bore the same proportion

to the precipitation which it does at present, we should almost

certainly have the peak above the permanent snow line, that level

below which all the winter's fall melts away. These propositions are

stated with some care, for the reason that the student should perceive

how delicate may be--indeed, commonly is--the balance of forces which

make the difference between a seasonal and a perennial snow covering.

As soon as the snow outlasts the summer, the region which it occupies

is sterilized to life. From the time the snow begins to hold over the

warm period until it finally disappears, that field has to be reckoned

out of the habitable earth, not only to man, but to the lowliest

organisms.[6]

[Footnote 6: In certain fields of permanent snow, particularly near their

boundaries, some very lowly forms of vegetable life may develop on a

frozen surface, drawing their sustenance from the air, and supplied with

water by the melting which takes place during the summertime. These

forms include the rare phenomenon termed red snow.]

If the snow in a glaciated region lay where it fell, the result would

be a constant elevation of the deposit year by year in proportion to

the annual excess of deposition over the melting or evaporation of the

material. But no sooner does the deposit attain any considerable

thickness than it begins to move in the directions of least

resistance, in accordance with laws which the students of glaciers are

just beginning to discern. In small part this motion is accomplished

by avalanches or snow slides, phenomena which are in a way important,

and therefore merit description. Immediately after a heavy snowfall,

in regions where the slopes are steep, it often happens that the

deposit which at first clung to the surface on which it lay becomes so

heavy that it tends to slide down the slope; a trifling action, the

slipping, indeed, of a single flake, may begin the movement, which at

first is gradual and only involves a little of the snow. Gathering

velocity, and with the materials heaped together from the junction of

that already in motion with that about to be moved, the avalanche in

sliding a few hundred feet down the slope may become a deep stream of

snow-ice, moving with great celerity. At this stage it begins to break

off masses of ice from the glaciers over which it may flow, or even to

move large stones. Armed with these, it rends the underlying earth.

After it has flowed a mile it may have taken up so much earth and

material that it appears like a river of mud. Owing to the fact that

the energy which bears it downward is through friction converted into

heat, a partial melting of the mass may take place, which converts it

into what we call slush, or a mixture of snow and water. Finally, the

torrent is precipitated into the bottom of a valley, where in time the

frozen water melts away, leaving only the stony matter which it bore

as a monument to show the termination of its flow.

It was the good fortune of the writer to see in the Swiss Oberland one

very great avalanche, which came from the high country through a

descent of several thousand feet to the surface of the Upper

Grindelwald Glacier. The first sign of the action was a vague tremor

of the air, like that of a great organ pipe when it begins to vibrate,

but before the pulsations come swiftly enough to make an audible note.

It was impossible to tell when this tremor came, but the wary guide,

noting it before his charge could perceive anything unusual, made

haste for the middle of the glacier. The vibration swelled to a roar,

but the seat of the sound amid the echoing cliffs was indeterminable.

Finally, from a valley high up on the southern face of the glacier,

there leaped forth first a great stone, which sprang with successive

rebounds to the floor of ice. Then in succession other stones and

masses of ice which had outrun the flood came thicker and thicker,

until at the end of about thirty seconds the steep front of the

avalanche appeared like a swift-moving wall. Attaining the cliffs, it

shot forth as a great cataract, which during the continuance of the

flow--which lasted for several minutes--heaped a great mound of

commingled stones and ice upon the surface of the glacier. The mass

thus brought down the steep was estimated at about three thousand

cubic yards, of which probably the fiftieth part was rock material. An

avalanche of this volume is unusual, and the proportion of stony

matter borne down exceptionally great; but by these sudden motions of

the frozen water a large part of the snow deposited above the zone of

complete melting is taken to the lower valleys, where it may disappear

in the summer season, and much of the erosion accomplished in the

mountains is brought about by these falls.

In all Alpine regions avalanches are among the most dreaded accidents.

Their occurrence, however, being dependent upon the shape of the

surface, it is generally possible to determine in an accurate way the

liability of their happening in any particular field. The Swiss take

precaution to protect themselves from their ravages as other folk do

to procure immunity from floods. Thus the authorities of many of the

mountain hamlets maintain extensive forests on the sides of the

villages whence the downfall may be expected, experience having shown

that there is no other means so well calculated to break the blow

which these great snowfalls can deliver, as thick-set trees which,

though they are broken down for some distance, gradually arrest the

stream.

As long as the region occupied by permanent snow is limited to sharp

mountain peaks, relief by the precipitation of large masses to the

level below the snow line is easily accomplished, but manifestly this

kind of a discharge can only be effective from a very small field.

Where the relief is not brought about by these tumbles of snow,

another mode of gravitative action accomplishes the result, though in

a more roundabout way, through the mechanism of glaciers.

We have already noted the fact that the winter's snow upon our

hillsides undergoes a movement in the direction of the slope. What we

have now to describe in a rather long story concerning glaciers rests

upon movements of the same nature, though they are in certain features

peculiarly dependent on the continuity of the action from year to

year. It is desirable, however, that the student should see that there

is at the foundation no more mystery in glacial motion than there is

in the gradual descent of the snow after it has lain a week on a

hillside. It is only in the scale and continuity of the action that

the greatest glacial envelope exceeds those of our temporary

winters--in fact, whenever the snow falls the earth it covers enters

upon an ice period which differs only in degree from that from which

our hemisphere is just escaping.

Where the reader is so fortunate as to be able to visit a region of

glaciers, he had best begin his study of their majestic phenomena by

ascending to those upper realms where the snow accumulates from year

to year. He will there find the natural irregularities of the rock

surface in a measure evened over by a vast sheet of snow, from which

only the summits of the greater mountains rise. He may soon satisfy

himself that this sheet is of great depth, for here and there it is

intersected by profound crevices. If the visit is made in the season

when snow falls, which is commonly during most of the year, he may

observe, as before noted in our winter's snow, that the deposit,

though at first flaky, attains at a short distance below the surface a

somewhat granular character, though the shotlike grains fall apart

when disturbed. Yet deeper, ordinarily a few feet below the surface,

these granules are more or less cemented together; the mass thus loses

the quality of snow, and begins to appear like a whitish ice. Looking

down one of the crevices, where the light penetrates to the depth of a

hundred feet or more, he may see that the bluish hue somewhat

increases with the depth. A trace of this colour is often visible even

in the surface snow on the glacier, and sometimes also in our ordinary

winter fields. In a hole made with a stick a foot or more in depth a

faint cerulean glimmer may generally be discerned; but the increased

blueness of the ice as we go down is conspicuous, and readily leads us

to the conclusion that the air, to which, as we before noted, the

whiteness of the snow is due, is working out of the mass as the

process of compaction goes on. In a glacial district this snow mass

above the melting line is called the _névé_.

Remembering that the excess of snow beyond the melting in a _névé_

district amounts, it may be, to some feet of material each year, we

easily come to the conclusion that the mass works down the slope in

the manner which it does even where the coating is impermanent. This

supposition is easily confirmed: by observing the field we find that

the sheet is everywhere drawing away from the cliffs, leaving a deep

fissure between the _névé_ and the precipices. This crevice is called

by the German-Swiss guides the _Bergschrund_. Passage over it is

often one of the most difficult feats to accomplish which the Alpine

explorer has to undertake. In fact, the very appearance of the

surface, which is that of a river with continuous down slopes, is

sufficient evidence that the mass is slowly flowing toward the

valleys. Following it down, we almost always come to a place where it

passes from the upper valleys to the deeper gorges which pierce the

skirts of the mountain. In going over this projection the mass of

snow-ice breaks to pieces, forming a crowd of blocks which march down

the slope with much more speed than they journeyed when united in the

higher-lying fields. In this condition and in this part of the

movement the snow-ice forms what are called the _seracs_, or curds, as

the word means in the French-Swiss dialect. Slipping and tumbling

down the steep slope on which the _seracs_ develop, the ice becomes

broken into bits, often of small size. These fragments are quickly

reknit into the body of ice, which we shall hereafter term the

glacier, and in this process the expulsion of the air goes on more

rapidly than before, and the mass assumes a more transparent icelike

quality.

