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Israel C. (Israel Cook) Russell.

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

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tion to which surface moraines are subjected. On Malaspina glacier the
formation of glacial tables is confined to the summer season. In winter
the surface of the glacier is snow-covered and differential melting cannot be
marked. The fact that glacial tables are seldom seen just after the snows
of winter disappear suggests that winter melting takes place to some extent,
but in a different manner from what it does in the summer. Just how the
blocks are dislodged from the pedestals in winter has not been observed.

While large objects lying on the surface of the glacier are elevated on
pedestals in the manner just described, smaller ones, as is well known, and
especially those of dark color, become heated by the sun, and melting
the ice beneath, sink into it. When small stones and dirt are gathered in
depressions on the surface of the glacier, or on a large scale, when
moulins become filled with fine debris and the adjacent surface is lowered
by melting, the material thus concentrated acts as do large boulders, and
protects the ice beneath. But as the gravel rises in reference to the
adjacent surface, the outer portion rolls down from the pedestal on all
sides, and the result is that a sharp cone of ice is formed, having a sheet
of gravel and dirt over its surface. These sand cones, as they are called,
sometimes attain a height of ten or twelve feet, and form conspicuous and
characteristic features of the glaciers over large areas.



o



GLACIERS OF ALASKA. 113

The surface of Malaspina glacier over many square miles, where free
from moraine, is covered with a coral-like crust, which results from the
alternate melting and freezing of the surface. The crevasses in this por-
tion of the vast plateau are seldom of large size, and owing to the melting
of their margins, are broad at the surface and contract rapidly downward.
They are in fact mere gashes, sometimes 10 or 20 feet deep, and are
apparently the remnants of larger crevasses formed in the glaciers which
flow down from the mountains. Deeper crevasses occur at certain locali-
ties about the border of the glacier, where the ice at the margin falls
away from the main mass ; but these are seldom conspicuous, as the ice in
the region where they occur is always heavily covered with debris and the
openings become filled with stones and boulders. The generally level
surface of the glacier and the absence of large crevasses indicate that the
ground on which it rests is comparatively even. Where the larger of the
tributary glaciers join it, however, ice falls occur, caused by steep descents
in the ground beneath. These falls are just at the lower limit of per-
petual snow, and are fully revealed only when melting has reached its
maximum and the snows of the winter have not yet begun to accumulate.

Moraines. From any commanding station overlooking Malaspina
glacier, one sees that the great central area of clear, white ice is bordered
on the south by a broad, dark band formed by boulders and stones. Out-
side of this and forming a belt concentric with it is a forest-covered area,
in many places four or five miles wide. The forest grows on a moraine,
which rests upon the ice of the glacier. In a general view by far the
greater part of the surface of the glacier is seen to be formed of clear ice,
but in crossing it one comes first to the forest and moraine-covered border,
which, owing to the great obstacles it presents to travel, impresses one as
being more extensive than it is in reality.

The moraines not only cover all of the outer border of the glacier, but
stream off from the mountain spurs projecting into it on the north. As
indicated on the accompanying map, one of these trains starting from a
spur of the Samovar hills crosses the entire breadth of the glacier and
joins the marginal moraine on its southern border. This long train of
stones and boulders is really a highly compound medial moraine formed
at the junction of the expanded extremities of the Seward and Agassiz
glaciers.

All of the glaciers which feed the great piedmont ice sheet are above
the snow line, and the debris they carry only appears at the surface after



114 GLACIERS OF NORTH AMERICA.

the ice descends to the region where the annual waste is in excess of the
annual supply. The stones and dirt previously contained in the glacier
are then concentrated at the surface, owing to the melting of the ice.
This is the history of all of the moraines on the glacier. They are formed
of the debris brought out of the mountains by the tributary alpine glaciers
and concentrated at the surface by reason of the melting of the ice.

Malaspina glacier in retreating has left irregular hillocks of coarse
debris which are now densely forest-covered. These deposits do not
form a continuous terminal moraine, however, but a series of irregular
ridges and hills having a somewhat common trend. They indicate a slow
general retreat without prolonged halts. The heaps of debris left as the
ice front retreated have a general parallelism with the present margin of
the glacier and are pitted with lake basins, but only their higher portions
are exposed above the general sheet of sand and gravel spread out by
streams draining the glacier.

