Recent Glacial Studies in Greenland
Scientific American supplement,
July 6, 1895
Abridged from a paper by T. C.
Chamberlin in the Bulletin of the Geological Society of America, Febuary,
1895 - Nature.
During the summer of 1894 Mr. Chamberlin was enabled
to devote some time to a personal study of the glaciation of Greenland,
and the results of his observations are so interesting that all geologists
who seek to interpret the records of the "Great Ice Age" will gladly make
acquaintance with them. Seldom has a geologist so experienced in the study
of glacial drifts and the problems connected with them had the advantage
of examining the behavior of ice in the Arctic regions.
His observations were specially directed to the way in
which a glacier gathers up detritus along its course, to the way in which
it carries it forward and finally puts it down. The main problem he sought
to solve was connected with the basal material of glaciers, debris which,
of course, is largely concealed.
In comparing the glaciation of Greenland with that of
the mainland of North America, he had to bear in mind that for the
most part the continental drift is spread over a vast plain. In Greenland
the ice fields rest mainly on plateaus fringed by rugged mountains, and
he sought for a tract free from such bordering elevations. This was found
at Inglefield Gulf, where the borderland is a plateau about 2000 feet above
sea level, and where the margin of the great ice sheet may be studied
on relatively smooth ground, on undulating ground and in lobes or
tongues that descend the valleys. Of the thirty or fourty glacial tongues
which descend toward Inglefield Gulf less thanone-third reach the shore,
and scarcely one-half of these discharge notable icebergs. The majority
terminate in valleys whose bottoms are formed of glacial debris an whose
lower gradients are moderate.
The fact great part of Greenland appears to consist of
ancient gneissic rocks renders the debris more or less stoney and
arenaceous: clayey material is rare. About Inglefield Gulf, however,
the older rocksare covered by thick layers of sandstone and shale, traversed
by basic igneous dikes. Hence it is possible there to tell
how late the erratics from this sedimentary series were introduced
into the ice, to ascertain what courses they pursued and the actions they
The margins of the Inglefield glaciers rise abruptly like
escarpments of rock 100 or 150 fett or more. the layers of ice are cut
sharply across,exposing their edges, and the formation of these scarps
is attributed to the lower inclination of the sun's rays, which strike
vertically and effectively against the edges of the glacier, whereas its
back is affected only by rays of low slant.
The stratification of the glaciers attrackted particular
attention. The ice was found to be almost as distinctly bedded and laminated
as a sedimentary rock. The vertical face was seen usually to present
two great divisions, an upper tract of thick, obscurely laminated layers
of nearly white ice and a lower laminated tract dicolored by debris.
At the base there is usuallya talus slope and sometimes there is a moraine.
In the lower portion of the ice there are here and there interstratified
layers of sand and silt, rubble and bowlders. these vary from a mere
film of silt to heterogeneous mixture of debris and ice
several feet thick. The detritus is usually arranged in definite and limited
horizons, the ice above and below being firm, clean and pure. Often a fragment
of rock or a bowlder of considerable dimensions will be several times thicker
than the silt layer, and it projects above and below into the clean ice.
The debris layers, though often regular and persistent, frequently thin
out and disappear. Lenses of debris also appear, and the layers are sometimes
doubled back apon themselves.
The laminae of the ice are sometimes very symmetric, straight
and parallel, but often wavy and undulating. In many instances they are
greatly curved or contorted. Thus, as Dr. E. von Drygalski has remarked,
they closely simulate the foliation and contortion of gneiss.
The debris belts, which are essentially parallel to the
base of the glacier, are confined chiefly to the lower 50 or 75 feet, but
they occur up to 100 feet and perhaps to 150 feet. They are more abundant
at the sides of the lobes than in the center; a notable portion of
the debris having evidently been introduced after the lobes were formed.
The the detritus appears most abundant in glacier lobes which descend as
cataracts or crowd between closley hugging cliffs.
In meeting obstacles the basel beds of the glacier
sometimes simply curve upward, carrying their debris with them over the
obstacle; at other times the laminae of ice are much crumpled. not only
the foliations of the ice twisted, but they are at times fractured and
faulted, and along the fault plane the laminae are affected by "drag,"as
in faulted rocks.
The general stratification of the ice had its initial
stages in the original snow falls; and the seasons doubtless developed
annual subdivisions. The more definite partings and the introduction of
layers of debris arose through a shearing movement between the lyers of
The actual process of intrusion of detritus was observed
in proximity to a large boss of rock which, protruding through the margin
of ice, had been partially cut away. Trains of cebris, apparently
rubbed from the surface of the rocky dome, were carried out almost horizontally
into the ice in its lee. Some of these were ahort. While others extended
several rods into the ice, passing into the body of it instead of
following its base. At one point the overthrust of the ice reached
such a degree as to carry the earthy layers obiquely across the thickness
of the glacier, producing a marked unconformity.
