GLACIERS AND THEIR WORK
A glacier may be defined as a mass of flowing ice. The motion may
not be that of flowage in the usually accepted sense of the term. A
discussion of the various theories of glacier motion will not here be
attempted. Glaciers form only in regions of perpetual snow, but they
commonly move down far below the line of perpetual snow of any given
region. In the polar regions they may form near sea level, while in the
tropics they form at altitudes of two to three miles, and there only
rarely. In southern Alaska, the lower limit of perpetual snow is about
5,000 feet above sea level, and many of the glaciers come down to sea
(Plate 4), while in the Alps, the lower limit of perpetual snow is at
about 9,000 feet, and the glaciers descend as much as 5,000 feet below
it.
In regions of perpetual snow there is a tendency for more or less snow
to accumulate faster than it can be removed by evaporation or melting.
As such snow accumulates it gradually undergoes a change, especially in
its lower parts, first into granulated snow (so-called "névé") and then
into solid ice. Snow drifts in the northern United States often undergo
similar transformation, after a few months first to névé, and then to
ice. This transformation seems to be brought about mainly by weight of
overlying snow which compacts the snow crystals; by rain or melting
snow percolating into the snow to freeze and fill spaces between the
snow crystals; and by the actual growth of the crystals themselves.
When ice of sufficient thickness has accumulated (probably at best
several hundred feet), the spreading action or flowage begins and a
glacier has developed. Renewed snowfalls over the gathering ground keep
up the supply of ice.
There are several types of glaciers: valley or alpine glaciers; cliff
or hanging glaciers; piedmont glaciers; ice caps; and continental ice
sheets. A valley or alpine glacier consists essentially of a stream
of ice slowly flowing down a valley and fed from a catchment basin of
snow within a region of perpetual snow. In the Alps, where glaciers of
this sort are very typically shown, they vary in length up to eight or
nine miles. Perhaps the grandest display of great valley glaciers is in
southern Alaska where they attain lengths up to forty or fifty miles
and widths of one or two miles (Plate 4).
Hanging or cliff glaciers are in many ways like valley glaciers, but
they are generally smaller; they develop in snow-filled basins above
the snow line usually on steep mountain sides; and they do not reach
down into well-defined valleys. Most of the glaciers of the Glacier
National Park in Montana and many of those in the Cascade Mountains are
of this type. Mount Rainier in Washington is one of the most remarkable
single large mountain peaks in the world, in regard to development of
glaciers over it. Great tongues of ice, starting mostly at 8,000 to
10,000 feet above sea level, flow down the sides of the mountain for
distances of to four and even six miles. The total area of ice in this
remarkable system of radiating glaciers on this one mountain is over
forty square miles. These Mount Rainier glaciers are in general best
classified as intermediate in type between valley and hanging glaciers.
[Illustration: Fig. 6.--Map of Mount Rainier, Washington, showing its
wonderful system of glaciers which covers more than 40 square miles.
Dotted portions represent moraines. (U. S. Geological Survey.)]
In some high latitude areas, as in Iceland and Spitzbergen, snow and
ice may accumulate on relatively level plains or plateaus and slowly
spread or flow radially from their centers. These are called ice caps.
Ordinary ice caps usually do not cover more than some hundreds of
square miles.
Continental glaciers or ice sheets are, in principle, much like ice
caps, but they are larger. Greenland is buried under an ice sheet of
moderate size (about 500,000 square miles), the motion being outward in
all directions toward the sea. Tongues of ice, like valley glaciers,
are commonly sent off from the main body of ice across the land
border of Greenland into the sea. The size of the great ice sheet of
Antarctica is not definitely known, but it covers probably at least
several million square miles. Two continental ice sheets of special
interest to the geologist are those which existed during the great
Ice Age of the Quaternary period. One of these then covered nearly
4,000,000 square miles of North America, while the other covered about
600,000 square miles of northern Europe. The main facts regarding
the Ice Age are given in a succeeding chapter. The facts brought out
in the present discussion of existing glaciers will greatly aid in
understanding the Ice Age.