The action of the ice in the pressures and strains to which it is

subjected in joining the main glacier and in the further part of its

course demand for their understanding a revision of those notions as

to rigidity and plasticity which we derive from our common experience

with objects. It is hard to believe that ice can be moulded by

pressure into any shape without fracturing, provided the motion is

slowly effected, while at the same time it is as brittle as ice to a

sudden blow. We see, however, a similar instance of contrasted

properties in the confection known as molasses candy, a stick of which

may be indefinitely bent if the flexure is slowly made, but will fly

to pieces like glass if sharply struck. Ice differs from the sugary

substance in many ways; especially we should note that while it may be

squeezed into any form, it can not be drawn out, but fractures on the

application of a very slight tension. The conditions of its movement

we will inquire into further on, when we have seen more of its action.

Entering on the lower part of its course, that where it flows into the

region below the snow line, the ice stream is now confined between the

walls of the valley, a channel which in most cases has been shaped

before the ice time, by a mountain torrent, or perhaps by a slower

flowing river. In this part of its course the likeness of a glacial

stream to one of fluid water is manifest. We see that it twists with

the turn of the gorge, widens where the confining walls are far apart,

and narrows where the space is constricted. Although the surface is

here and there broken by fractures, it is evident that the movement of

the frozen current, though slow, is tolerably free. By placing stakes

in a row across the axis of a glacier, and observing their movement

from day to day, or even from hour to hour if a good theodolite is

used for the purpose, we note that the movement of the stream is

fastest in the middle parts, as in the case of a river, and that it

slows toward either shore, though it often happens, as in a stream of

molten water, that the speediest part of the current is near one side.

Further observations have indicated that the movement is most rapid on

the surface and least at the bottom, in which the stream is also

riverlike. It is evident, in a word, that though the ice is not fluid

in strict sense, the bits of which it is made up move in substantially

the manner of fluids--that is, they freely slip over each other. We

will now turn our attention to some important features of a detailed

sort which glaciers exhibit.

If we visit a glacier during the part of the year when the winter

snows are upon it, it may appear to have a very uninterrupted surface.

But as the summer heat advances, the mask of the winter coating goes

away, and we may then see the structure of the ice. First of all we

note in all valley glaciers such as we are observing that the stream

is overlaid by a quantity of rocky waste, the greater part of which

has come down with the avalanches in the manner before described,

though a small part may have been worn from the bed over which the ice

flows. In many glaciers, particularly as we approach their

termination, this sheet of earth and rock materials often covers the

ice so completely that the novice in such regions finds it difficult

to believe that the ice is under his feet. If the explorer is minded

to take the rough scramble, he can often walk for miles on these

masses of stone without seeing, much less setting foot on any frozen

water. In some of the Alaskan glaciers this coating may bear a forest

growth. In general, this material, which is called moraine, is

distributed in bands parallel to the sides of the glaciers, and the

strips may amount to a half dozen or more. Those on the sides of the

ice have evidently been derived from the precipices which they have

passed. Those in the middle have arisen from the union of the moraines

formed in two or more tributary valleys.

[Illustration: Fig. 12.--Map of glaciers and moraines near Mont Blanc.]

Where the avalanches fall most plentifully, the stones lie buried with

the snow, and only melt out when the stream attains the region where

the annual waste of its surface exceeds the snowfall. In this section

we can see how the progressive melting gradually brings the rocky

_débris_ into plain view. Here and there we will find a boulder

perched on a pedestal of ice, which indicates a recent down-wearing of

the field. A frequent sound in these regions arises from the tumble of

the stones from their pedestals or the slipping of the masses from the

sharp ridge which is formed by the protection given to the ice through

the thick coating of detritus on its surface. These movements of the

moraines often distribute their waste over the glacier, so that in its

lower part we can no longer trace the contributions from the several

valleys, the whole area being covered by the _débris_. At the end of

the ice stream, where its forward motion is finally overcome by the

warmth which it encounters, it leaves in a rude heap, extending often

like a wall across the valley, all the coarse fragments which it

conveys. This accumulation, composed of all the lateral moraines which

have gathered on the ice by the fall of avalanches, is called the

terminal moraine. As the ice stream itself shrinks, a portion of the

detritus next the boundary wall is apt to be left clinging against

those slopes. It is from the presence of these heaps in valleys now

abandoned by glaciers that we obtain some information as to the former

greater extent of glacial action.

The next most noticeable feature is the crevasse. These fractures

often exist in very great numbers, and constitute a formidable barrier

in the explorer's way. The greater part of these ruptures below the

_serac_ zone run from the sides of the stream toward the centre

without attaining that region. These are commonly pointed up stream;

their formation is due to the fact that, owing to the swifter motion

in the central parts of the stream, the ice in that section draws away

from the material which is moving more slowly next the shore. As

before noted, these ice fractures when drawn out naturally form

fissures at right angles to the direction of the strain. In the middle

portions of the ice other fissures form, though more rarely, which

appear to depend on local strains brought about through the

irregularity of the surface over which the ice is flowing.

If the observer is fortunate, he may in his journey over the glacier

have a chance to see and hear what goes on when crevasses are formed.

First he will hear a deep, booming sound beneath his feet, which

merges into a more splintering note as the crevice, which begins at

the bottom or in the distance, comes upward or toward him. When the

sound is over, he may not be able to see a trace of the fracture,

which at first is very narrow. But if the break intersect any of the

numerous shallow pools which in a warm summer's day are apt to cover a

large part of the surface, he may note a line of bubbles rushing up

through the water, marking the escape of the air from the glacier,

some remnant of that which is imprisoned in the original snow. Even

where this indication is wanting, he can sometimes trace the crevice

by the hissing sound of the air streams where they issue from the ice.

If he will take time to note what goes on, he can usually in an hour

or two behold the first invisible crack widen until it may be half an

inch across. He may see how the surface water hastens down the

opening, a little river system being developed on the surface of the

ice as the streams make their way to one or more points of descent. In

doing this work they excavate a shaft which often becomes many feet in

diameter, down which their waters thunder to the base of the glacier.

This well-like opening is called a _moulin_, or mill, a name which, as

we shall see, is well deserved from the work which falling waters

accomplish. Although the institution of the _moulin_ shaft depends

upon the formation of a crevice, it often happens that as the ice

moves farther on its journey its walls are again thrust together,

soldered in the manner peculiar to ice, so that no trace of the

rupture remains except the shaft which it permitted to form. Like

everything else in the glacier, the _moulin_ slowly moves down the

slope, and remains open as long as it is the seat of descending waters

produced by the summer melting. When it ceases to be kept open from

the summer, its walls are squeezed together in the fashion that the

crevices are closed.

Forming here and there, and generally in considerable numbers, the

crevices of a glacier entrap a good deal of the morainal _débris_,

which falls through them to the bottom of the glacier. Smaller bits

are washed into the _moulin_, by the streams arising from the melting

ice, which is brought about by the warm sun of the summer, and

particularly by the warm rains of that season. On those glaciers

where, owing to the irregularity of the bottom over which the ice

flows, these fractures are very numerous, it may happen that all the

detritus brought upon the surface of the glacier by avalanches finds

its way to the floor of the ice.

