Israel C. (Israel Cook) Russell.

Glaciers of North America; a reading lesson for students of geography and geology online

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into the depths of the glacier with a deep roar, telling of caverns far
below the surface. The crevasses into which surface streams find their
way are frequently enlarged, and become well-like openings, or moulins,
as they are termed, which are sometimes several yards in diameter, and
of great depth. In many instances, these openings must penetrate to
the very bottom of a glacier. When this happens, the boulders and
stones that find their way into them are washed about, and are given
a rotary motion by the descending waters, so as to act as veritable mill-
stones, and grind the rocks beneath. The result of this action is the
formation of pits and holes in the rocks, resembling kettles, and termed
pot holes, in which the stones that made them may frequently be found.
These peculiar excavations are- well known in regions of former glaci-
ation. Typical examples in the Glacier garden, near Lucerne, Swit-
zerland, are familiar to many.

The surface melting of glaciers leads to the formation of broad, shal-
low lakes. These appear especially on the neves, and by the intensity
of their deep blue color impart an additional charm to the wintry scenes
reflected from their surfaces. The shores of such lakes afford favorable
camping places for glacier explorers, since water, the only necessity of
camp life to be found in such regions, can there be had without the


expense of time and fuel necessary to procure it by melting snow.
Many times during two expeditions conducted by the writer on the
broad neve fields' of southern Alaska, we had occasion to pitch our
tent by the shores of these snow-bound lakes, and fully appreciated the
advantages they afforded. In other instances, when necessity required
us to camp on greatly crevassed snow, our water supply was sometimes
obtained from the crevasses by means of a bucket attached to a line.

The water formed on the surfaces of glaciers, and draining from the
land surrounded by them, or lying in front and sloping towards them,
finds its way into the ice and escapes by tunnels situated either at the
bottom of the glacier or in the ice itself. At the ends of alpine glaciers,
and about the margins of both piedmont and continental ice sheets, there
are ice caverns from which flow turbid streams of ice-cold water. (Fig. B,
Plate 17.) The archways are the mouths of tunnels into which one
can sometimes penetrate for a long distance. The streams issuing from
such openings are supplied by both surface and basal melting, and pos-
sibly also by subglacial springs. These tunnels appear in all stages of
glacial growth, and are kept open even when ice sheets reach great dimen-
sions. On Malaspina glaciers, the course of such tunnels can in some
instances be followed for miles, by listening to the muffled roar of the
rivers rushing along through ice caverns far below the surface. Some of
the tunnels, through which the waters formed by the melting of glaciers
escape, are known to be situated on the underlying rock, but in other
instances the openings traverse the ice itself, perhaps several hundred feet
above its bottom. The tunnels through the body of the ice are thought
to have originated from crevasses which allowed the surface water to
escape from one break to another, and maintain a continuous passage-way.
But observations proving this to be the true explanation are wanting.
In the sides of deep crevasses in the Malaspina glaciers one some-
times discovers a circular opening several feet in diameter, which reveals
the position of an abandoned tunnel. In traversing the extremely rough
outer margin of the glacier referred to, these openings were at times
of great assistance, as they allow an explorer to pass from one deep
valley in the ice to another, and thus avoid a steep climb over moraine-
covered ice.

The drainage of glaciers, particularly those of the piedmont and conti-
nental types, is of special geological interest, for the reason that vast
quantities of mud, sand, gravel, etc., are carried into the tunnels through
which the sub- and englacial streams flow, and either left on the bottoms of


the channels, or swept out at the margins of the ice and deposited in part
over the adjacent land. The sediments now forming about the border of
Malaspina glacier are of great volume, and of more geological interest
than even the abandoned moraines left by the slowly retreating ice mass.
As will be described, thousands of acres of dense forest are there being
overwhelmed and buried by the deposits of streams, that pour out from
the ice, heavily freighted with sediment and even sweeping along large