The blocks of stone forming the moraines now resting on the ice are
of all sizes up to 20 or 30 feet in diameter, but those of large dimen-
sions are not common. The stones are rough and angular except when
composed of material like granite, which on weathering forms oval
and rounded boulders of disintegration. So far as has been observed,
very few of the stones on the glacier have polished or striated surfaces.
The material of which the moraines are composed is of many kinds, but
individual ridges frequently consist of fragments of the same variety
of rock, the special kind in each case depending on the source of the
thread in the great ice current which brought the fragments from the
mountains. *

In many instances, particularly near the outer border of the ice sheet,
there are large quantities of tenacious clay, filled with angular stones,
which is so soft, especially during heavy rains, that one may sink waist
deep in the treacherous mass. Sometimes blocks of stone a foot or
more square float on the liquid mud and lure the unwary traveler to
disaster.

On the eastern margin of the ice sheet adjacent to Yakutat bay, where
the frontal slope is low, there are broad deposits of sand and well-rounded
gravel which has been spread out over the ice. On the extreme margin
of the glacier this deposit merges with hillocks and irregular knolls of the
same kind of material, some of which rise a hundred feet above the nearest
exposure of ice and are clothed with dense forests. The debris is so
abundant and the ice ends in such a low slope that it is frequently im-



GLACIERS OF ALASKA. 115

possible to determine where the glacier actually terminates. The water-
worn material here referred to as resting on the glacier has been brought
out of tunnels in the ice, as will be noticed further on.

Surface of the Fringing- Moraines. A peculiar and interesting
feature of the moraine on the stagnant border of Malaspina glacier is
furnished by the lakelets that occur everywhere upon it. These are found
in great numbers both in the forest-covered moraine and in the outer
border of the barren moraine. They are usually rudely circular, and have
steep walls of dirty ice which slope toward the water at high angles, but
are undercut at the bottom, so that the basins in vertical cross-section
have something of an hour-glass form. The walls are frequently from 50 to
100 feet high, with a slope of 40 to 50 degrees, and sometimes are nearly
perpendicular. Near the water's edge the banks are undercut so as to leave
a ridge projecting over the water. The upper edge of the walls is formed
of the sheet of debris which covers the glacier, and the melting of the ice
beneath causes this material to roll and slide down the ice slopes and
plunge into the waters below. The lakes are usually less than 100 feet
in diameter, but larger ones are by no means uncommon, several being ob-
served which were 150 or 200 yards across. Their waters are always
turbid, owing to the mud which is carried into them by small avalanches
and by the rills that trickle from their sides. The rattle of stones falling
into them is frequently heard while traveling over the glacier, and is
especially noticeable on warm days, when the ice is melting rapidly, but is
even more marked during heavy rains. The crater-like walls inclosing the
lakes are seldom of uniform height, but frequently rise into pinnacles.
Between the pinnacles there are occasionally low saddles, through which
in some instances the lakes overflow. Frequently there are two low
saddles nearly opposite to each other, which suggests that the lakes were
formed by the widening of crevasses. The stones and dirt which fall into
them, owing to the melting of the walls, gradually fill their bottoms. In-
stances are numerous where the waters have escaped through crevasses or
openings in the bottom of the basin, leaving an exceedingly rough depres-
sion, with a heavy deposit of debris at the bottom.

As the general surface of the glacier is lowered by melting, the
partially filled holes gradually disappear, and their floors, owing to the
deep accumulation of debris on them, which protects the ice from melting,
become elevated above the surrounding surface, in the same manner that
glacial tables are formed. The debris covering these elevations slides down



116



GLACIERS OF NORTH AMERICA.



their sides as melting progresses ; and finally a rugged pyramid of ice,
covered with a thin coating of debris, occupies the place of the former
lake. These pyramids frequently have a height of 60 or 80 feet, and are
sometimes nearly conical in shape. They resemble "sand cones," but are
of much greater size and are sheathed with coarser debris. The sand cones




FIG. 9. LAKELET ox MALASPIXA GLACIER, ALASKA.

are usually, if not always, formed and melted away during a single
season, while the debris pyramids require several seasons for their cycle
of change.