Although it is difficult to learn what is going on at the under

surface of the glacier, it is possible directly and indirectly to

ascertain much concerning the peculiar and important work which is

there done. The intrepid explorer may work his way in through the

lateral fissures, and even with care safely descend some of the

fissures which penetrate the central parts of a shallow ice stream.

There, it may be at the depth of a hundred feet or more, he will find

a quantity of stones, some of which may be in size like to a small

house held in the body of the ice, but with one side resting upon the

bed rock. He may be so fortunate as to see the stone actually in

process of cutting a groove in the bed rock as it is urged forward by

the motion of the glacier. The cutting is not altogether in the fixed

material, for the boulder itself is also worn and scored in the work.

Smaller pebbles are caught in the space between the erratic and the

motionless rock and ground to bits. If in his explorations the student

finds his way to the part of the floor on which the waters of a

_moulin_ fall, he may have a chance to observe how the stones set in

motion serve to cut the bed rock, forming elongated potholes much as

in the case of ordinary waterfalls, or at the base of those shafts

which afford the beginnings of limestone caverns.

The best way to penetrate beneath the glacier is through the arch of

the stream which always flows from the terminal face of the ice river.

Even in winter time every large glacier discharges at its end a

considerable brook, the waters of which have been melted from the ice

in small part by the outflow of the earth's heat; mainly, however, by

the warmth produced in the friction of the ice on itself and on its

bottom--in other words, by the conversion of that energy of position,

of which we have often to speak, into heat. In the summer time this

subglacial stream is swollen by the surface waters descending through

the crevices and the _moulins_ which come from them, so that the

outflow often forms a considerable river, and thus excavates in the

ice a large or at least a long cavern, the base of which is the bed

rock. In the autumn, when the superficial melting ceases, this gallery

can often be penetrated for a considerable distance, and affords an

excellent way to the secrets of the under ice. The observer may here

see quantities of the rock material held in the grip of the ice, and

forced to a rude journey over the bare foundation stones. Now and then

he may find the glacial mass in large measure made up of stones, the

admixture extending many feet above the bottom of the cavern, perhaps

to the very top of the arch. He may perchance find that these stones

are crushing each other where they are in contact. The result will be

brought about by the difference in the rate of advance of the ice,

which moves the faster the higher it is above the surface over which

it drags, and thus forces the stones on one level over those below.

Where the waters of the subglacial stream have swept the bed rock

clean of _débris_ its surface is scored, grooved, and here and there

polished in a manner which is accomplished only by ice action, though

some likeness to it is afforded where stones have been swept over for

ages by blowing sand. Here and there, often in a way which interrupts

the cavern journey, the shrunken stream, unable to carry forward the

_débris_, deposits the material in the chamber, sometimes filling the

arch so completely that the waters are forced to make a detour. This

action is particularly interesting, for the reason that in regions

whence glaciers have disappeared the deposits formed in the old ice

arches often afford singularly perfect moulds of those caverns which

were produced by the ancient subglacial streams. These moulds are

termed _eskers_.

If the observer be attentive, he will note the fact that the waters

emerging from beneath the considerable glacier are very much charged

with mud. If he will take a glass of the water at the point of escape,

he will often find, on permitting it to settle, that the sediment

amounts to as much as one twentieth of the volume. While the greater

part of this detritus will descend to the bottom of the vessel in the

course of a day, a portion of it does not thus fall. He may also note

that this mud is not of the yellowish hue which he is accustomed to

behold in the materials laid down by ordinary rivers, but has a

whitish colour. Further study will reveal the fact that the difference

is due to the lack of oxidation in the case of the glacial detritus.

River muds forming slowly and during long-continued exposure to the

action of the air have their contained iron much oxidized, which gives

them a part of their darkened appearance. Moreover, they are somewhat

coloured with decayed vegetable matter. The waste from beneath the

glacier has been quickly separated from the bed rock, all the faces of

the grains are freshly fractured, and there is no admixture of organic

matter. The faces of the particles thus reflect light in substantially

the same way as powdered glass or pulverized ice, and consequently

appear white.

A little observation will show the student that this very muddy

character of waters emerging from beneath the glacier is essentially

peculiar to such streams as we have described. Ascending any of the

principal valleys of Switzerland, he may note that some of the streams

flow waters which carry little sediment even in times when they are

much swollen, while others at all seasons have the whitish colour. A

little further exploration, or the use of a good map, will show him

that the pellucid streams receive no contributions of glacial water,

while those which look as if they were charged with milk come, in part

at least, from the ice arches. From some studies which the writer has

made in Swiss valleys, it appears that the amount of erosion

accomplished on equal areas of similar rock by the descent of the

waters in the form of a glacier or in that of ordinary torrents

differs greatly. Moving in the form of ice, or in the state of

ice-confined streams, the mass of water applies very many times as

much of its energy of position to grinding and bearing away the rocks

as is accomplished where the water descends in its fluid state.

The effect of the intense ice action above noted is rapidly to wear

away the rocks of the valley in which the glacier is situated. This

work is done not only in a larger measure but in a different way from

that accomplished by torrents. In the case of the latter, the stream

bed is embarrassed by the rubbish which comes into it; only here and

there can it attack the bed rock by forcing the stones over its

surface. Only in a few days of heavy rain each year is its work at all

effective; the greater part of the energy of position of its waters is

expended in the endless twistings and turnings of its stream, which

result only in the development of heat which flies away into the

atmosphere. In the ice stream, owing to its slow movement and to the

detritus which it forces along the bottom, a vastly greater part of

the energy which impels it down the slope is applied to rock cutting.

None of the boulders, even if they are yards in diameter, obstruct its

motion; small and great alike are to it good instruments wherewith to

attack the bed rocks. The fragments are never left to waste by

atmospheric decay, but are to a very great extent used up in

mechanical work, while the most of the detritus which comes to a

torrent is left in a coarse state when it is delivered to the stream;

the larger part of that which the glacier transports is worn out in

its journey. To a great extent it is used up in attacking the bed

rock. In most cases the _débris_ in the terminal moraine is evidently

but a small part of what entered the ice during its journey from the

uplands; the greater part has been worn out in the rude experiences to

which it has been subjected.

It is evident that even in the regions now most extensively occupied

by glaciers the drainage systems have been shaped by the movement of

ordinary streams--in other words, ice action is almost everywhere,

even in the regions about the poles, an incidental feature in the work

of water, coming in only to modify the topography, which is mainly

moulded by the action of fluid water. When, owing to climatal changes,

a valley such as those of the Alps is occupied by a glacial stream,

the new current proceeds at once, according to its evident needs, to

modify the shape of its channel. An ordinary torrent, because of the

swiftness of its motion, which may, in general, be estimated at from

three to five miles an hour, can convey away the precipitation over a

very narrow bed. Therefore its channel is usually not a hundredth part

as wide as the gorge or valley in which it lies. But when the

discharge takes place by a glacier, the speed of which rarely exceeds

four or five feet a day, the ice stream because of its slow motion has

to fill the trough from side to side, it has to be some thousand times

as deep and wide as the torrent. The result is that as soon as the

glacial condition arises in a country the ice streams proceed to

change the old V-shaped torrent beds into those which have a broad

U-like form. The practised eye can in a way judge how long a valley

has been subjected to glacial action by the extent to which it has

been widened by this process.

