THE GLACIER’S SURFACE FEATURES AND THE DEPOSITS UPON ITS BED
=The glacier flow.=—The downward flow of the ice within a mountain
glacier has been the subject of many investigations and the topic of
many heated discussions since the time when Louis Agassiz and his
companions set a line of stakes across the Aar glacier and numbered the
surface bowlders in preparation for repeated observations. Their first
observation was that the line of stakes, which had run straight across
the glacier, was distorted into a curve which was convex downstream
(Fig. 416, A´), thus showing that the surface layers have more rapid
motion in proportion as they are distant from the side margins.
Summarizing these and later studies, it may be stated that the glacier
increases its rate of motion from its side margin towards its center
line, from its bed upwards towards its surface, and below the névé the
velocity is greatest where the fall is greatest and also wherever the
cross section diminishes. In all these particulars, then, the ice of
the glacier behaves like a stream of water. The average rate of flow of
Alpine glaciers varies from a few inches to a few feet per day, and is
greater during the warm summer season. The Muir glacier of Alaska has
been shown to move at the rate of about seven feet per day.
[Illustration:
FIG. 416.—Diagram to illustrate the migrations of lines of stakes
crossing a glacier, due to its surface movement, _A_, original position
of lines; _A´_, later positions; _a_ and _a´_, original and distorted
forms of a square section of the glacier surface near its margin; _r_,
_r´_, diagonal crevasses.]
In traveling from the névé downward to the glacier foot, the snow not
only changes into ice, but it undergoes a granulating process with
continued increase in the size of the nodules until at the foot of
the glacier these may be picked out of the partially melted ice as
articulating balls the size of the fist or larger. Glacier ice has
therefore a structure quite different from that of lake ice, since the
latter is developed in parallel needles perpendicular to the freezing
surface.
=Crevasses and séracs.=—Prominent surface indications of glacier
movement are found in the open cracks or _crevasses_, which are the
marks of its yielding to tensional stresses. Crevasses are apt to run
either directly across the glacier, wherever there is a steep descent
upon its bed, or diagonally, running in from the margin and directed
up-glacier (_r_, _r_, _r_, of Fig. 416), though they occasionally run
longitudinally with the glacier when there is a rock terrace at the
side of the valley beneath the ice. The diagonal crevasses at the
glacier margin are due to the more sluggish movement where the ice is
held back by friction upon the walls of the valley, as will be clear
from Fig. 416. The square _a_ has by this movement been distorted into
the lozenge _a´_, so that the line _xy_ has been extended into _x´y´_,
with the obvious tendency to open cracks in the direction _ss_.
Every glacier surface below its névé is marked by steps or terraces,
which are well understood to overlie corresponding steps of the cascade
stairway to be seen in all vacated glacier valleys (plate 19). The
steep risers of these steps are usually marked by parallel crevasses
which cross the glacier. Under the rays of the sun, which strike them
more from one side than from the other, the slices into which the ice
is divided are transformed into sharpened blades and needles which are
known as _séracs_ (Fig. 401, p. 376, and Fig. 417).
[Illustration:
FIG. 417.—Transverse crevasses at the fall below a glacier step
transformed by unsymmetrical melting into séracs.]
The numerous crevasses tell us that the ice is many times wrenched
apart during its journey down the glacier. This has been illustrated by
somewhat grewsome incidents connected with accidents to Alpinists, but
as they illustrate in some measure both the mode and the rate of motion
of Swiss glaciers, they are worthy of our consideration.
=Bodies given up by the Glacier _des Bossons_.=—In the year 1820,
during one of the earlier ascents of Mont Blanc, three guides were
buried beneath an avalanche near the _Rochers Rouges_ in the névé
of the Glacier des Bossons (Fig. 418). In 1858 Dr. Forbes, who had
measured the rate of flow of a number of Alpine glaciers, predicted
that the bodies of the victims of this accident would be given up by
the glacier after being entombed from thirty-five to forty years. In
the year 1861, or forty-one years after the disaster, the heads of
the three guides, separated from their bodies, with some hands and
fragments of clothing, appeared at the foot of the Glacier des Bossons,
and in such a state of preservation that they were easily recognized
by a guide who had known them in life. Inasmuch as these fragments of
the bodies had required forty-one years to travel in the ice the three
thousand meters which separate the place of the accident from the foot
of the glacier, the rate of movement was twenty centimeters, or eight
inches, per day.
[Illustration:
FIG. 418.—View of the _Glacier des Bossons_ upon the slopes of Mont
Blanc showing the position of accidents to Alpinists and the place of
reappearance of their bodies.]