How fast do glaciers flow? Based upon many observations, we may say
that an average rate of flow for the glaciers of the world is not more
than a few feet per day. A very exceptional case is a large glacier,
branching off as a tongue from the ice sheet of Greenland, which is
said to move sixty to seventy-five feet per day. Some of the great
Alaskan glaciers have been found to flow from four to forty feet per
day. Most glaciers of the Alps move only one to two feet per day. A
glacier advances only when the rate of motion is greater than the rate
of melting of its lower end and vice versa in the case of retreat. Thus
it is true, though seemingly paradoxical, to assert that a glacier has
a constant forward motion even when it is retreating by melting.
By watching the changing position of marked objects placed in the ice,
it has been proved that, in a valley glacier, the top moves faster
than the bottom; the middle moves faster than the sides; the rate of
motion increases with thickness of ice, slope of floor over which it
moves, and temperature.
Ice, like molasses candy, tends to crack when subjected to a relatively
sudden force, and where the ice rides over a salient on the bed of
the glacier, transverse cracks or fissures often develop. Due to more
rapid motion of the central part of a valley glacier, stresses and
strains are set up and crevasses are formed, usually pointing obliquely
upstream. Where the ice tends to spread laterally in a broad portion
of a valley, longitudinal cracks may develop. Crevasses vary in size
up to several feet in width and hundreds of feet in depth. Owing to
the forward motion of the ice, old fissures tend to close up and new
ones form, and, aided by uneven melting, the surface of a glacier is
generally very rough.
Like running water, ice may have considerable erosive power when it is
properly supplied with tools. The total erosive effect which has been,
and is now being, accomplished by ice compared with that of running
water is, however, slight. One of the main processes by which ice
erosion is accomplished is "corrasion" due to the rubbing or grinding
action of hard rock fragments frozen into the bottom and sides of the
glacier. Thick ice, shod with hard rock fragments and flowing through a
deep, narrow valley of soft rock, is especially powerful as an erosive
agent because the abrasive tools are supplied; the work to be done is
easy; and the deep ice causes great pressure on the bottom and lower
sides of the valley. Rock surfaces which have been thus subjected to
ice erosion are characteristically smoothed and more or less scratched,
striated, or ground due to the corrosive effects of small and large
rock fragments. This affords one of the best means of proving the
former presence of a glacier over a region or in a valley. A typical
V-shaped stream cut (eroded) valley is changed into one with a U-shaped
profile or cross section by glacier erosion (Plate 5).
Another important process of ice erosion is "plucking," which consists
in pushing among already more or less loosened joint blocks by the
pressure of the moving ice. The pressure thus exerted, especially by
a deep valley glacier, may be enormous. This process was an important
factor in the development of the famous Yosemite Valley, a very brief
account of whose history it will now be instructive to give.
[Illustration: Plate 3.--The Gorge of Niagara River Below the Great
Falls. The strata (containing fossils) were accumulated on the bottom
of the Silurian sea which overspread the region at least 18,000,000
years ago. Since the Ice Age or within 20,000 to 40,000 years, the
river has carved out the gorge. (_Courtesy of the Haines Photo Company,
Conneaut, Ohio._)]
[Illustration: Plate 4.--(_a_) A Winding Stream in the St. Lawrence
Valley of New York. Due to its low velocity the stream cuts its channel
down very little, but it swings or "meanders" slowly from one side of
its valley to the other, developing a wide flood plain. The stream once
flowed against the valley wall shown at the middle left. (_Photo by the
author._)]
[Illustration: Plate 4.--(_b_) Davidson Glacier, Alaska. This glacier
is at work slowly grinding down the valley floor and cutting back its
walls, thus changing the original stream-cut, V-shaped profile, like
that of Plate 5. (_Photo by Wright, U. S. Geological Survey._)]
The Yosemite Valley, about 7 miles long, less than one mile wide,
and from 2,000 to 4,000 feet deep, lies on the western slope of the
Sierra Nevada Mountains of California. Great cliffs of granite,
mostly from 1,000 to over 3,000 feet high, bound the valley on either
side. The floor of the valley is wide and remarkably flat (Plate 6).