In the valleys of Switzerland and other mountain districts which have

been attentively studied it is evident that glacial action has played

a considerable part in determining their forms. But the work has been

limited to that part of the basin in which the ice is abundantly

provided with cutting tools in the stone which have found their way to

the base of the stream. In the region of the _névé_, where the

contributions of rocky matter to the surface of the deposit made from

the few bare cliffs which rise above the sheet of snow is small, the

snow-ice does no cutting of any consequence. Where it passes over the

steep at the head of the deep valley into which it drains, and is

riven into the _seracs_, such stony matter as it may have gathered is

allowed to fall to the bottom, and so comes into a position where it

may do effective work. From this _serac_ section downward the now

distinct ice river, being in general below the snow line, has

everywhere cliffs, on either side from which the contributions of rock

material are abundant. Hence this part of the glacier, though it is

the wasting portion of its length, does all the cutting work of any

consequence which is performed. It is there that the underrunning

streams become charged with sediment, which, as we have noted, they

bear in surprising quantities, and it is therefore in this section of

the valley that the impress of the ice work is the strongest. Its

effect is not only to widen the valley and deepen it, but also to

advance the deep section farther up the stream and its tributaries.

The step in the stream beds which we find at the _seracs_ appears to

mark the point in the course of the glacier where, owing to the

falling of stones to its base, as well as to its swifter movements and

the firmer state of the ice, it does effective wearing.

There are many other features connected with glaciers which richly

repay the study of those who have a mind to explore in the manner of

the physicist interested in ice actions the difficult problems which

they afford; but as these matters are not important from the point of

view of this work, no mention of them will here be made. We will now

turn our attention to that other group of glaciers commonly termed

continental, which now exist about either pole, and which at various

times in the earth's history have extended far toward the equator,

mantling over vast extents of land and shallow sea. The difference

between the ice streams of the mountains and those which we term

continental depends solely on the areas of the fields and the depth of

the accumulation. In an ordinary Alpine region the _névé_ districts,

where the snow gathers, are relatively small. Owing to the rather

steep slopes, the frozen water is rapidly discharged into the lower

valleys, where it melts away. Both in the _névé_ and in the distinct

glacier of the lower grounds there are, particularly in the latter,

projecting peaks, from which quantities of stone are brought down by

avalanches or in ordinary rock falls, so that the ice is abundantly

supplied with cutting tools, which work from its surface down to its

depths.

As the glacial accumulation grows in depth there are fewer peaks

emerging from it, and the streams which it feeds rise the higher until

they mantle over the divides between the valleys. Thus by

imperceptible stages valley glaciers pass to the larger form, usually

but incorrectly termed continental. We can, indeed, in going from the

mountains in the tropics to the poles, note every step in this

transition, until in Greenland we attain the greatest ice mass in the

world, unless that about the southern pole be more extensive. In the

Greenland glacier the ice sheet covers a vast extent of what is

probably a mountain country, which is certainly of this nature in the

southern part of the island, where alone we find portions of the earth

not completely covered by the deep envelope. Thanks to the labours of

certain hardy explorers, among whom Nansen deserves the foremost

place, we now know something as to the conditions of this vast ice

field, for it has been crossed from shore to shore. The results of

these studies are most interesting, for they afford us a clew as to

the conditions which prevail over a large part of the earth during the

Glacial period from which the planet is just escaping, and in the

earlier ages when glaciation was likewise extensive. We shall

therefore consider in a somewhat detailed way the features which the

Greenland glacier presents.

Starting from the eastern shore of that land, if we may thus term a

region which presents itself mainly in the form of ice, we find next

the shore a coast line not completely covered with ice and snow, but

here and there exhibiting peaks which indicate that if the frozen

mantle were removed the country would appear deeply intersected with

fiords in the manner exhibited in the regions to the south of

Greenland or the Scandinavian peninsula. The ice comes down to the

sea through the valleys, often facing the ocean for great distances

with its frozen cliffs. Entering on this seaward portion of the

glacier, the observer finds that for some distance from the coast line

the ice is more or less rifted with crevices, the formation of which

is doubtless due to irregularities of the rock bottom over which it

moves. These ruptures are so frequent that for some miles back it is

very difficult to find a safe way. Finally, however, a point is

attained where these breaks rather suddenly disappear, and thence

inward the ice rises at the rate of upward slope of a few feet to the

mile in a broad, nearly smooth incline. In the central portion of the

region for a considerable part of the territory the ice has very

little slope. Thence it declines toward the other shore, exhibiting

the same features as were found on the eastern versant until near the

coast, when again the surface is beset with crevices which continue to

the margin of the sea.

Although the explorations of the central field of Greenland are as yet

incomplete, several of these excursions into or across the interior

have been made, and the identity of the observations is such that we

can safely assume the whole region to be of one type. We can

furthermore run no risk in assuming that what we find in Greenland, at

least so far as the unbroken nature of the central ice field is

concerned, is what must exist in every land where the glacial envelope

becomes very deep. In Greenland it seems likely that the depth of the

ice is on the average more than half a mile, and in the central part

of the realm the sheet may well have a much greater profundity; it may

be nearly a mile deep. The most striking feature--that of a vast

unbroken expanse, bordered by a region where the ice is ruptured--is

traceable wherever very extensive and presumably deep deposits of ice

have been examined. As we shall see hereafter, these features teach us

much as to the conditions of glacial action--a matter which we shall

have to examine after we have completed our general survey as to the

changes which occur during glacial periods.

In the present state of that wonderful complex of actions which we

term climate, glaciers are everywhere, so far as our observations

enable us to judge, generally in process of decrease. In Switzerland,

although the ancients even in Roman days were in contact with the ice,

they were so unobservant that they did not even remark that the ice

was in motion. Only during the last two centuries have we any

observations of a historic sort which are of value to the geologist.

Fortunately, however, the signs written on the rock tell the story,

except for its measurement in terms of years, as clearly as any

records could give it. From this testimony of the rocks we perceive

that in the geological yesterday, though it may have been some tens of

thousands of years ago, the Swiss glaciers, vastly thickened, and with

their horizontal area immensely expanded, stretched over the Alpine

country, so that only here and there did any of the sharper peaks rise

above the surface. These vast glaciers, almost continually united on

their margins, extended so far that every portion of what is now the

Swiss Republic was covered by them. Their front lay on the southern

lowlands of Germany, on the Jura district of France; on the south, it

stretched across the valley of the Po as far as near Milan. We know

this old ice front by the accumulations of rock _débris_ which were

brought to it from the interior of the mountain realm. We can

recognise the peculiar kinds of stone, and with perfect certainty

trace them to the bed rock whence they were riven. Moreover, we can

follow back through the same evidence the stages of retreat of the

glaciers, until they lost their broad continental character and

assumed something like their present valley form. Up the valley of any

of the great rivers, as, for instance, that of the Rhône above the

lake of Geneva, we note successive terminal moraines which clearly

indicate stages in the retreat of the ice when for a time it ceased to

go backward, or even made a slight temporary readvance. It is easily

seen that on such occasions the stones carried to the ice front would

be accumulated in a heap, while during the time when day by day the

glacier was retreating the rock waste would be left broadcast over the

valley.

As we go up from the course of the glacial streams we note that the

successive moraines have their materials in a progressively less

decayed state. Far away from the heap now forming, and in proportion

to the distance, the stones have in a measure rotted, and the heaps

which they compose are often covered with soil and occupied by

forests. Within a few miles of the ice front the stones still have a

fresh aspect. When we arrive within, say, half a mile of the moraine

now building, we come to the part of the glacial retreat of which we

have some written or traditional account. This is in general to the

effect that the wasting of the glaciers is going on in this century as

it went on in the past. Occasionally periods of heavy snow would

refresh the ice streams, so that for a little time they pushed their

fronts farther down the valley. The writer has seen during one of

these temporary advances the interesting spectacle of ice destroying

and overturning the soil of a small field which had been planted in

grain.