The chief characteristic of the streams that emerge from beneath
glaciers is their peculiar turbidity or milkiness. In exploring regions
where the glaciers are small arid hidden in sheltered recesses about high
peaks, one is frequently enabled to discover them by noting the character
of the waters flowing from the mountains. Upland streams not fed by
melting glaciers are usually clear and sparkling, except during storms,
while those born in ice caverns are rendered opalescent, and have a pecu-
liar greenish-yellow tint, on account of the extremely fine material sus-
pended in them. This fine rock flour, as it is termed, is retained in sus-
pension even after the streams emerge from the highlands and flow through
adjacent plains. Deposits of fine sediment of peculiar geological interest
are formed by such streams, and enable one to interpret similar accumula-
tions termed loess, left about the margins of ice sheets that have now passed
away, and along the stream channels leading from them. The bluffs of fine,
yellowish, clay-like material along the Mississippi and Missouri are of this


The preceding paragraphs contain, I believe, an enumeration of the
principal characteristics of glaciers. Although it is difficult, and perhaps
impossible, to frame a concise definition of a glacier which will embrace
all ice bodies that should be properly included, and exclude other accumu-
lations of snow and ice to which the name should not be applied, yet it
seems safe to assert that any considerable mass of snow and ice which
presents a number of the characteristics referred to above may with
propriety be included in the term.

As a provisional definition, it may be said that a glacier is an ice body
originating from the consolidation of snow in regions where secular accu-
mulation exceeds melting and evaporation, i.e. above the snow line, and
flowing to regions where waste exceeds supply, i.e. below the snow line.
Accompanying these primary conditions are many secondary phenomena


dependent upon environment, such as the grain of the ice, crevasses, melt-
ing, laminations, dirt bands, moraines, glacier tables, ice pyramids, sand
cones, etc., which may or may not be present. Glaciers, even of large
size, may exist in which few and perhaps none of these details can be dis-
covered. We may conceive of a glacier as flowing through a channel so
even and so well adjusted to its progress that no crevasses will be formed.
So little debris may reach its surface that moraines and all accompanying
details will be absent. The most persistent features of an ice stream
are, perhaps, the slow movement or downward flow in both the neve and
ice regions, the stratification of the neve, and the laminated structure
and grain of the glacier proper. Yet even these important characteristics
may not be readily discernible, even in ice sheets that are unques-
tionably true glaciers. Although the brief definition given above may
assist one in obtaining an idea of what constitutes a glacier, it is mani-
festly open to qualifications and exceptions. If we consider the snow line
as defining the boundary between the neve and the glacier proper, it is
evident that there must be numerous exceptions to the rule. As before
remarked, during certain years, and at times for many years in succession,
the snow line is much lower than at other times, and may even completely
conceal the hard ice which usually protrudes below the neve. Again, an
ice stream may end in the sea, and be broken off and float away as bergs,
before the division into neve and glacier proper is distinguishable on the
surface. One of the most characteristic features of glaciers is their slow
flowing motion, yet in their old age this may cease, so that the limits
between a true ice stream and an inert ice mass may be indefinite, and
perhaps impossible to define.

From what has been learned concerning glaciers it is evident that they
form one of the transition phases in the history of drainage in many
regions, and that the variations they present, like genera and species in
the organic kingdom, cannot be limited by hard and fast lines, but should
be classified by means of comparisons with typical examples. From snow,
hail, and frozen mists, usually on elevated regions, the granular ice-snow
of a neve is formed. By pressure and alternate softening and refreezing
the neve is changed into compact glacial ice, but the plane of separa-
tion is indefinite, and no one can say where in a vertical section, the neve
ends and the true glacial ice begins. Both the neve and the glacier
proper are wasted by melting when the temperature is above 32 of
the Fahrenheit scale, and the solid drainage is transformed to a liquid



Worn and Striated Rock Surfaces. The movement of glacial ice
causes friction and leads to the grinding, smoothing, and scratching of
the rocks over which it passes. The intensity of this grinding can be
appreciated to some extent by considering the force with which a thick
ice mass presses on the rocks beneath. The weight of a cubic foot of ice
is about fifty-seven pounds ; hence a glacier 1000 feet thick, which is by
no means the maximum, would exert a pressure on its bed of twenty-eight
tons to the square foot. A movement of ice charged with sand and
stones under such a pressure cannot fail to produce abrasion of the
rocks beneath.