Just prior to the Ice Age, by the processes of erosion already set
forth, the Merced River had carved out a great steep-sided V-shaped
canyon commonly from 1,000 to 3,000 feet deep. During the Ice Age,
two glaciers joined to form an extra deep powerful glacier, which
flowed through a deep part of the Merced Canyon and modified it into
the Yosemite Valley, essentially as we see it to-day. Because the ice
was shod with many fragments of hard rock (granite), and the pressure
at the bottom and lower sides of the glacier (several thousand feet
thick) was so great, the V-shaped stream-cut canyon was changed to a
U-shaped canyon with very steep to even vertical walls. A factor of
great importance which notably aided the erosive power of the glacier
in this case was the existence of an unusual number of large vertical
joint cracks in the granite in this local region. The plucking action
of the ice was thus very greatly facilitated and great slabs of rock,
separated by the vertical joints, especially toward the lower sides
and bottom of the valley, were pushed away one after another by the
ice. When the ice disappeared, great precipitous joint faces from 1,000
to 3,000 feet high were left along the valley sides. At its lower
end the glacier left a dam of glacial débris (moraine) across the
valley, thus causing a lake to form over the valley floor. The wide
flat bottom of the valley was caused by filling up of the lake with
sediment. The uniqueness of the Yosemite Valley is, then, due to a
remarkable combination of several main factors; one, the presence of
a large swift river well supplied with tools which carved out a deep
V-shaped canyon; two, a mighty glacier which plowed its way through
this canyon and converted it by erosion into a U-shaped canyon;
three, the weakening of the rock by many joint cracks, thus greatly
facilitating the ice erosion; and four, a postglacial lake covering
the valley floor which became filled with sediment. As a result of the
ice work, several streams, tributary to the main stream (Merced River)
which flows through the bottom of the valley, were forced to plunge
over great vertical rock walls (joint faces), thus producing high and
beautiful true waterfalls, including the very high Upper Yosemite Fall
where Yosemite Creek makes a straight drop of 1,430 feet. A tributary
valley like that of Yosemite Creek, which ends abruptly well above
the main valley, is known as a "hanging" valley. The valley of Bridal
Veil Creek is another good example. (See Plate 6.) Valleys which were
once occupied by active glaciers are generally characterized by their
U-shaped cross sections and their hanging (tributary) valleys, but
the great height and steepness of the valley walls in Yosemite are
exceptional.
A type of glacial erosion which is of special interest is the
sculpturing of so-called "cirques" or "amphitheaters" in mountains
within the region of perpetual snow. Where the main mass of snow and
ice in the catchment basin or gathering ground of a valley glacier
pulls away from the snow and névé on the upper slopes, the rock wall
is more or less exposed in the deep crevasse. During warm days water
fills the joint cracks in the rocks down in this crevasse (so-called
"Bergschrund"), and during cold nights the water freezes and forces
the blocks of rock apart. This is greatest toward the bottom of the
crevasse and so, by this excavating or quarrying process, vertical or
very steep walls are developed around a great bowlike basin or cirque.
Such cirques, now free from glacial ice, with precipitous walls 500 to
2,000 feet high and one-fourth of a mile to one-half of a mile across,
are common in the Sierra Nevada and Cascade Ranges and in the Rocky
Mountains.
What becomes of the materials eroded by the ice? An answer to this
question involves at least a brief discussion of the deposition of
glacial débris, this constituting an important feature of the work
of ice. The débris transported by a glacier is carried either on
its surface or within it, or pushed along under it. It is generally
heterogeneous material ranging from the finest clay through sand and
gravel, to bowlders of many tons' weight. Various types of glacial
deposits are abundantly illustrated by débris left strewn over much
of the northeastern United States and some reference to these will be
made.
Most valley glaciers carry considerable débris on their surfaces, this
representing material which falls or is carried down from the valley
walls upon the margins of the ice, thus forming marginal moraines.
When two glaciers flow together, one marginal moraine from each will
coalesce to form a medial moraine. The material carried along at the
bottom of a glacier is called the ground moraine. Where it contains
much very fine grained material with pebbles or bowlders scattered
through its mass, it is called "till" or "bowlder clay." The pebbles or
bowlders of the ground moraine are commonly facetted and striated as a
result of having been rubbed against the bedrock on which the glacier
moved. Ground moraine material is the most extensively developed of
all glacial deposits. It is so widely scattered over the glaciated
northeastern portion of the United States that most of the soils
consist of it, having been left strewn over the country during the
melting of the vast ice sheet.
