THE GLACIERS OF MOUNTAIN AND CONTINENT
=Conditions essential to glaciation.=—Wherever for a sufficiently
protracted period the annual snowfall of a district is in excess of
the snow that is melted, a residue must remain from each season to be
added to that of succeeding ones. Eventually so much snow will have
accumulated that under its own weight and in obedience to its peculiar
properties, a movement will begin within the mass tending to spread
it and so to reduce the slope of its upper surface (Frontispiece
plate). The conditions favorable to glaciation are, therefore, heavy
precipitation and low annual temperature. If the precipitation is
scanty, the small snowfall is soon melted; and if the temperature be
too high, the moisture is precipitated not in the form of snow but as
rain. It is important here to keep in mind that snow is a poor heat
conductor and itself protects its deeper layers from melting.
=The snow-line.=—Because of the low temperatures glaciers should
be most abundant or most extensive in high latitudes and in high
altitudes. The largest are found in polar and subpolar regions, and
they are elsewhere encountered only at considerable elevations.
The largest glaciers are the vast sheets of ice which inwrap the
continents of Greenland and Antarctica, but glaciers of large size
are to be found upon other large land masses of the Arctic, as well
as in Alaska, in the southern Andes, and in New Zealand. Much smaller
glaciers are characteristic of certain highlands within temperate and
tropical regions, but because of specially favorable conditions both of
altitude and precipitation the Himalayas, although in relatively low
latitudes, nourish glaciers of large proportions. In general, it may
be said that the nourishing grounds of glaciers are largely restricted
to those areas where snow covers the ground throughout the year. The
lower margin of such areas is designated the _snow line_, and varies
but little from the line on which the average summer temperature is at
the freezing point of water—the so-called _summer isotherm of 32°
Fahrenheit_. Within the tropics this line may rise as high as 18,000
feet above the sea, whereas in polar latitudes it descends to sea level.
=Importance of mountain barriers in initiating glaciers.=—The
precipitation within any district depends, however, not alone upon the
amount of moisture which is brought to it in the clouds, but upon the
amount which is abstracted before the clouds have passed over it. The
capacity of space to hold moisture increases with its temperature, and
hence any lowering of this temperature will reduce the capacity. If
lowered sufficiently, the point of complete saturation will be reached
and further cooling must result in precipitation. Hence, anything which
forces an air current to rise into more rarefied zones above, will
lower the pressure upon it and so bring about a cooling effect in which
no heat is abstracted. This so-called _adiabatic refrigeration_ of a
gas may be illustrated by the cool current which issues in a jet from
a warm expanded rubber tire after the cock has been opened; or even
better, by the instant solidification at extreme low temperatures of
such normal gases as carbonic acid when they are allowed to issue under
heavy pressure from a small orifice.
As applied to moisture-laden and near-surface winds, the effective
agents of adiabatic cooling are the upland areas upon the continents,
and especially the ranges of mountains. These barriers force the moving
clouds to rise, cool, and deposit their moisture. It is, therefore, the
highland barriers which face the oncoming, moisture-laden winds that
receive the heaviest precipitation. Within temperate regions, because
of the prevalence of westerly winds, those barriers which face the
western shores receive the heaviest fall. Within the tropics, on the
other hand, it is the barriers facing the eastern shores which, because
of the easterly “trades”, are most favorable to precipitation.
Thus it is in the Sierra Nevadas of California, and not in the Rockies
or the Appalachians, that the glaciers of the United States are found.
The highland of the Swiss Alps lying likewise athwart the “westerlies”
of the temperate zone acquires the moisture for nourishment of its
glaciers from the western ocean—here the Atlantic (Fig. 291). Within
the tropics the conditions are reversed, and it is in general the
ranges which lie nearer the eastern coasts that are the more favored.
If no barrier is found upon this coast, the clouds may travel over
vast stretches of country before being arrested by mountains and robbed
of their moisture. Thus in tropical Brazil the glaciers are found in
the Andes upon the Pacific coast though nourished by clouds from the
Atlantic.
[Illustration: FIG. 291.—Map showing the distribution of existing
glaciers, and the two important wind poles of the earth.]
=Sensitiveness of glaciers to temperature changes.=—How sensitive
is the adjustment between snow precipitation and temperature may be
strikingly illustrated by the statement on excellent authority that if
the average annual temperature of the air within the Scottish Highlands
should be lowered by only three degrees Fahrenheit, small glaciers
would be the result; and a moderate temperature fall within the region
surrounding the Laurentian lakes of North America would bring on
glaciation, otherwise expressed as a depression of the snow line of the
region.
=The cycle of glaciation.=—Though to-day buried beneath its ice
mantle, it is known that Greenland had more than once in earlier
geological ages a notably mild climate, and in some future age it may
revert to this condition. In other regions, also, we have evidence
that such a rotation of climatic changes has been successively
accomplished, the climate having steadily increased in severity towards
a culminating point, and been followed by a reverse series of changes.