As will be shown in a future chapter devoted to theories of glacial
motion, the precise mechanics of glacial flow is not clearly understood.
It is well known, however, that the ice is not forced along as a rigid body.
If such were the case, the grinding would be far more intense than is now
believed to occur. It is, also, known that the flow of glacial ice is at least
analogous to the flow of what are commonly considered plastic solids, as
pitch, for example. In an ice stream the movement is most rapid at the
surface at a distance from its borders, and decreases toward the bottom
and sides, where the friction is greatest. Under similar conditions the
movement of clear ice is greater than when it is charged with debris.
The study of glaciers has shown, also, that sometimes the ice is sheared,
and a forward movement is accomplished by a thrust of the upper portion
of the mass over the lower portion. However accomplished, the fact
remains that there is frequently a movement of even the sand-charged
layers at the bottom, and that friction does occur between the ice and the
underlying rock.

The conditions governing the flow of glaciers are so complicated that
varying results are to be expected. When the bottom layer is heavily
charged with de'bris, and, especially when containing a large proportion
of gravel and stones, the friction is increased, and may possibly become
so great that the bottom layer will be practically stagnant and allow
the clearer ice above to flow over it ; or a shearing of the mass may
result, and the lower portion remain stationary for a time, while the
upper portion moves on. Probably the most favorable conditions for
rock abrasion are when the bottom of a glacier is lightly charged with
sand, and the surface of contact with the rocks beneath is lubricated with
water. That glaciers abrade the rocks over which they pass, as already


stated, there is abundant evidence. At the lower end, and along the
sides of many alpine glaciers, the ice charged with sand and stones may
be seen in direct contact with the smooth, polished, and striated rock
surfaces. Below glaciers that have recently retreated, and where the
surface is still bare of vegetation, records similar to those just mentioned
may be observed in thousands of localities. The same is true, also, over
vast regions that are known to have been formerly glaciated ; while on
adjacent areas, where the conditions are similar, excepting that they were
not occupied by ice, the peculiar and not easily mistaken evidences of ice
abrasion are lacking. We have, therefore, both positive and negative
evidence pointing to the conclusion that glaciers abrade the rocks over
which they flow.

Smoothed and Striated Rock Surfaces not Produced by Glaciers.

There are markings that simulate glacial polishing and striation, and
might be mistaken for them, but are produced by other agencies. Kiver
ice, especially when swept along by freshets, sometimes scratches and
striates the rocky ledges with which it comes in contact, but this action is
confined within narrow vertical limits, and the marks produced are by no
means so regular, or so deeply engraved, as those frequently made by
glaciers. The abrasion of river ice was observed by the writer under
favorable conditions along the Yukon river, but it did not appear as if the
smoothing and striation produced in that way, except, perhaps, when only
limited exposures were observable, could be easily mistaken for the work
of glaciers.

The action of floe ice on the shores of lakes and northern oceans, when
driven landward by wind pressure, on shelving beaches, makes the nearest
approach to glacial abrasion and striation that is known. Except that the
action of floe ice is confined to narrow vertical limits, it is difficult to
understand how the planing and striation it produces on the rocks beneath
could, in the absence of other data, be distinguished from the work of
glaciers. Glaciers smooth and striate vertical walls, as well as flat sur-
faces, however, and make these and other records at all elevations from
the surface of the sea and to a limited extent even below sea level
up to the summits of lofty mountains. It is to be expected, also, that
the records of floe ice would be accompanied by other evidence, such as
deposits of clay and sand containing marine or lacustral shells, and topo-
graphic features due to the abrasion and deposition produced by waves
and currents. When a considerable body of evidence is in hand in connec-


tion with the abrasion of rock surfaces in a given locality, there usually
remains no room for doubting in what way the planing and striation were