When a glacier remains practically stationary for some time, more or
less material which it carries is piled up at its lower end to form
a terminal moraine. Repeated pauses during general glacier retreat
permit the accumulations of so-called recessional moraines. A wonderful
display of recessional moraines occurs from the Great Lakes south,
where they are festooned one within another and remain almost exactly
as they were formed during pauses in retreat of great lobes of ice
during the closing stages of the Ice Age. A great terminal moraine
marks the southernmost limit of the ice sheet during the Ice Age,
a very fine illustration being the ridge of low irregular hills
extending the whole length of Long Island. Some of the material in that
morainic ridge was transported by the ice from northern New England.
Considerable rock débris is transported within the ice, and such
"englacial" material in part results from rock débris which falls
on the surface in the catchment basin and becomes buried under new
snowfalls which change to ice, and in part from material which falls
into the crevasses in the glacier farther down the valley. Marked
objects thrown into the catchment basin have, after many years, emerged
at or near the end of the glacier; thus the rate of motion can be very
accurately told. A very remarkable case of transportation through the
body of a glacier is the following: In 1820, three men were buried
under an avalanche in the catchment basin of the Bossons Glacier in
the Alps. Forty-one years later several parts of the bodies, including
the three heads together with some pieces of clothing, emerged at the
foot of the glacier after traveling most of its length at the rate of
eight inches per day. The heads were so perfectly preserved after their
remarkable journey in cold storage that they were clearly recognized by
former friends!
Where a valley floor slopes downward away from the end of a glacier,
waters emerging from the ice, heavily loaded with rock débris, cause
more or less deposition of the débris on the valley floor often for
miles beyond the ice front. Such a deposit is called a "valley train."
When the ice front pauses for a considerable time upon a rather flat
surface, the débris-laden waters emerging from the ice develop an
"outwash plain" by deposition of sediment rather uniformly over the
flat surface. A very fine example is the plain which constitutes most
of the southern half of Long Island just beyond the southern limit of
the great terminal moraine ridge.
A type of glacial deposit of particular interest is the "drumlin"
which is, in reality, only a special form of ground moraine material
(commonly till), and, therefore, essentially unstratified. Typical
drumlins are low, rounded mounds of till with roughly elliptical bases
and steeper fronts facing the direction from which the ice flowed.
Their long axes are always parallel to the direction of ice movement.
In height they commonly range from 50 to 200 feet. Their mode of origin
is not yet definitely known, but they form near the margins of broad
lobes of ice either by erosion of earlier glacial deposits, or by
accumulation beneath the ice under peculiarly favorable conditions,
as perhaps in the longitudinal crevasses. One of the finest and most
extensive exhibitions of drumlins in the world is in western New York
between Syracuse and Rochester. Thousands of drumlins there rise above
the general level of the Ontario plain, the New York Central Railroad
passing through the very midst of them. Drumlins are also abundant in
eastern Wisconsin.
Another type of glacial deposit in the form of low hills is the "kame"
which, unlike the drumlin, always consists of more or less stratified
material. Kames are seldom over 200 feet high, and they are of various
shapes. In many cases they form irregular groups of hills, and in
other cases fairly well defined kame ridges. Kames form as deposits
from débris-laden streams emerging from the margins of glaciers, the
water sometimes rising as great fountains because of the pressure. Such
deposits are now actually in process of formation along the edge of
the great Malaspina Glacier of Alaska. Kames are commonly associated
with terminal and recessional moraines. "Eskers" are similar except
that they are long winding low ridges of stratified material deposited
by débris-laden streams, probably in longitudinal fissures in the ice
near its margin. (See Plate 20.)
Glacial bowlders, or "erratics" are blocks of rock or bowlders left
strewn over the country during the melting of the ice. They vary in
size from small pebbles to those of many tons of weight, and most of
them were derived from ledges of relatively hard, resistant rocks. (See
Plate 20.) Erratics have very commonly been carried a few miles from
their parent ledges, while more rarely they have traveled even hundreds
of miles. They are extremely abundant in New York and New England,
many occurring even high up on the mountains. In some cases erratics
of ten or more tons' weight have been left in such remarkably balanced
positions on bedrock that a child can cause one of them to swing back
and forth slightly. Such a bowlder is literally a "rocking stone."
In the Adirondack Mountains the writer recently observed a rounded
erratic of very hard rock fourteen feet in diameter resting in a very
remarkably balanced position on top of another large round glacial
bowlder.