[Illustration: FIG. 419.—Lines of flow upon the surface of the
Hintereisferner glacier in the Alps (after Hess).]
Various separated parts of the body of Captain Arkwright, who had been
lost in 1866 upon the névé of the same glacier, reappeared at its foot
after entombment in the ice for a period of thirty-one years. To-day
the time of reappearance of portions of the bodies of persons lost
upon Mont Blanc is rather accurately predicted, so that friends repair
to Chamonix to await the giving up of its victims by the Glacier des
Bossons.
[Illustration:
FIG. 420.—Lateral and medial moraines of the _Mer de glace_ and its
tributary ice streams.]
=The moraines.=—The horns and comb ridges which rise above the glacier
surface are continually subject to frost weathering, and from time to
time drop their separated fragments upon the glacier. Falling as these
do from considerable heights, they reach the ice under a high velocity,
and rebounding, sometimes travel well out upon its surface before
coming to a temporary rest. Upon a fresh snow surface of the névé
their tracks may sometimes be followed with the eye for considerable
distances, and their fall is a constant menace to Alpine climbers.
Below the névé the larger number of such fragments remain near the
cliff, and the lines of flow of the ice within the glacier surface are
such that blocks which reach points farther out upon the glacier are
later gathered in beneath the cliff at the side (Fig. 419). The ridge
of angular rock débris which thus forms at the side of the glacier is
called a _lateral moraine_ (see Fig. 411, p. 385, and Fig. 420).
At the junction of two glacier streams, the lateral moraines are
joined, and there move out upon the ice surface of the resultant
glacier as a _medial moraine_. Thus from the number of medial moraines
upon a glacier surface it is possible to say that the important
tributary glaciers number one more (Fig. 420).
[Illustration: FIG. 421.—Ideal cross-section of a mountain glacier to
show the position of moraines and other peculiarities characteristic of
the surface of the bed.]
The plucking and abrading processes in operation beneath the glacier,
quarry the rock upon its bed, and after shaping and smoothing the
separated rock fragments, these are incorporated within the lower
layers of the ice as _englacial_ rock débris. In spaces favorable for
its accumulation, a portion of this material, together with much finer
débris and rock flour, is left behind as a ground moraine upon the bed
of the glacier (see Fig. 421).
[Illustration: FIG. 422.—Fragments of rock of different sizes, to
bring out their different effects upon the melting of the glacier
surface.]
At the foot of the glacier the relatively angular rock débris, which
has been carried upon the surface, and the soled and polished englacial
material from near the bottom, are alike deposited in a common marginal
ridge known as the _terminal_ or _end moraine_ (plate 21 B).
┌──────────────────────────────────────────────────────────────────────┐
│ PLATE 21. │
│ │
│ [Illustration: _A._ View of the Harvard Glacier, Alaska, showing the │
│ characteristic terraces (after U. S. Grant).] │
│ │
│ [Illustration: _B._ The terminal moraine at the foot of a mountain │
│ glacier (after George Kinney).] │
└──────────────────────────────────────────────────────────────────────┘
=Selective melting upon the glacier surface.=—The white surface of the
glacier generally reflects a large proportion of the sun’s rays which
reach it, and its more rapid melting is largely accomplished through
the agency of rock fragments spread upon its surface. Such fragments,
however, promote or retard the melting process in inverse proportion to
their size up to a certain limit, and above that size their action is
always to protect the glacier from the sun. This nice adjustment to the
size of the rock fragments will be clear from examination of Fig. 422,
for rock is a poor conductor of heat, and in even the longest summer
day a thin outer layer only is appreciably warmed. Large rock blocks,
grouped in the medial and lateral moraines, hold back the process of
lowering the glacier surface during the summer, so that late in the
season these moraines stand fifty feet or more above the glacier as
armored ice ridges.
[Illustration:
FIG. 423.—Small glacier table upon the surface of the Great Aletsch
glacier in 1908.]
Isolated and large rock slabs, as the season advances, may come to form
the capping of an ice pedestal which they overhang and are known as
_glacier tables_ (Fig. 423). Such tables the sun attacks more upon one
side than upon the other, so that the slab inclines more and more to
the south and may eventually slip down until its edges rest against the
glacier surface. Rounded bowlders, which less frequently become perched
upon ice pedestals, may, from a similar process, slide down upon the
southern side and leave a pyramid of ice furrowed upon this side and
known as an _ice pyramid_.