Special Features of Glaciated Surfaces. The minor changes
produced on rock surfaces by the movement of ice over them are
so numerous that attention can only be directed at this time to those
that are most common and most characteristic. The details of
these wonderful inscriptions can only be appreciated by studying the

Rock surfaces that have been subjected to the grinding of an ice sheet,
or crossed by even a small alpine glacier, are frequently found to be
worn and the angles and prominences rounded and planed away. All
weathered and oxidized portions of the preglacial surface are removed,
and the fresh hard rock exhibits a polish approaching that given by
marble- workers to finished monuments. The hardest and finest-grained
rocks receive the most brilliant polish. Limestone, granite, and quartzite,
especially, are frequently so highly burnished that they glitter in the
sunlight with dazzling brilliancy. On such surfaces there are usually
scratches and grooves, frequently in long, parallel lines, which show the
direction in which the ice moved over them. These markings vary in
size from delicate, hair-like lines, such as might be made by a crystal
point, to heavy grooves and gouges, a foot and sometimes several feet
deep, which frequently run in one general direction for many yards
and even several rods, and indicate by their straightness and evenness
that the engine which made them was one of great power and moved
steadily in a continuous direction. In regions formerly occupied by
continental glaciers, particularly, two, and possibly three, well-defined
series of parallel striations are sometimes observable on the same sur-
face, crossing each other at varying angles. The most probable explana-
tion of these double or triple, inscriptions is that the direction of the
ice current varied with the growth and decline of the glacier which
made them, or that the ice flowed in great swirls or eddies, as in the
case of the Malaspina glacier, and that the direction of these currents
changed with variations in the volume of the glacier, or perhaps with
variations in the amount of debris in the ice. On small areas the
parallel striations appear straight, but if one could examine square miles
of surface it would probably be found that the lines are frequently
portions of broad curves.


Occasionally the more strongly marked glacial grooves in resistant
rocks, like hard limestone and quartzite, exhibit curved or semilunar
cracks, which cross the furrows from side to side at quite uniform
intervals of a fraction of an inch up to an inch or more, and are con-
vex in the direction of the former ice movement. These "chatter
marks " are thought to have been formed by pebbles that were checked
in their movement by friction, and when the force became sufficient to
carry them onward, were forced forward suddenly, perhaps turning over,
and struck the rock with such force as to produce cracks. A similar
action may be observed in sliding bodies, as when the wheels of a car
slide on the track and a jar is felt when they slip and are arrested.
These peculiar semilunar cracks are not confined to bottoms of grooves,
however, but appear on flat surfaces, where they are sometimes two or
three inches or more in length, and are separated by intervals fully as
great. These larger cracks, or " disrupted gouges," as Chamberlin has
called them, are concave toward the point of the compass from which
the ice came.

Another characteristic feature of glaciated surfaces is observed when
hard knobs occur in rock, as, for example, when limestone is charged
with small masses of chert, or with silicified shells and corals. In
such instances the hard portions are left in relief by the abrasion of
the softer matrix. Starting from each elevation there are frequently
raised ridges, tapering to a point in the direction of the ice movement,
and showing the manner in which the soft rock in the lee of the prom-
inences was protected. On the opposite side of such knobs, i.e. on
the side from which the ice came, the rock is sometimes worn into a
furrow, which bends around the obstruction, and from its form indi-
cates that the ice behaved as a plastic body and moulded itself to the
surface over which it flowed.

Many other features of ice-worn surfaces might be enumerated,
but in an elementary introduction it is perhaps better not to burden
the reader with details. 1

1 In the report on " The Rock Scorings of the Great Ice Invasion," by T. C. Chamber-
lin, in the 7th Annual Report, U.S. Geological Survey, the reader will find many illustra-
tions of ice abrasion, accompanied by clear and concise explanations of the manner of
their formation, which will enable him to interpret such inscriptions for himself wherever



The morainal material carried by glaciers either on their surfaces
or within their mass, is left when they melt, and forms accumulations
to which, in part, the term moraine is still applied. The characteristics
of such abandoned moraines are frequently well exhibited in mountain
valleys from which glaciers have recently retreated. The most common
of these deposits are briefly described below.