In another instance similar features were observed below
an ice cataract. Tongues of debris, having their origin in the bowlder
clay below the glacier, were seen to reach out into the basal portion of
the ice as though they were being introduced into it by the differential
movement of the layers upon each other thus when the ice is forced
over a prominence it settles down a little in its lee, and is then protected
somewhat from the thrust of the ice behind. The next ice that passes
over, being prevented by the former portion from the settling
down at once, is thrust forward over it. This is accomplished by the bending
and doubling of the layers, and also by distinct shearing. At length,
however, the first layer is compelled by the general friction
to move somewhat forward, and in time to join the common moving mass,
carrying the overthrust layer of debris between it and the ice layer above.
It appears obvious that the ice in the lee of a
rockyprominence moves more slowly than that above; hence the doubling of
the laminae upon themselves. Moreover, there is a gradation from laminae
that simply suffered doubling up to layers that obviously sheared upon
each other and produced manifest unconformity by overthrust.
Evidence showed that the more solid (blue) bands in the
ice are produced by exceptional pressure in moving over rugosities, and
that their position in the ice is parallel to the ice movement; while
at the same time blue bands may be developed nearly at right angles, after
the manner of slaty cleavage.
Summarizing the above conclusions, it appears that stratification
originated in tha inequalities of deposition, emphasized by intercurrent
winds, rains, and surface meltings; that the incipient statification may
have been intensified by the the ordinary processes of consolidation; That
the shearing of the strata upon each other still further emphasized
the stratification, and developed new horizons under favorable conditions;
that basal inequalities introduced new planes of stratification, accompanied
by earthly debris, and that this process extended itself so far as
even to form very minute laminae.
There is involved in the foregoing conceptions the idea
of a ice layer acting as a unit of movement; at any rate, there is recognized
individuality of movement in the layer. This view involves the idea of
rigidity rather than viscosity. The introduction of earthly material into
the ice layers involves the idea of thrust rather than pull. The picture
is not that of gravitation pulling a thick, stiff liquid down the lee side
of a rocky prominence, but of rigid body thrusting itself over the crest
by means of a force in the rear.
The extreme fragility of the ice is difficult to harmonize
with the idea of viscosity. Wherever the ice passed over an undulation
of even moderate dimensions, it was abundantly crevassed. Ther was no indication
that the bowlders decend through the ice as heavy substances descend through
viscous bodies. The rigidity did not prevent contortions and foldings of
the laminations such as take place in crystalline rocks, but faulting and
vein structures also occure, and there seems no more occasion to assume
viscosity in the one case than in the other. Even if a certain measure
of viscosity be admitted, it does not follow that viscosity was an essential
agency of motion. The crystalline body may readily be made to change
its form by the removal of particles from one portion by melting, and their
attachment at other points by congelation; but not, apparently, by the
flowing of crystallized particles over each other in their crystalline
It has been already pointed out that much basal material
is carried in the lower layers of ice. It was also a matter of frequent
observation that debris lies under the ice. Apparently the ice sometimes
pushes this along, and sometimes slides over it. At the end of the
glacier the debris within the ice is freed by melting, and accumulates
as a talus slope. This sometimes protects the basal layers from melting,
and they become at length incorporated in the growing accumulation.
It appeared, from the stages presented by the several
glaciers, that where the ice is slowly advancing, the talus slope
gradually grows, forward and constitutes an embankment, upon which the
glacier advances. It therby grades up its own pathway in advance. On seeing
this process, one is at no loss to understand how ice can advance over
fields of sand or soil without in any way disrupting them. it buries them
before it advances upon them.
Where the frontal material accumulates in a large mass,
it opposes such a degree of resistance to the ice that its layers are curved
upward on the inner slope; and if th glacier subsequently advaces,
the ice rides up over the moraine. Several such instances were observed,
but none was seen where the ice showed any competency to push even its
own debris, in notable quantity, in front of it. The ice is weaker than
the moraine as a whole.
Great quantities of snow are carried by winds from
the region of the great ice cap, and this snow may be lodged in immense
heaps in the lee of the terminal moraines. Such a border drift may have
a breadth of from 1,000 to 3,000 feet. It becomes solidified after
the fashion of a glacier, and may serve to arrest or deflect the
main ice, for it was observed that the basal layers of the ice in places
curved upward on encountering the resistance of this wind-drifted accumulation.
The rate of movement of the majority of the glaciers
was found to be exceedingly slow, though a few which produce large
icebergs are notable exceptions.
The amount of drift on the territory once occupied, but
now free of ice, was scanty. At some points there are considerable accumulations
of drift within a mile or two of the present ice front, but over much of
the area no great moraines, nor any thick mantles of drift, were to be
seen. There was but moderate evidence of glacial action; the land was gently
rounded, but not greatly moulded. In this area of Southern Greenland
tracts of angular, unsubdued topography alternate with rounded, flowing
contours. The inference was drawn that the ice formerly
so extended itself as to reach the present coast for about half its extent,
while in the remaining portion the ice fell ahort. Thus the conclusion
seems unavoidable that the ice of Greenland, on its western side, at least,
has never advanced very greatly beyond its normal boarder in recent geologic
times. This carries with it the dismissal of the hypothesis that the glaciation
of the mainland of North America had its source in Greenland.
There is no ground to question the former elevation of
Greenland, but it would appear that this was not coincident with
conditions favoring glaciation.
H. B. W.
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