June 6, 1895] 



NATURE 



'39 



RECENT GLACIAL STUDIES 

 GREE.XLANOy 



IN 



T~\L'KIX(; the summer of 1894. Mr. Chambcrlin was cnahlcil ^ 

 ^^^ to devote some time to a ]>ersonal study of the glaciation of I 

 (Ireenlaml, and the results of his observations arc so interesting, 

 that all j^eologists who seek to interpret the records of the "' (Ireat 

 Ice Age,' will gladly make acquaintance with them. SeUlom 

 has a geologist so experienced in the study of glacial clriftsand of 

 the problems connected with ihem, had the advantage of exam- 

 ining the behaviour of ice in the .Vrctic regions. 



His observations were specially directed to the way in which a 

 glacier gathers up detritus ahmg its course, to the way in which 

 it carries it forward and finally puts it down. The main jjroblem 

 he sought to solve, was connected with the basal material of 

 glaciers, deljris which, of course, is largely concealed. 



In comparing the glaciation of (Ircenland with that of the 

 mainland of North .\merica, he had to bear in mind, that for 

 ihe most |Mrt the continental drift is spread over a vast pKiin. 

 In Greenland the ice-fields rest mainly on plateaus fringed by 

 rugged mountains, and he sought for a tract free from such bor- 

 dering elevations. This was found at Inglefiekl Gulf, where the 

 Iwrderland is a plateau about 2000 feet above sea-level, and w here 

 the margin of thegreat ice-sheet may be studied on relatively smooth 

 ground, on un<lulating ground, and in lobes or tongues that descend 

 the valleys. Of thethirtyorforty glacial tongues which descend to- 

 wards Inglefiekl Ciulf, less than one-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, and whose lower gradients are moderate. 



The fact that great part of Greenland a])pears to consist of 

 ■ancient gneissic rocks, renders the debris more or less stt^ny and 

 .arenaceous ; clayey material is rare. About Inglefiekl ( iulf, 

 however, the older rocks are covered by thick layers of sand- 

 •stone and shale, traversed by basic igneous dykes. Hence it is 

 p<jssible there to tell how late the erratics from this sedimentary 

 series were introducetl 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 1 50 feet 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 attracted particular attention. 

 The ice was found to he almost as distinctly bedded and 

 laminated as a sedimentary rock. The vertical face was .seen 

 visually to present two great divisions — an upper tract of thick, 

 obscurely laminated layers (if nearly white ice, and a lower 

 laminated tract discoloured by debris. At the iKise there is 

 usually a 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 boulders. These vary from a 

 mere film of silt to a 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 boulder of con- 

 siderable 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 anil disappear. Lenses of debris also appear, and the layers 

 are sontetimes doubled back upon themselves. 



The laminie of the ice are sometimes very symmetric, straight, 

 and |>arallel, but often wavy and undulating. In many instances 

 ihey are greatly curved or contorted. Thus, as I3r. E. von 

 Drygalski has remarked, they closely simulate the foliation and 

 cimlortion of gneiss. 



The ilebris-belts, which are essentially parallel to the base i>f 

 the glacier, are confined chiefly to the lower 50 or 75 feet, but 

 they occur u]) to 100 feet and, |)erhaps, to 150 feet. They are 

 more abun<lant at the sides of the lobes than in the centre : a 

 notable iiortion of the debris having evidently been introduced 

 after the lobes were formed. Thus the detritus ajipears mi>st 

 abundant in gl.icier-lobes which descend as catar.icts, or crowd 

 between closely hugging cliffs. 



In meeting obstacles the bas;tl beds of the glacier sometimes 

 simply curve U|iwards, carrying their ilebris with them over the 

 obstacle ; at other times, the laminie of ice are much crumpled. 



1 .\bridgc'd from .1 paper by T. C. Chamberlin, in the Bulletin uf tlic 

 CcoIogical Sticiety of .\merica, February 1 895, 



NO. 