Lateral Moraines. The ddbris accumulated on the borders of an
ice stream, and constituting the lateral moraines of a living glacier, is
left when the ice melts and appears as a ridge or terrace at varying
elevations. Steep-sided mountain valleys are frequently bordered on
either side by ridges of this character, which may be situated 1000 feet
or more above the bottom of the trough and clearly traceable for miles.
On the precipitous sides of such valleys, above the highest of the
abandoned moraines, the slopes are usually rough and irregular, and
bear evidence of the work of streams and rills descending from higher
elevations, as well as other results of atmospheric waste ; while below
the horizon referred to the relief is subdued, and the valley has the
smooth and flowing contours characteristic of ice work. Moraines of
this character are frequently similar to stream terraces, but usually
have a raised outer margin, and besides are composed of angular and
unassorted material.

Terminal Moraines. At various stages in the retreat of an ice
stream, the lateral moraines on its sides are united by a terminal
moraine, which crosses the abandoned bed of the glacier and forms a
somewhat regular and usually crescent-shaped pile of stones, gravel, and
sand, which is convex down stream and in many instances 100 feet or more
in thickness. Between successive terminal moraines the bottom of the
trough may be deeply filled with morainal material, deposited without
special arrangement, and in many instances evidently accumulated
beneath the ice as a "ground moraine." These low spaces between
well-defined terminal moraines are frequently occupied by lakes or by
grassy meadows, and furnish some of the most charming features of
mountain scenery.

Morainal Embankments. When a glacier is prolonged beyond
the entrance of a mountain valley and reaches an adjacent plain, it


may expand and end in a semicircular ice foot, or preserve its stream-
like form and finally melt without expanding laterally. The marked
contrast in the behavior of different glaciers in this respect depends on
the relative abundance of debris in their lateral and in their terminal
moraines. When the debris on the margins of a glacier is small in
volume, the ice has freedom to expand on getting free from the valley
through which it descended, but when the margins of the prolonged
stream are more heavily charged with debris than its extremity, lateral
expansion is checked, while the clear ice at the extremity flows on.
The ice advances between the stagnant borders of the stream to a greater
or less distance, depending upon the supply from the higher mountains ;
and when it retreats, the heavy lateral moraines are left as parallel
ridges with steep slopes on each side. These ridges frequently
resemble great railroad embankments. The best examples of structures
of this character that have been described are situated at the east base
of the Sierra Nevada, in Mono valley, California. 1 Their general appear-
ance is shown in Plate 4.

Morainal embankments, like lateral moraines on the sides of a valley,
may be united by terminal moraines so as to form lake basins. When
the terminal moraines are composed of coarse material and are too
open to retain water, or when they have been breached by overflowing
streams, grassy meadows and forest-covered parks, frequently of great
beauty, occupy the spaces between them which were formerly filled by
the retreating ice stream.

Frontal Moraines. Moraines left by piedmont and continental
glaciers are of the same general character as those deposited by alpine
glaciers, but are frequently of vast extent. The frontal moraines of
continental glaciers corresponding to the terminal moraines of local
ice streams, are in some instances a score or more of miles broad and
not only hundreds but thousands of miles long. Where two lobes of
a continental glacier come together their frontal moraines are united
and form what is known as an interlobate moraine. The best known
examples are in the upper Mississippi valley, and mark the junction of
the larger marginal extensions, or lobes, of the Pleistocene ice sheet of
that region.

!" Quaternary History of Mono Valley, California,' 1 8th Annual Keport, U. S. Geo-
logical Survey, pp. 360-368, Pis. 25, 26.


Till. Besides the irregular piles and ridges of unassorted debris
composing the moraines formed about the margins of glaciers, there
are accumulations of clay, filled at times with stones and boulders,

Online LibraryIsrael C. (Israel Cook) RussellGlaciers of North America; a reading lesson for students of geography and geology → online text (page 3 of 24)