It should be noted that these temporary advances of the ice are not

due to the snowfall of the winter or winters immediately preceding the

forward movement. So slow is the journey of the ice from the _névé_

field to the end of a long glacier that it may require centuries for

the store accumulated in the uplands to affect the terminal portion of

the stream. We know that the bodies of the unhappy men who have been

lost in the crevices of the glacier are borne forward at a uniform and

tolerably computable rate until they emerge at the front, where the

ice melts away. In at least one case the remains have appeared after

many years in the _débris_ which is contributed to the moraine. On

account of this slow feeding of the glacial stream, we naturally may

expect to find, as we do, in fact, that a great snowfall of many

years ago, and likewise a period when the winter's contribution has

been slight, would influence the position of the terminal point of the

ice stream at different times, according to its length. If the length

of the flow be five miles, it may require twenty or thirty years for

the effect to be evident; while if the stream be ten miles long, the

influence may not be noted in less than threescore years. Thus it

comes about that at the present time in the same glacial district some

streams may be advancing while others are receding, though, on the

whole, the ice is generally in process of shrinkage. If the present

rate of retreat should be maintained, it seems certain that at the end

of three centuries the Swiss glaciers as a whole will not have

anything like their present area, and many of the smaller streams will

entirely disappear.

Following the method of the illustrious Louis Agassiz, who first

attentively traced the evidence which shows the geologically recent

great extension of glaciers by studying the evidence of the action in

fields they no longer occupy, geologists have now inspected a large

part of the land areas with a view to finding the proofs of such ice

work. So far as these indications are concerned, the indications which

they have had to trace are generally of a very unmistakable character.

Rarely, indeed, does a skilled student of such phenomena have to

search in any region for more than a day before he obtains indubitable

evidence which will enable him to determine whether or not the field

has recently been occupied by an enduring ice sheet--one which

survives the summer season and therefore deserves the name of glacier.

The indications which he has to consider consist in the direction and

manner in which the surface materials have been carried, the physical

conditions of these materials, the shape of the surface of the

underlying rock as regards its general contour, and the presence or

absence of scratches and groovings on its surface. As these records of

ice action are of first importance in dealing with this problem, and

as they afford excellent subjects for the study of those who dwell in

glaciated regions, we shall note them in some detail.

The geologist recognises several ways in which materials may be

transported on the surface of the earth. They may be cast forth by

volcanoes, making their journey by being shot through the air, or by

flowing in lava streams; it is always easy at a glance, save in very

rare instances, to determine whether fragments have thus been

conveyed. Again, the detritus may be moved by the wind; this action is

limited; it only affects dust, sand, and very small pebbles, and is

easily discriminated. The carriage may be effected by river or marine

currents; here, again, the size of the fragments moved is small, and

the order of their arrangement distinctly traceable. The fragments may

be conveyed by ice rafts; here, too, the observer can usually limit

the probabilities he has to consider by ascertaining, as he can

generally do, whether the region which he is observing has been below

a sea or lake. In a word, the before-mentioned agents of

transportation are of somewhat exceptional influence, and in most

cases can, as explanations of rock transportation, be readily

excluded. When, therefore, the geologist finds a country abundantly

covered with sand, pebbles, and boulders arranged in an irregular way,

he has generally only to inquire whether the material has been carried

by rivers or by glaciers. This discrimination can be quickly and

critically effected. In the first place, he notes that rivers only in

their torrent sections can carry large fragments of rock, and that in

all cases the fragments move down hill. Further, that where deposits

are formed, they have more or less the form of alluvial deposits. If

now the observations show that the rock waste occupying the surface of

any region has been carried up hill and down, across the valleys,

particularly if there are here and there traces of frontal moraines,

the geologist is entitled to suppose--he may, indeed, be sure--that

the carriage has been effected by a glacial sheet.

Important corroborative evidence of ice action is generally to be

found by inspecting the bed rock below the detritus, which indicates

glacial action. Even if it be somewhat decayed, as is apt to be the

case where the ice sheet long since passed away, the bed rock is

likely to have a warped surface; it is cast into ridges and furrows of

a broad, flowing aspect, such as liquid water never produces, which,

indeed, can only be created by an ice sheet moving over the surface,

cutting its bed in proportion to the hardness of the material.

Furthermore, if the bed rock have a firm texture, and be not too much

decayed, we almost always find upon it grooves or scratches, channels

carved by the stones embedded in the body of the ice, and drawn by its

motion over the fixed material. Thus the proof of glacial extension in

the last ice epoch is made so clear that accurate maps can be prepared

showing the realm of its action. This task is as yet incomplete,

although it is already far advanced.

While the study of glaciers began in Europe, inquiries concerning

their ancient extension have been carried further and with more

accuracy in North America than in any other part of the world. We may

therefore well begin our description of the limits of the ice sheets

with this continent. Imagining a seafarer to have approached America

by the North Atlantic, as did the Scandinavians, and that his voyage

came perhaps a hundred thousand years or more before that of Leif

Ericsson, he would have found an ice front long before he attained the

present shores of the land. This front may have extended from south of

Greenland, off the shores of the present Grand Banks of Newfoundland,

thence and westward to central or southern New Jersey. This cliff of

ice was formed by a sheet which lay on the bottom of the sea. On the

New Jersey coast the ice wall left the sea and entered on the body of

the continent. We will now suppose that the explorer, animated with

the valiant scientific spirit which leads the men of our day to seek

the poles, undertook a land journey along the ice front across the

continent. From the New Jersey coast the traveller would have passed

through central Pennsylvania, where, although there probably detached

outlying glaciers lying to the southward as far as central Virginia,

the main front extended westward into the Ohio Valley. In southern

Ohio a tongue of the ice projected southwardly until it crossed the

Ohio River, where Cincinnati now lies, extending a few miles to the

southward of the stream. Thence it deflected northwardly, crossing the

Mississippi, and again the Missouri, with a tongue or lobe which went

far southward in that State. Then again turning to the northwest, it

followed in general the northern part of the Missouri basin until it

came to within sight of the Rocky Mountains. There the ice front of

the main glacier followed the trend of the mountains at some distance

from their face for an unknown extent to the northward. In the

Cordilleras, as far south as southern Colorado, and probably in the

Sierra Nevada to south of San Francisco, the mountain centres

developed local glaciers, which in some places were of very great

size, perhaps exceeding any of those which now exist in Switzerland.

It will thus be seen that nearly one half of the present land area of

North America was beneath a glacial covering, though, as before noted,

the region about the Gulf of Mexico may have swayed upward when the

northern portion of the land was borne down by the vast load of ice

which rested upon it. Notwithstanding this possible addition to the

land, our imaginary explorer would have found the portion of the

continent fit for the occupancy of life not more than half as great as

it is at present.

In the Eurasian continent there was no such continuous ice sheet as in

North America, but the glaciers developed from a number of different

centres, each moving out upon the lowlands, or, if its position was

southern, being limited to a particular mountain field. One of these

centres included Scandinavia, northern Germany, Great Britain about as

far south as London, and a large part of Ireland, the ice covering the

intermediate seas and extending to the westward, so that the passage

of the North Atlantic was greatly restricted between this ice front

and that of North America. Another centre, before noted, was formed in

the Alps; yet another, of considerable area, in the Pyrenees; other

less studied fields existed in the Apennines, in the Caucasus, the

Ural, and the other mountains of northern Asia. Curiously enough,

however, the great region of plains in Siberia does not appear to have

been occupied by a continuous ice sheet, though the similar region in

North America was deeply embedded in a glacier. Coincident with this

development of ice in the eastern part of the continent, the ice

streams of the Himalayan Mountains, some of which are among the

greatest of our upland glaciers, appear to have undergone but a

moderate extension. Many other of the Eurasian highlands were probably

ice-bound during the last Glacial period, but our knowledge concerning

these local fields is as yet imperfect.