Like the lakelets to which they owe their origin, the debris pyramids
are confined to the stagnant portions of the glacier and play an important
part in the breaking up and comminution of the material forming the
marginal moraines. Owing to the sliding of the boulders and stones into
the lakelets and their subsequent fall from the sides of the pyramids, they



GLACIERS OF ALASKA. 117

are broken and crushed, so that the outer portion of the glacier, where the
process has been going on longest, is covered with finer debris and contains
more clay and sand than the inner portions.

Just how the holes containing glacial lakelets originate it is difficult to
say, but their formation seems to be initiated, as already suggested, by
the melting back of the sides of crevasses. Breaks in the general sheet of
debris covering the glacier expose the ice beneath to the action of the sun
and rain, which causes it to melt and the crevasses to broaden. The
openings become partially filled with water, and lakelets are formed.
The waves wash the debris from the ice about the margin of the lakelets,
thus exposing it to the direct attack of the water, which melts it more
rapidly than higher portions of the slopes are melted by the sun and rain.
It is in this manner that the characteristic hour-glass form of the basins
originates. The lakelets are confined to the outer or stagnant portion of
the glacier, for the reason that motion in the ice would produce crevasses
through which the water would escape. Where glacial lakelets occur in
great numbers it is evident that the ice must be nearly or quite stationary,
otherwise the basins could not exist for a series of years. The lakelets
and the pyramids resulting from them are the most characteristic features
of the outer border of the glacier. The number of each must be many
thousand. They occur not only in the outer portion of the barren mo-
raine, but also throughout the forest-covered area still nearer the outer
margin of the glacier. Large quantities of trees and bushes fall into them
with the debris that slides from their sides, and tree trunks, roots, and soil
thus become buried in the moraines.

Forests on the Moraines. The outer and consequently older portions
of the fringing moraines are covered with vegetation, which in places, par-
ticularly near the outer margin of the belt, has all the characteristics of
old forests. It consists principally of spruce, alder, and cotton wood trees,
and a great variety of shrubs, bushes, and ferns. In many places the ice
beneath the dense forest is not less than a thousand feet thick. The vege-
tation is confined principally to the border of the Seward lobe. Near
Yahtse river the belt is five miles broad, but decreases toward the east and
is absent at the Sitkagi bluffs, where the glacier is being eaten away by
the sea. It is only on the stagnant borders of the ice sheet that forests
occur. Both glacial lakelets and forests on the moraines are absent where
the ice has motion. The forest-covered portion is, by estimate, between
20 and 25 square miles in area.



118 GLACIERS OF NORTH AMERICA.

Character of the Outer Margin. The southern margin of Malaspina
glacier, between the Yahtse and Point Manby, is abrupt, and forms a
bluff that varies in height from 140 to 300 feet or more. The bluff is
so steep in most places and is so heavily encumbered with fallen trees
and boulders that it is with difficulty one can climb it. Often the
trouble in ascending is increased by landslides, which have piled the
superficial material in confused heaps, and in other instances the melting
of the ice beneath the vegetation has left concealed pitfalls into which
one may drop without warning. The bluff formed by the margin of the
glacier, when not washed by the sea, is boldest and steepest where the
covering of vegetation is most dense. Where the covering consists of
stones and dirt without vegetation, however, the margin may still be
bold. This is illustrated between the mouth of the Yahtse and Icy cape,
where the ice is concealed beneath a general sheet of debris, but , has a
bold convex margin which rises abruptly from the desolate, torrent-swept
waste at its base.

When the glacier meets the sea the ice is cut away at the water-level,
and blocks fall from above, leaving perpendicular cliffs of clear ice. At
Icy cape there is a bold headland of this nature from which bergs are
continually falling with a thunderous roar that may be heard fully twenty
miles away. On the crest of the cliffs of clear blue ice there is a dark
band formed by the edge of the sheet of debris covering the glacier, and
showing that the moraine which blackens its surface along its outer
margin is entirely superficial. At Sitkagi bluffs the glacier is again
washed by the sea, but the base of the ice is there just above the water-
level and recession is slow. The bluffs are heavily covered with stones
and dirt, and icebergs do not form.