Fine dirt when scattered over the glacier surface is, on the other
hand, most effective in lowering its level by melting. Use was made
of this knowledge to lower the great drifts of snow which had to be
removed each season during the construction of the new Bergen railway
of southern Norway. Each dirt particle, being warmed throughout by
the sun’s rays, melts its way rapidly into the glacier surface until
the _dust well_ which it has formed is so deep that the slanting
rays of the sun no longer reach it. When the dirt particles are near
together, the thin walls which separate the dust wells are attacked
from the sides in the warm air of summer days, thus producing from a
patch of dirt upon the glacier surface a _bath tub_ (Fig. 424 _d_). At
night the water which fills these basins is frozen to form a lining
of ice needles projecting inward from the wall, and this, repeated in
succeeding nights, may entirely close the basin with water ice and
produce the familiar _glacier star_ (Fig. 424 _c_).
[Illustration:
FIG. 424.—Effects of differential melting and subsequent refreezing
upon the glacier surface. _a_, dust wells; _b_, glacier _tub_ produced
by melting about a group of scattered dust particles; _c_, glacier star
produced when the inclosed water of the glacier well has frozen in
successive nights; _d_, “bath tub.”]
If the dirt upon the glacier surface, instead of being scattered, is
so disposed as to make a patch completely covering the ice to the
thickness of an inch or more, the effect is altogether different.
Protecting as it now does the ice below, a local ice hillock rises upon
its site as the surrounding surface is lowered, and as this grows in
height its declivities increase and a portion of the dirt slides down
the side. The final product of this shaping is an almost perfectly
conical ice hill encased in dirt and known as a _débris_, _sand_, or
_dirt cone_ (Fig. 425). The novice in glacier study is apt to assume
that these black cones contain only dirt, but is rudely awakened to the
reality when he attempts to kick them to pieces. Both glacier tubs and
débris cones may assume large dimensions; as, for example, in Alaska,
where they may be properly described as lakes and hills.
[Illustration: FIG. 425.—Dirt cone and one with its casing in part
removed. Victoria glacier (after Sherzer).]
A patch of hard and dense snow which is less easily melted than that
upon which it rests may lead to the formation of snow cones upon
the glacier surface similar in size and shape to the better known
débris cones. Such cones of snow have, with doubtful propriety, been
designated “penitents”, for it is pretty clear that the interesting
bowed snow figures, which really resemble penitents and which were
first described from the southern Andes under the name of _nieves
penitentes_, are of somewhat different character.
[Illustration:
FIG. 426.—Schematic diagram to show the manner of formation of glacier
cornices.]
One further ice feature shaped by differential melting around rock
particles remains to be mentioned. Wherever the seasonal snowfalls
of the névé are exposed in crevasses, they are generally found to be
separated by layers of dirt, and lines of pebbles similarly separate
those ice layers which are revealed at the foot of the glacier. In
either case, if the sun’s rays can reach these layers in an opened
crevasse, the half-buried rock fragments are warmed by the sun upon
their exposed surfaces and slowly melt their way down the ice surface,
thus removing from it a thin layer of snow or ice and causing that part
above the pebble layer to project like a cornice. This process will go
on until the overhanging cornice protects the pebbles from any further
warming by the sun, but each lower pebble layer that is reached by the
sun will produce an additional cornice, so that the original surface
may at the bottom have been retired by the process a number of inches.
These features are described as _glacier cornices_ (Fig. 426).
[Illustration: FIG. 427.—Superglacial stream upon the Great Aletsch
glacier.]
=Glacier drainage.=—Already in the early morning of every warm summer
day, active melting has begun upon the surface of the Swiss glaciers.
Rills of icy water soon make their way along depressions upon the
surface, and are joined to one another so that they sometimes form
brooks of considerable size (Fig. 427). Such streams continue their
serpentine courses until these are intersected by a crevasse down which
the waters plunge in a whirling vortex which soon develops a vertical
shaft of circular section within the ice. Such shafts with their
descending columns of whirling water are the well-known _moulins_, or
“_mills_”, which may be detected from a distance by their gurgling
sounds. The first plunge of the water may not reach to the bottom of
the glacier, in which case the stream finds a passageway below the
surface but above the floor until another crevasse is encountered and
a new plunge made, here perhaps to the bottom. Once upon the valley
floor the stream is joined by others, and pursues its course within an
ice tunnel of its own making (Fig. 421, p. 394) until it issues at the
glacier front.