1336, VOL. 52] 



Not only are the foliations of the ice twisted, but they are at 

 times fractured ami faidted, and along the fault-plane the laminie 

 are affected by '• ilrag,'' as in faulted rocks. 



The general stratification of the ice had its initial stages in the 

 original snow-falls : and the sea.sons <loubtless developed annual 

 subdivisions. The more definite partings and the introduction of 

 the layers of debris, arose through a shearing movement between 

 the layers of ice. 



The actual jirtKess of intrusion of detritus was observed in 

 proximity to a large boss of rock which, protruding through the 

 margin of the ice, had been partially cut away. Trains of 

 debris, apparently rubbed from the surface of the rocky dome, 

 were carried out almost horizontally into the ice in its lee. Some 

 of these were short, while others extemled several ro<ls into the 

 ice, passing into the body of it instead of following its Ijase. .\t 

 one jjoint the overthrust of the ice reached such a degree as to 

 carry the earthy layers obliquely across the thickness of the 

 glacier, producing a marked unconformity. 



In another insiance similar features were observed Ijelow an 

 ice-cataract. Tongues of <icbris, having their origin in the 

 boulder-clay below the glacier, were seen to reach out into the 

 ba.sal ])ortion of the ice as though they were being introduced 

 into it by the differential movement of the layers U]>on each 

 other. Thus w hen 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 jasses over, being 

 prevented by the former portion from settling down at ijnce, is 

 thrust forward over it. This is accomplished by the bending and 

 <loubling 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 mo\"ing 

 mass, carrying the overthnist layer of debris between it and the 

 ice-layer above. 



It appears obvious that the ice in the lee of a rocky prominence 

 moves more slow ly than that above ; hence the doubling of the 

 lamin;e upon themselves. Moreover, there is a gradation from 

 laminar that simply suffered doubling up, to layers that obviously 

 sheared ujx>n each other anfl produced manifest unconformity by 

 overthrust. 



Evidence showed that the more solid (blue) bands in the ice 

 are pri;iduced by exceptional pressure in moving over rugosities, 

 and that their ]>osition in the ice is jmrallel to the ice-movement ; 

 while at the same lime blue bands may be develo]x;d nearly at 

 right-angles, after the manner of slaty cleavage. 



Summarising the above conclusions, it appears that stratifica- 

 tion originated in the inetjualities of deposition, emphasised by 

 intercurrent winds, rains, and surface meltings : that the incipient 

 stratification may have been intensified by the ordinary processes 

 of c.insolidation : that the shearing iif the strata upon e.ich other 

 still further emphasised the stratification, ami developed new- 

 horizons under favourable conditions; that basal inequalities 

 introduced new jilanes of stratification, accompanied by earthy 

 debris, and that this process extendetl itself so far as even to form 

 very minute lamin;v. 



There is involved in the foregoing conceptions the idea of an 

 ice-layer acting as a unit of movement ; at any rate, there is 

 recognised individuality of niovemenl in the layer. This view 

 involves the idea of rigidity rathei than viscosity. The intro- 

 duction of earthy material into the ice-layers involves the idea 

 of thrust rather than pull. The picture is not that of gravita- 

 tion pidling a thick, stiff liquid down the lee side of a rocky 

 prominence, but of a rigid body thrusting itself over the crest 

 by means of a force in the rear. 



The extreme fragility of the ice is difficult to harmonise with 

 the idea of viscosity. Wherever the ice i».ssed over an undula- 

 tion of even moderate dimensions, it was abundantly crevassed. 

 There was no indication that boulders descend through the ice 

 as heavy substances descend through viscous bo<lies. The 

 rigidity did not prevent contortions and foldings of the lamina- 

 tions, such as lake place in crystalline rocks, but faidting and 

 vein-structures also occur : and there seems no more occasion to 

 assume viscosity in the one ca.se 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 Ije made to change its form by the removal or 

 particles from one ixjrtion by melting, and their attachment at 

 other points by congelation ; but not, apparently, by the flowing 

 of crystallised jxirticles over each other in their crystalline 

 condition^ 



It has been already |)oinletl out that much basal material is 