At the heads of the gorges in the margin of the glacier leading to the
mouths of tunnels, the dirt-covered ice forms bold cliffs which are most
precipitous at the heads of the reentrant angles. The eastern margin of
the ice sheet facing Yakutat bay is low, and covered to a large extent
with water-worn debris. The ridges on the glacier formed by moraines
are there at right angles to the margin of the ice, and are bare of vege-
tation. The reason for the exceptionally low slope of the eastern margin
of the ice sheet seems to be that the current in the ice is there eastward,
and the glacier is melting back without leaving a stagnant border.

Marginal Lakes. The water bodies here referred to are called
marginal lakes, for the reason that they are peculiar to the margins of



GLACIERS OF ALASKA. 119

glaciers. Where rocks border an ice field or project through it they
become heated, especially on southern exposures, and, radiating heat to
the adjacent ice, cause it to melt. A depression is thus formed along the
margin of the ice which becomes a line of drainage. Water flowing
through such a channel accelerates the melting of the ice, at least until
a heavy coating of debris has accumulated. When a steep mountain
spur projects into an ice field, the lines of drainage on each side converge
and frequently unite at its extremity, forming a lake, from which the
water usually escapes through a tunnel in the ice. Typical instances of
lakes of this character occur at Terrace point, at the south end of the
Hitchcock range, and again about the base of the Chaix hills.

When a stream flows along the side of a glacier a movement in the
ice or the sliding of stones and dirt from its surface sometimes obstructs
the drainage and causes the formation of another variety of marginal
lakes. In such instances the imprisoned waters usually rise until
they can find an outlet across the barrier, and then cut a channel
through it.

A glacier in flowing past the base of a mountain frequently obstructs
the drainage of lateral valleys, and causes lakes to form. These usually
find outlets, as in the case of lakes at the end of mountain spurs, through
a subglacial or englacial tunnel, and are filled or emptied according as
the tunnel through which the waters escape affords free drainage, or is
obstructed. Several examples of this variety of marginal lakes occur
011 the west and south sides of the Chaix hills. They correspond in
the mode of their formation with the well-known Merjelen lake of
Switzerland.

Other variations in the manner in which glaciers obstruct drainage
might be enumerated, but those mentioned cover all of the examples thus
far observed about Malaspina glacier. The conditions which lead to the
formation of the marginal lakes are unstable, and the records which the
lakes leave in the form of terraces, deltas, etc., are consequently irregular.
When streams flow into one of these lakes, deltas and horizontally
stratified lake beds are formed as in ordinary water bodies ; but, as the
lakes are subject to many fluctuations, the elevations at which the records
are made are continually changing, and in instances like those about
Malaspina glacier, where the retaining ice body is constantly diminishing,
may occupy a wide vertical interval.

Drainage begins on the southeast side of Chaix hills at Moore's
nunatak, where, during the time of our visit, there were two small lakes,



120 GLACIERS OF NORTH AMERICA.

walled in on nearly all sides by the moraine-covered ice of Malaspina
glacier. The water filling these basins comes principally from the high
ice fall at the north, where the glacier descends over a projecting spur
running east from Moore's nunatak. The water escaped from the first
lake across a confused mass of debris which had slid from the ice bluff
bordering the stream, and formed a temporary dam. Below the dam the
water soon disappeared beneath deeply crevassed and heavily moraine-
covered ice, and came to light once more at the mouth of a tunnel about
a mile to the southwest. The second lake, at the time of our visit, had
almost disappeared, but its former extent was plainly marked by a barren
sand-flat, many acres in extent, and by terraces along its western border.
The lake occupied a small embayment in the hills, the outlet of which
had been closed by the ice flowing past it. Below the second lake the
stream flows along the base of densely wooded knolls, and has a steep,
moraine-covered bluff of ice for its left bank. About a mile below, it
turns a sharp projection of rocks and cuts deeply into its left bank, which
stands as an overhanging bluff of dirty ice, over 100 feet high. The
stream then flows nearly due west for some three miles to Crater lake. On
its right bank is a terrace about 150 feet high, which skirts the base of
the Chaix hills, and marks the position of the stream at a former stage.
The terrace is about 100 yards broad, and above it are two other terraces
on the mountain slope, one at an elevation of 50 feet, and the other at
75 feet, above the broad terrace. The upper terraces were only observed
at one locality, and were probably due to deposits formed in a marginal
lake at the end of a mountain spur.