Such a complete period may be called a _cycle of glaciation_. While
the climate is steadily becoming more rigorous, we have to do with
an _advancing hemicycle_ of glaciation, but after the culminating
point has been reached, the period of amelioration of climate is the
_receding hemicycle_.
=The advancing hemicycle.=—There is little reason to doubt that
whatever be the cause of the climatic changes which bring on glacial
conditions, these changes come on by insensible gradations. The
first visible evidence of the increased severity of the climate is
the longer persistence of the winter snows, at first within the more
elevated districts. In such positions drifts must eventually continue
throughout the warm season and so contribute to the snow accumulations
of the succeeding winter. This point once reached, small glaciers are
inevitable, even should the average temperature fall no further, for
the snow left over in each season must steadily increase the depth of
the deposits until the weight brings about an internal motion of the
mass from higher to lower levels.
[Illustration: FIG. 292.—An Alaskan glacier spreading out at the foot
of the range which nourishes it.]
The inherited depressions of the upland—the gentle hollows at the
heads of rivers—will first be filled, and so the valleys below become
the natural channels for the outflow of the early glaciers. With a
continued lowering of the annual temperature and consequent increased
snowfall, the early glaciers become more and more amply nourished. Snow
and ice will, therefore, cover larger areas of the upland, and the
glaciers will push their fronts farther down the valleys before they
are wasted in the warm air of the lower levels. As the valleys become
thus more completely invested by the glacier they are likewise filled
to greater and greater depths, and they may thus submerge portions of
the walls that separate adjacent valleys. Reaching at last the front
of the upland area, the glaciers may now be so well nourished at
their heads that they push out upon the flatter foreland and without
restraint from retaining walls spread broadly upon it (Fig. 292).
[Illustration: FIG. 293.—Surface of a glacier whose upper layers
spread with slight restraint from retaining walls. Surface of the
Folgefond, an ice cap of southern Norway.]
The culmination of the progressive climatic change may ere this have
been reached and milder conditions have ensued. If, however, the
severity of the climate should be still further increased, the expanded
fronts of neighboring glaciers will coalesce to form a common ice fan
or apron along the foot of the upland (Plate 18 B). This could hardly
take place without a still further deepening of the ice within the
valleys above, and, probably, a progressive submergence of the lower
crests in the valley walls. This may even continue until all parts of
the upland area have been buried. The snow and ice now take the form
of a covering cap or carapace, and the upper portions being no longer
restrained at the sides, now spread into a broad dome, as would a
viscous liquid like thick molasses when poured out upon the floor (Fig.
293). The lower zones of the mass and the thinner marginal portions
still have their motion to a greater or less extent controlled by the
irregularity of the rock floor against which they rest.
The reverse series of changes in the glacier is inaugurated by an
amelioration of the climate, and here, therefore, the advancing
hemicycle becomes merged in the receding hemicycle of glaciation.
=Continental and mountain glaciers contrasted.=—The time when the
rock surface becomes submerged beneath the glacier is, as regards
both the surface forms and the erosive work, a critical point of much
significance; for the ice cap and larger continental glacier obviously
protect the rock surface from the action of those chemical and
mechanical processes in which the atmosphere enters as chief agent, and
which are collectively known as weathering processes. Until submergence
is accomplished, larger or smaller portions of the rock surface project
either through or between the ice masses and are, therefore, exposed to
direct attack by the weather (see below, p. 370).
[Illustration: FIG. 294.—Section through a mountain glacier (in solid
black), showing how its surface is determined by the irregularities in
the rock basement (after Hess).]
Snow which falls in the mountains is not allowed to remain long where
it falls. By the first high wind it is swept off the more elevated and
exposed surfaces and collected under eddies in any existing hollows,
but especially those upon the lee slopes of the range. We are to learn
that glaciers carve the mountains by enlarging the hollows which they
find and producing great basins for the collection of their snows; but
with the initiation of glaciation the inherited hollows are in most
cases the unimportant depressions at the heads of streams. Whatever
they may be and however formed, the snow first fills those hollows
which are sheltered from the wind, and as it accumulates and becomes
distributed as ice, assumes a surface of its own that is dependent upon
the form and the position of the basin which it occupies (see Fig. 294).
[Illustration: FIG. 295.—Profile across the largest of the Icelandic
ice caps, with the vertical scale greatly exaggerated (after Thoroddsen
and Spethmann).]
When the quantity of accumulated snow is so great that all hollows of
the rock surface are filled, its own surface is no longer controlled
by retaining rock walls, and it now assumes a form largely independent
of the irregularities in the upland. Experience shows that this surface
is approximately that of a flat dome or shield, and as it covers all
the upland, save where the ice thins upon its margins, this type of
glacier is called an _ice cap_ (Fig. 295). All types of glacier in
which rock projects above the highest levels of the ice and snow are
known as _mountain glaciers_.