In the southern hemisphere the lands are of less extent and, on the

whole, less studied than in the northern realm. Here and there where

glaciers exist, as in New Zealand and in the southern part of South

America, observant travellers have noticed that these ice fields have

recently shrunk away. Whether the time of greatest extension and of

retreat coincided with that of the ice sheets in the north is not yet

determined; the problem, indeed, is one of some difficulty, and may

long remain undecided. It seems, however, probable that the glaciers

of the southern hemisphere, like those in the north, are in process of

retreat. If this be true, then their time of greatest extension was

probably the same as that of the ice sheets about the southern pole.

From certain imperfect reports which we have concerning evidences of

glaciation in Central America and in the Andean district in the

northern part of South America, it seems possible that at one time the

upland ice along the Cordilleran chain existed from point to point

along that system of elevations, so that the widest interval between

the fields of permanent snow with their attendant glaciers did not

much exceed a thousand miles.

Observing the present gradual retreat of those ice remnants which

remain mere shreds and patches of the ancient fields, it seems at

first sight likely that the extension and recession of the great

glaciers took place with exceeding slowness. Measured in terms of

human life, in the manner in which we gauge matters of man's history,

this process was doubtless slow. There are reasons, however, to

believe that the coming and going were, in a geological sense, swift;

they may have, indeed, been for a part of the time of startling

rapidity. Going back to the time of geological yesterday, before the

ice began its development in the northern hemisphere, all the evidence

we can find appears to indicate a temperate climate extending far

toward the north pole. The Miocene deposits found within twelve

degrees, or a little more than seven hundred miles, of the north pole,

and fairly within the realm of lowest temperature which now exists on

the earth, show by the plant remains which they contain that the

conditions permitted the growth of forests, the plants having a

tolerably close resemblance to those which now freely develop in the

southern portion of the Mississippi Valley. Among them there are

species which had the habit of retaining their broad, rather soft

leaves throughout the winter season. The climate appears, in a word,

to have been one where the mean annual temperature must have been

thirty degrees or more higher than the present average of that realm.

Although such conditions near the sea level are not inconsistent with

the supposition that glaciers existed in the higher mountains of the

north, they clearly deny the possibility of the realm being occupied

by continental glaciers.

Although the Pliocene deposits formed in high latitudes have to a

great extent been swept away by the subsequent glacial wearing, they

indicate by their fossils a climatal change in the direction of

greater cold. We trace this change, though obscurely, in a

progressive manner to a point where the records are interrupted, and

the next interpretable indication we have is that the ice sheet had

extended to somewhere near the limits which we have noted. We are then

driven to seek what we can concerning the sojourn of the ice on the

land by the amount of wearing which it has inflicted upon the areas

which it occupied. This evidence has a certain, though, as we shall

see, a limited value.

When the students of glacial action first began the great task of

interpreting these records, they were led to suppose that the amount

of rock cutting which was done by the ice was very great. Observing

what goes on, in the manner we have noted, beneath a valley glacier

such as those of Switzerland, they saw that the ice work went on

rapidly, and concluded that if the ice remained long at work in a

region it must do a vast deal of erosion. They were right in a part of

their premises, but, as we shall see, probably in another part wrong.

Looking carefully over the field where the ice has operated, we note

that, though at first sight the area appears to have lost all trace of

its preglacial river topography, this aspect is due mainly to the

irregular way in which the glacial waste is laid down. Close study

shows us that we may generally trace the old stream valleys down to

those which were no larger than brooks. It is true that these channels

are generally and in many places almost altogether filled in with

rubbish, but a close study of the question has convinced the writer,

and this against a previous view, that the amount of erosion in New

England and Canada, where the work was probably as great as anywhere,

has not on the average exceeded a hundred feet, and probably was much

less than that amount.

Even in the region north of Lake Ontario, over which the ice was deep

and remained for a long time, the amount of erosion is singularly

small. Thus north of Kingston the little valleys in the limestone

rocks which were cut by the preglacial streams, though somewhat

encumbered with drift, remain almost as distinct as they are on

similar strata in central Kentucky, well south of the field which the

ice occupied. In fact, the ice sheet appears to have done the greatest

part of its work and to have affected the surface most in the belt of

country a few hundred miles in width around the edges of the sheet. It

was to be expected that in a continental glacier, as in those of

mountain valleys, the most of the _débris_ should be accumulated about

the margin where the materials dropped from the ice. But why the

cutting action should be greatest in that marginal field is not at

first sight clear. To explain this and other features as best we may,

we shall now consider the probable history of the great ice march in

advance and retreat, and then take up the conditions which brought

about its development and its disappearance.

Ice is in many ways the most remarkable substance with which the

physicist has to deal, and among its eminent peculiarities is that it

expands in freezing, while the rule is that substances contract in

passing from the fluid to the solid state. On this account frozen

water acts in a unique manner when subjected to pressure. For each

additional atmosphere of pressure--a weight amounting to about fifteen

pounds to the square inch--the temperature at which the ice will melt

is lowered to the amount of sixteen thousandths of a degree

centigrade. If we take a piece of ice at the temperature of freezing

and put upon it a sufficient weight, we inevitably bring about a small

amount of melting. Where we can examine the mass under favourable

conditions, we can see the fluid gather along the lines of the

crystals or other bits of which the ice is composed. We readily note

this action by bringing two pieces of ice together with a slight

pressure; when the pressure is removed, they will adhere. The adhesion

is brought about not by any stickiness of the materials, for the

substance has no such property. It is accomplished by melting along

the line of contact, which forms a film of water, that at once

refreezes when the pressure is withdrawn. When a firm snowball is

made by even pressing snow, innumerable similar adhesions grow up in

the manner described. The fact is that, given ice at the temperature

at which it ordinarily forms, pressure upon it will necessarily

develop melting.

The consequences of pressure melting as above described are in

glaciers extremely complicated. Because the ice is built into the

glacier at a temperature considerably below the freezing point, it

requires a great thickness of the mass before the superincumbent

weight is sufficient to bring about melting in its lower parts. If we

knew the height at which a thermometer would have stood in the surface

ice of the ancient glacier which covered the northern part of North

America, we could with some accuracy compute how thick it must have

been before the effect of pressure alone would have brought about

melting; but even then we should have to reckon the temperature

derived from the grinding of the ice over the floor and the crushing

of rocks there effected, as well as the heat which is constantly

though slowly coming forth from the earth's interior. The result is

that we can only say that at some depth, probably less than a mile,

the slowly accumulating ice would acquire such a temperature that,

subjected to the weight above it, the material next the bottom would

become molten, or at least converted into a sludgelike state, in which

it could not rub against the bottom, or move stones in the manner of

ordinary glaciers.

As fast as the ice assumed this liquid or softened state, it would be

squeezed out toward the region where, because of the thinning of the

glacier, it would enter a field where pressure melting did not occur.

It would then resume the solid state, and thence journey to the margin

of the ice in the ordinary manner. We thus can imagine how such a

glacier as occupied the northern part of this continent could have

moved from the central parts toward its periphery, as we can not do if

we assume that the glacier everywhere lay upon the bed rock. There is

no slope from Lake Erie to the Ohio River at Cincinnati. Knowing that

the ice moved down this line, there are but two methods of accounting

for its motion: either the slope of the upper surface to the northward

was so steep that the mass would have been thus urged down, the upper

parts dragging the bottom along with them, or the ice sheet for the

greater part of its extent rested upon pressure-molten water, or

sludge ice, which was easily squeezed out toward the front. The first

supposition appears inadmissible, for the reason that the ice would

have to be many miles deep at Hudson Bay in order that its upper

surface should have slope enough to overcome the rigidity of the

material and bring about the movement. We know that any such depth is

not supposable.