The terraces left by streams flowing between a moraine-covered glacier
and a precipitous mountain slope are peculiar and readily distinguishable
from other similar topographic features. The channels become filled
principally with debris which slides down the bank of ice. This material
is angular and unassorted, but when it is brought within the reach of
flowing waters soon becomes r.ounded and worn. On the margin of the
channel, adjacent to the glacier, there is usually a heavy deposit of unas-
sorted debris, which rests partly upon the ice and forms the actual border
of the stream. When the glacier is lowered by melting, the stream
abandons its former channel and repeats the process of terrace-building at
a lower level. The material forming the terrace at the base of Chaix
hills is largely composed of blue clay filled with both angular and rounded
stones and boulders, but its elevated border is almost entirely of angular
debris. The drainage from the mountain slope above the terrace is



GLACIERS OF ALASKA. 121

obstructed by the elevated border referred to, and swamps and lagoons
have formed back of it. In the material forming the terraces there are
many tree trunks, and growing upon its surface there is a forest of large
spruce trees.

At the extreme southern end of the Chaix hills the drainage from the
northeast, which we have been tracing, joins another stream from the
northwest and forms Lake Castani. This lake, like the one at Terrace
point, is at the south end of a precipitous mountain ridge projecting into
the glacier, and drains through a tunnel in the ice. The stream flowing
from it is known as the Yahtse, and flows for six or eight miles beneath
the ice before emerging at its southern margin. Large quantities of both
coarse and fine material are being carried into Lake Castani by tributary
streams, and are there deposited as deltas and lake beds. When the lake
is drained, as sometimes happens, vast quantities of this material must be
carried into the tunnel through which the waters escape.

On the west side of Chaix hills are several other marginal lakes of the
same general character as those just described. The one next northwest
of Lake Castani occupies a long, narrow valley between two outstanding
mountain ridges, and is retained by the glacier which blocks the end of
the recess thus formed. This lake was clear of ice July, 1891, and of a
dark blue color, showing that it received little drainage from the glacier.
Other lakes on the northwest side of the Chaix hills are of a similar
nature, and during my visit were heavily blocked with floating ice. On
the north side of Chaix hills there are other small water bodies occupying
embayments, and retained by the glacier which flows past their entrances.
The water from all these lakes escapes through tunnels.

The lakes to which attention has been directed are especially interest-
ing, as they illustrate one phase of deposition depending upon glaciation,
and suggest that a great ice sheet like that which formerly covered New
England very likely gave origin to marginal lakes, the records of which
should be found on steep mountain slopes.

Drainage. The drainage of the Malaspina glacier is essentially
englacial or subglacial. There is no surface drainage excepting in a few
localities, principally on its northern border, where there is a slight sur-
face slope, but even in such places the streams are short and soon plunge
into a crevasse or a moulin and join the drainage beneath.

On the lower portions of the alpine glaciers, tributary to the main ice
sheet, there are sometimes small streams coursing along in ice channels,



122 GLACIERS OF NORTH AMERICA.

but these are short-lived. On the borders of the tributary glaciers there
are frequently important streams flowing between the ice and the adjacent
mountain slope, but when these come down to the Malaspina glacier they
flow into tunnels and are lost to view.

Along the southern margin of the glacier, between the Yahtse and
Point Manby, there are hundreds of streams which pour out of the escarp-
ment formed by the border of the glacier, or rise like great fountains from
the gravel and boulders accumulated at its base. All of these are brown
and heavy with sediment and overloaded with boulders and stones. The
largest and most remarkable of these springs is the one indicated on the


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Online LibraryIsrael C. (Israel Cook) RussellGlaciers of North America; a reading lesson for students of geography and geology → online text (page 13 of 24)