[Illustration: FIG. 296.—Ideal section across a continental glacier,
with the vertical scale and the projecting rock masses of the marginal
zone greatly magnified.]
The flat domes of ice which mantle the continents of Greenland and
Antarctica, though resembling in form the smaller ice cap, are yet
because of their vast size so distinct from them, particularly in the
manner of their nourishment, that they belong in a separate class
described as _inland ice_ or _continental glaciers_. Though they have
some affinities with ice caps, they are most sharply differentiated
from all types of mountain glaciers. Of them it is true that the
lithosphere projects through them only in the neighborhood of their
margins (Fig. 296), whereas in the case of mountain glaciers rock may
project at any level but _always above the highest snow surface_. Ice
caps may be regarded as intermediate between the two main classes of
mountain and continental glaciers (Fig. 297). Because of the large rôle
which continental glaciers have played in geological history, it is
thought best to consider them first, leaving for later discussion the
no less interesting but less important mountain glaciers.
[Illustration: FIG. 297.—View of the Eyriks-Jökull, an ice-cap of
Iceland (after Grossman).]
=The nourishment of glaciers.=—The life of a glacier is dependent upon
the continued deposition of snow in aggregate amount in excess of that
which is lost by melting or by other depleting processes. Whenever, on
the other hand, the waste exceeds the precipitation, the glacier is in
a receding condition and must eventually disappear, if such conditions
are sufficiently long continued. The source of the snow is the water
of the ocean evaporated into the atmosphere and transported over the
land in the form of clouds. We are to learn that the changes which
this moisture undergoes before its delivery to the glacier are notably
different for the classes of continental and mountain glacier.
=The upper and lower cloud zones of the atmosphere.=—Before we can
comprehend the nature of the processes by which glaciers are nourished,
it will be necessary to review the results of recent studies made upon
the earth’s atmospheric envelope. It must be kept in mind that the
sun’s rays are chiefly effective in warming the atmosphere through
being first absorbed by some solid body such as rock or water and
their heat then communicated by contact to the immediately adjacent
air layers. The layers thus warmed being now lighter than before,
they rise and are replaced by colder air, which in its turn is warmed
and likewise set in upward motion. Such currents developed in the air
by contact with warmer solid bodies constitute the process known as
convection.
[Illustration: FIG. 298.—The zones of the lower atmosphere as revealed
by recent kite and balloon explorations.]
To a relatively small degree the atmosphere is heated by the direct
absorption of the sun’s rays which pass through it. Since air has
weight, it compresses the lower layers near the earth, and hence as we
ascend from the earth’s surface the air becomes continually lighter.
Convection currents must, therefore, adjust themselves by the air
expanding as it rises. But expansion of a gas always results in its
cooling, as every one must have observed who has placed his finger in
the air current which escapes from the open valve of a warm rubber
tire. Dry air is cooled a degree Fahrenheit for every six hundred feet
of ascent in the atmosphere. At a height of about seven miles above
the earth’s surface all rising air currents have cooled to about 68°
below the zero of the Fahrenheit scale, and exploration with balloons
has shown that the currents rise no farther. At this level they move
horizontally, just as rising vapor spreads out in a room beneath the
ceiling. Above this level, as far as exploration has gone, or to a
height of more than twelve miles, the temperature remains nearly
constant, and this upper zone is, therefore, called the _isothermal_
or the _advective zone_—the uniform temperature zone of the lower
atmosphere. Beneath the convective ceiling the process of convection is
characteristic, and this zone is therefore described as the _convective
zone_ (Fig. 298).
A large part of the moisture which rises from the ocean’s surface is
condensed to vapor before it has ascended three miles, and in this form
it makes its transit over land as fleecy or stratiform clouds—the
so-called cumulus and stratus clouds and their many intermediate
varieties (see Frontispiece). This lower layer within the convective
zone is, therefore, a moist one overlaid by a relatively drier middle
layer of the convective zone. That moisture which rises above the lower
cloud layer is congealed by adiabatic cooling to fine ice needles
visible as the so-called cirrus clouds which float as feathery fronds
beneath the convective ceiling (see frontispiece at right upper corner
of picture). Thus we have within the convective zone an upper layer
more or less charged with water in the form of ice needles. It is the
clouds of the lower zone whose moisture in the form of vapor supplies
the nourishment of mountain glaciers, and the high cirrus clouds whose
congealed moisture, after interesting transformations, is responsible
for the continued existence of continental glaciers.
As we are to see, there are other noteworthy differences between
continental and mountain glaciers, in the manner of their sculpture
of the lithosphere, so that long after they have disappeared the
characters of each are easily identified in their handiwork. How the
lower clouds are forced upward and so compelled to give up their
moisture to feed the mountain glaciers, and how the upper clouds are
pulled downward to nourish the glaciers of continents, can be best
understood after the characteristics of each glacier class have been
studied.