The recent studies in Greenland supply us with strong corroborative

evidence for the support of the view which is here urged. The wide

central field of that area, where the ice has an exceeding slight

declivity, and is unruptured by crevices, can not be explained except

on the supposition that it rests on pressure-molten water. The thinner

section next the shore, where the glacier is broken up by those

irregular movements which its wrestle with the bottom inevitably

induces, shows that there it is in contact with the bed rock, for it

behaves exactly as do the valley glaciers of like thickness.

The view above suggested as to the condition of continental glaciers

enables us to explain not only their movements, but the relatively

slight amount of wearing which they brought about on the lands they

occupied. Beginning to develop in mountain regions, or near the poles

on the lowlands, these sheets, as soon as they attained the thickness

where the ice at their bottom became molten, would rapidly advance for

great distances until they attained districts where the melting

exceeded the supply of frozen material. In this excursion only the

marginal portion of the glacier would do erosive work. This would

evidently be continued for the greatest amount of time near the front

or outer rim of the ice field, for there, we may presume, that for

the longest time the cutting rim would rest upon the bed rock of the

country. As the ice receded, this rim would fall back; thus in the

retreat as in the advance the whole of the field would be subjected to

a certain amount of erosion. On this supposition we should expect to

find that the front of a continental glacier, fed with pressure-molten

water from all its interior district, which became converted into ice,

would attain much warmer regions than the valley streams, where all

the flow took place in the state of ice, and, furthermore, that the

speed of the going on the margin would be much more rapid than in the

Alpine streams. These suppositions are well borne out by the study of

existing continental ice sheets, which move with singular rapidity at

their fronts, and by the ancient glaciers, which evidently extended

into rather warm fields. Thus, when the ice front lay at the site of

Cincinnati, at six hundred feet above the sea, there were no glaciers

in the mountains of North Carolina, though those rise more than five

thousand feet higher in the air, and are less than two hundred miles

farther south. It is therefore evident that the continental glacier at

this time pushed southward into a comparatively warm country in a way

that no stream moving in the manner of a valley glacier could possibly

have done.

The continental glaciers manage in many cases to convey detritus from

a great distance. Thus, when the ice sheet advanced southwardly from

the regions north of the Great Lakes, they conveyed quantities of the

_débris_ from that section as far south as the Ohio River. In part

this rubbish was dragged forward by the ice as the sheet advanced; in

part it was urged onward by the streams of liquid water formed by the

ordinary process of ice melting. Such subglacial rivers appear to have

been formed along the margins of all the great glaciers. We can

sometimes trace their course by the excavation which they have made,

but more commonly by the long ridges of stratified sand and gravel

which were packed into the caverns excavated by these subglacial

rivers, which are known to glacialists as _eskers_, or as serpent

kames. In many cases we can trace where these streams flowed up stream

in the old river valleys until they discharged over their head waters.

Thus in the valley of the Genesee, which now flows from Pennsylvania,

where it heads against the tributaries of the Ohio and Susquehanna, to

Lake Ontario, there was during the Glacial epoch a considerable river

which discharged its waters into those of the Ohio and the Susquehanna

over the falls at the head of its course.

[Illustration: _Front of Muir Glacier, showing ice entering the sea;

also small icebergs._]

The effect of widespread glacial action on a country such as North

America appears to have been, in the first place, to disturb the

attitude of the land by bearing down portions of its surface, a

process which led to the uprising of other parts which lay beyond the

realm of the ice. Within the field of glaciation, so far as the ice

rested bodily on the surface, the rocks were rapidly worn away. A

great deal of the _débris_ was ground to fine powder, and went far

with the waters of the under-running streams. A large part was

entangled in the ice, and moved forward toward the front of the

glacier, where it was either dropped at the margin or, during the

recession of the glacier, was laid upon the surface as the ice melted

away. The result of this erosion and transportation has been to change

the conditions of the surface both as regards soil and drainage. As

the reader has doubtless perceived, ordinary soil is, outside of the

river valleys, derived from the rock beneath where it lies. In

glaciated districts the material is commonly brought from a

considerable distance, often from miles away. These ice-made soils are

rarely very fertile, but they commonly have a great endurance for

tillage, and this for the reason that the earth is refreshed by the

decay of the pebbles which they contain. Moreover, while the tillable

earth of other regions usually has a limited depth, verging downward

into the semisoil or subsoil which represent the little changed bed

rocks, glacial deposits can generally be ploughed as deeply as may

prove desirable.

The drainage of a country recently affected by glaciers is always

imperfect. Owing to the irregular erosion of the bed rocks, and to the

yet more irregular deposition of the detritus, there are very numerous

lakes which are only slowly filled up or by erosion provided with

drainage channels. Though several thousand years have passed by since

the ice disappeared from North America, the greater part of the area

of these fresh-water basins remains, the greater number of them,

mostly those of small size, have become closed.

Where an ice stream descends into the sea or into a large lake, the

depth of which is about as great as the ice is thick, the relative

lightness of the ice tends to make it float, and it shortly breaks off

from the parent mass, forming an iceberg. Where, as is generally the

case in those glaciers which enter the ocean, a current sweeps by the

place where the berg is formed, it may enter upon a journey which may

carry the mass thousands of miles from its origin. The bergs separated

from the Greenland glaciers, and from those about the south pole, are

often of very great size; sometimes, indeed, they are some thousand

feet in thickness, and have a length of several miles. It often

happens that these bergs are formed of ice, which contains in its

lower part a large amount of rock _débris_. As the submerged portion

of the glacier melts in the sea water, these stones are gradually

dropped to the bottom, so that the cargo of one berg may be strewed

along a line many hundred miles in length. It occasionally happens

that the ice mass melts more slowly in those parts which are in the

air than in its under-water portions. It thus becomes top-heavy and

overturns, in which case such stony matter as remains attains a

position where it may be conveyed for a greater distance than if the

glacier were not capsized. It is likely, indeed, that now and then

fragments of rock from Greenland are dropped on the ocean floor in the

part of the Atlantic which is traversed by steamers between our

Atlantic ports and Great Britain.

Except for the risks which they bring to navigators, icebergs have no

considerable importance. It is true they somewhat affect the

temperature of sea and air, and they also serve to convey fragments of

stone far out to sea in a way that no other agent can effect; but, on

the whole, their influence on the conditions of the earth is

inconsiderable.

Icebergs in certain cases afford interesting indices as to the motion

of oceanic currents, which, though moving swiftly at a depth below the

surface, do not manifest themselves on the plain of the sea. Thus in

the region about Greenland, particularly in Davis Strait, bergs have

been seen forcing their way southward at considerable speed through

ordinary surface ice, which was either at rest or moving in the

opposite direction. The train of these bergs, which moves upward from

the south polar continent, west of Patagonia, indicates also in a very

emphatic way the existence of a very strong northward-setting current

in that part of the ocean.

* * * * *

We have now to consider the causes which could bring about such great

extensions of the ice sheet as occurred in the last Glacial period.

Here again we are upon the confines of geological knowledge, and in a

field where there are no well-cleared ways for the understanding. In

facing this problem, we should first note that those who are of the

opinion that a Glacial period means a very cold climate in the regions

where the ice attained its extension are probably in error. Natural as

it may seem to look for exceeding cold as the cause of glaciation, the

facts show us that we can not hold this view. In Siberia and in the

parts of North America bordering on the Arctic Sea the average cold is

so intense that the ground is permanently frozen--as it is, for

instance, in the Klondike district--to the depth of hundreds of feet,

only the surface thawing out during the warm summers. All this region

is cold enough for glaciers, but there is not sufficient snowfall to

maintain them. On the other hand, in Greenland, and in a less though

conspicuous degree in Scandinavia, where the waters of the North

Atlantic somewhat diminish the rigour of the cold, and at the same

time bring about a more abundant snowfall, the two actions being

intimately related, we have very extensive glaciers. Such facts, which

could be very much extended, make it clear that the climate of glacial

periods must have been characterized by a great snowfall, and not by

the most intense cold.

