Recent Glacial Studies in Greenland
Scientific American supplement, No. 1018
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 suffered.
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 ice.
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 condition.
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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