The coarser of the rock débris which was gathered up by the stream
upon the glacier surface is deposited within the tunnel in imperfect
assortment (gravel and sand), while all finer material and that lifted
from the floor (rock flour) is retained in suspension and gives to the
escaping stream its opaque white appearance. This _glacier milk_ may
generally be traced far down the valleys or out upon the foreland,
and is often the traveler’s first indication that a range which he is
approaching supports glaciers.
[Illustration:
FIG. 428.—Ideal form of the surface left on the site of the apron of
a piedmont glacier. _M_, moraine; _T_, outwash; _C_, basin usually
occupied by a lake; _D_, drumlins (after Penck).]
=Deposits within the vacated valley.=—For every excavation of the
higher portions of the upland through glacial sculpture, there is a
corresponding deposit of the excavated materials in lower levels. So
far as these materials are deposited directly by the ice, they form
the lateral, medial, ground, and terminal moraines already described.
A considerable proportion of them are, however, deposited by the
water outside the terminal moraine; but as with the shrinking glacier
the ice front retires in halting movements over the area earlier
ice-covered, the terminal moraines are ranged along the vacated valley
as _recessional moraines_, each with a _valley train_ of outwash below.
About the apron of the piedmont glacier, such deposits are particularly
heavy (Fig. 428). During the “ice age” the Swiss glaciers extended down
the valleys below the existing ice remnants and spread upon the Swiss
foreland as great piedmont glaciers such as may now be seen in Alaska.
To-day we find there moraines and glacial outwash, a lake in the middle
of the apron site, and sometimes a group of radiating drumlins like
those found within the ice lobes of the continental glacier in southern
Wisconsin (Fig. 429, and Fig. 344, p. 317).
[Illustration:
FIG. 429.—Moraines and drumlins about Lake Constance upon the site of
the earlier piedmont glacier of the Upper Rhine. The white area outside
the outermost moraine is buried in glacial outwash (after Penck and
Brückner).]
Behind the recessional moraines within the glaciated valley are found
the valley moraine lakes (Fig. 448, p. 413), in association with the
rock basin lakes due to glacial sculpture (Fig. 447, p. 412). After
the glacier has vacated its valley, the precipitous side walls become
the prey of frostwork and are the scenes of disastrous avalanches or
landslides. Within the cirques, drifts of snow are nourished long after
the ice has disappeared, and as a consequence the amphitheater walls
succumb to the process of solifluxion (p. 153).
Diversions and reversals of drainage, which are so characteristic of
the work of continental glaciers, are hardly less common to glaciated
mountain districts. Many of our most beautiful waterfalls have resulted
from either the temporary or permanent obstruction of earlier valleys
above the falls. The famous Yosemite Falls offers an interesting
illustration of the shifting of an earlier waterfall, itself no doubt
due to ice blocking in a still earlier glaciation (plate 22 B).
=Marks of the earlier occupation of mountains by glaciers.=—It is
well that we should now bring together within a small compass those
evidences by which the existence of earlier mountain glaciers may be
proven in any district. These marks are so deeply stamped upon the
landscape that no one need err in their interpretation.
MARKS OF MOUNTAIN GLACIERS
_High-level sculpture._ The grooved upland with its cirques, or the
fretted upland with its cirques, cols, horns, and comb ridges.
_Low-level sculpture._ The U-shaped main valley, the hanging side
valleys with their ribbon falls, the glacier staircase with its rock
bars and gorges, the rounded, polished, and striated rock floor.
_Deposits._ The recessional moraines of till and the valley trains of
sand and gravel, the soled erratic blocks derived always from higher
levels of the valley.
_Lakes._ The valley moraine lakes and the chains of rock basin lakes.
READING REFERENCES FOR CHAPTER XXVIII
Glacier movement:—
L. AGASSIZ. Nouvelles Études et Expériences sur les Glaciers Actuels,
etc., Paris, 1847, pp. 435-539.
H. HESS. Die Gletscher, Braunschweig, 1904, pp. 115-150.
H. F. REID. The Mechanics of Glaciers, Jour. Geol., vol. 4, 1896, pp.
912-928; Glacier Bay and Its Glaciers, 16th Ann. Rept. U. S Geol.
Surv., Pt. i, 1898, pp. 445-448.
┌─────────────────────────────────────────────────────────────────────┐
│ PLATE 22. │
│ │
│ [Illustration: _A._ Model of the vicinity of Chicago, showing the │
│ position of the ancient beaches and the outlet of the former Lake │
│ Chicago.] │
│ │
│ [Illustration: _B._ Map of Yosemite Falls and its earlier site near │
│ Eagle Peak (after F. E. Matthes).] │
└─────────────────────────────────────────────────────────────────────┘