It is evident that what would be necessary again to envelop the boreal

parts of North America with a glacial sheet would not be a

considerable decrease of heat, but an increase in the winter's

contribution of frozen water. Even if the heat released by this

snowfall elevated the average temperature of the winter, as it

doubtless would in a considerable measure, it would not melt off the

snow. That snowfall tends to warm the air by setting free the heat

which was engaged in keeping the water in a state of vapour is

familiarly shown by the warming which attends an ordinary snowstorm.

Even if the fall begin with a temperature of about 0° Fahr., the air

is pretty sure to rise to near the freezing point.

It is evident that no great change of temperature is required in order

to bring about a very considerable increase in the amount of snowfall.

In the ordinary succession of seasons we often note the occurrence of

winters during which the precipitation of snow is much above the

average, though it can not be explained by a considerable climatal

change. We have to account for these departures from the normal

weather by supposing that the atmospheric currents bring in more than

the usual amount of moisture from the sea during the period when great

falls of snow occur. In fact, in explaining variations in the humidity

of the land, whether those of a constant nature or those that are to

be termed accidental, we have always to look to those features which

determine the importation of vapour from the great field of the ocean

where it enters the air. We should furthermore note that these

peculiarities of climate are dependent upon rather slight geographic

accidents. Thus the snowfall of northern Europe, which serves to

maintain the glaciation of that region, and, curiously enough, in some

measure its general warmth, depends upon the movement of the Gulf

Stream from the tropics to high latitudes. If by any geographical

change, such as would occur if Central America were lowered so as to

make a free passage for its waters to the westward, the glaciers of

Greenland and of Scandinavia would disappear, and at the same time the

temperature of those would be greatly lowered. Thus the most evident

cause of glaciation must be sought in those alterations of the land

which affect the movement of the oceanic currents.

Applying this principle to the northern hemisphere, we can in a way

imagine a change which would probably bring about a return of such an

ice period as that from which the boreal realm is now escaping. Let us

suppose that the region of not very high land about Bering Strait

should sink down so as to afford the Kuro Siwo, or North Pacific

equivalent of our Gulf Stream, an opportunity to enter the Arctic Sea

with something like the freedom with which the North Atlantic current

is allowed to penetrate to high latitudes. It seems likely that this

Pacific current, which in volume and warmth is comparable to that of

the Atlantic, would so far elevate the temperature of the arctic

waters that their wide field would be the seat of a great evaporation.

Noting once again the fact that the Greenland glaciers, as well as

those of Norway, are supplied from seas warmed by the Gulf Stream, we

should expect the result of this change would be to develop similar

ice fields on all the lands near that ocean.

Applying the data gathered by Dr. Croll for the Gulf Stream, it seems

likely that the average annual temperature induced in the Arctic Sea

by the free entrance of the Japan current would be between 20° and 30°

Fahr. This would convert this wide realm of waters into a field of

great evaporation, vastly increasing the annual precipitation. It

seems also certain that the greater part of this precipitation would

be in the form of snow. It appears to the writer that this cause alone

may be sufficient to account for the last Glacial period in the

northern hemisphere. As to the probability that the region about

Bering Strait may have been lowered in the manner required by this

view, it may be said that recent studies on the region about Mount St.

Elias show that during or just after the ice epoch the shores in that

portion of Alaska were at least four thousand feet lower than at

present. As this is but a little way from the land which we should

have to suppose to be lowered in order to admit the Japan current, we

could fairly conclude that the required change occurred. As for the

cause of the land movement, geologists are still in doubt. They know,

however, that the attitudes of the land are exceedingly unstable, and

that the shores rarely for any considerable time maintain their

position. It is probable that these swayings of the earth's surface

are due to ever-changing combinations of the weight in different parts

of the crust and the strains arising from the contraction of its inner

parts.

In the larger operations of Nature the effects which we behold,

however simple, are rarely the products of a single cause. In fact,

there are few actions so limited that they can fairly be referred to

one influence. It is therefore proper to state that there are many

other actions besides those above noted which probably enter into

those complicated equations which determine the climatal conditions of

the earth. To have these would carry us into difficult and speculative

inquiries.

As before remarked, all the regions which have been subjected to

glaciation are still each year brought temporarily into the glacial

state. This fact serves to show us that the changes necessary to

produce great ice sheets are not necessarily of a startling nature,

however great the consequences may be. Assuming, then, that relatively

slight alterations of climate may cause the ice sheet to come and go,

we may say that all the influences which have been suggested by the

students of glaciation, and various other slighter causes which can

not be here noted, may have co-operated to produce the peculiar

result. In this equation geographic change has affected the course of

the ocean currents, and has probably been the most influential, or at

least the commonest, cause to which we must attribute the extension of

ice sheets. Next, alterations of the solar heat may be looked to as a

change-bringing action; unfortunately, however, we have no direct

evidence that this is an efficient cause. Thirdly, the variations in

the eccentricity of the earth's orbit, combined with the precession of

the equinoxes and the rotation of the apsides, may be regarded as

operative. The last of all, changes in the constitution of the

atmosphere, have to be taken into account. To these must be added, as

before remarked, many less important actions which influence this

marvellously delicate machine, the work of which is expressed in the

phenomena assembled under the name of climate.

Evidence is slowly accumulating which serves to show that glacial

periods of greater or less importance have been of frequent occurrence

at all stages in the history of the earth of which we have a distinct

record. As these accidents write their history upon the ground alone,

and in a way impermanently, it is difficult to trace the ice times of

ancient geological periods. The scratches on the bed rocks, and the

accumulations of detritus formed as the ice disappeared, have alike

been worn away by the agents of decay. Nevertheless, we can trace here

and there in the older strata accumulations of pebbly matter often

containing large boulders, which clearly were shaped and brought

together by glacial action. These are found in some instances far

south of the region occupied by the glaciers during the last ice

epoch. They occur in rocks of the Cambrian or Silurian age in eastern

Tennessee and western North Carolina; they are also found in India

beyond the limits to which glaciers have attained in modern times.

In closing this inadequate account of glacial action, a story which

for its complete telling would require many volumes, it is well for

the reader to consider once again how slight are the changes of

climate which may alternately withdraw large parts of the land from

the uses of life, and again quickly restore the fields to the service

of plants and animals. He may well imagine that these changes, by

driving living creatures to and fro, profoundly affect the history of

their development. This matter will be dealt with in the volume

concerning the history of organic beings.

When the ice went off from the northern part of this continent, the

surface of the country, which had been borne down by the weight of the

glacier, still remained depressed to a considerable depth below the

level of the sea, the depression varying from somewhere about one

hundred feet in southern New England to a thousand feet or more in

high latitudes. Over this region, which lay beneath the level of the

sea, the glacier, when it became thin enough to float, was doubtless

broken up into icebergs, in the manner which we now behold along the

coast of Greenland. Where the shore was swept by a strong current,

these bergs doubtless drifted away; but along the most of the coast

line they appear to have lain thickly grouped next the shores,

gradually delivering their loads of stones and finer _débris_ to the

bottom. These masses of floating ice in many cases seem to have

prevented the sea waves from attaining the shore, and thus hindered

the formation of those beaches which in their present elevated

condition enable us to interpret the old position of the sea along

coast lines which have been recently elevated. Here and there,

however, from New Jersey to Greenland, we find bits of these ancient

shores which clearly tell the story of that down-sinking of the land

beneath the burden of the ice which is such an instructive feature in

the history of that period.