Studies on the Rhone Glacier, 1927.--The Structure of the Ice in a Compressed Zone on the...

Studies on the Rhone Glacier, 1927.--The Structure of the Ice in a Compressed Zone on theSouth-Eastern Part of the GlacierAuthor(s): George SlaterSource: Proceedings of the Royal Society of London. Series B, Containing Papers of aBiological Character, Vol. 106, No. 743 (Apr. 2, 1930), pp. 203-215Published by: The Royal SocietyStable URL: http://www.jstor.org/stable/81477 .

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203

55I.3II.I2 (234.3)

Studies on the Rhone Glacier, 1927.-The Structure of the Ice in a Compressed Zone on the South-Eastern Part of the Glacier.

By GEORGE SLATER, D.Sc., D.I.C., A.R.C.S.

(Communicated by Prof. W. W. Watts, F.R.S.-Received January 25, 1930.)

[PLATES 19-21.]

T. Introduction; General Questions Involved.

The following paper embodies the results of a, study of the Rhone Glacier in the summer of 1927. The object of the investigation was to map the structures developed in a selected zone which had been subjected to the influence of concentrated pressure. In the Alpine area this problem has been overshadowed by investigation of the wider problems of the physics of a glacier as a whole, as evidenced by the large amount of research accomplished, notably on the viscosity, the granular structure, and the movement of the ice (9 and 12). Deeley, for example, discussing the genesis of movement of a glacier, writes as follows (4):

"Throughout its course a glacier is urged along, not by a pressure from above, but by gravity acting upon each molecule of ice during its whole course

No doubt thrust does play some part in the movement but it is quite a subordinate feature and only produces local effects" (1895).

The point of view of a geologist regarding ice-motion has been given by Lamplugh, who in 1903 wrote as follows (6):-

"The much discussed problem as to the physics of ice motion is not of immediate consequence to the field-worker in glacial geology, who sees all round him the evidence that the moving mass behaved to all intents and purposes as a plastic body, be the cause what it miay."

But to the student of disturbed Pleistocene Drift deposits the behaviour of ice in a zone of compression is of great importance. Up to the latter part of the 19th century information on this subject was largely based on inferences derived from the morphological study of structures in disturbedl areas of glacial drift, such as Cromer (2), M0en in Denmark (14), and Riigen in Germany (7). Clement Reid, for example, speaking of the Norfolk Contorted Drift deposits, came to the following conclusion (1882) (2):-



204 G. Slater.

The extent of the disturbance at Triningham also points to the employ-

ment of enormo-us force, as well as steady pressure. Though much has

yet to be explained as to the cause of the flow of an ice-sheet, the

acceptation of this agency seems the only way of accounting for these

phenomena."

The demand of geologists for enormous force to account for the phenomena

was consistently opposed by physicists, as being alien to the modus operand,

of ice action. Towards the end of the 19th century, fresh light was thrown

on the problem of the behaviour of ice under compression, as a result of investi-

gations by American geologists in Greenland (3), and of later work in Spits-

bergen by Norwegian and British geologists (5 and 11). The principles

elicited by their work are comparatively simple :--Obstruction to movement of

a glacier results in the formation of a zone of compression. The ice seeks relief

from this pressure (1) by the development of thrust-planes associated with

upward m-ovement, and (2) by a second movement, at right angles to the main

*direction of pressure, caused by tension. These movements are betrayed in

Arctic glaciers by the englacial material incorporated in them, which acquires

aind reveals the structures of the ice.

Such structures are strikingly analogous to those exhibited by disturbed

Pleistocene Drift deposits; as, for instance, those of the Ipswich district;

and these may therefore be regarded as glacial pseudo-morphs (13). They

are not the result of " enormous " ice-sheet pressure brought to bear on flat-

lying deposits, previously laid down, but material carried on and, in a glacier,

contorted and twisted by its movement, imbricated by the thrust-planes formed

on the ice-ward slope of obstructions to ice-movement, and finally laid down

without much re-arrangement and without loss of its structure when the ice

melted. Neither are the great masses of chalk and other rocks included in the

drift due to shifting of such material in mass from its outcrop, but to the

moulding by ice of separate lenticles of country-rock. Thus the evidence of the drift forms a supplement and sequel to that derived

from existing (Arctic and Antarctic) glaciers, the geologist being able to study the final results of varying processes operating in moving viscous bodies over

long periods of time. It, therefore, becomes necessary to study in more detail

the physics of compressed ice in Alpine glaciers, and for this purpose the

writer selected a section of the iRhone Glacier. The detailed trigonometrical

work accomplished by Swiss workers since 1874 in this glacier has provided a

mass of data on the differential movement of ice, which has furniished an



Studies on the Rhone Glacier. 205

admirable starting point for the elucidation of the special problem now dealt with.

II. The South-eastern Embaymnent of the Rhone Glacier.

The area selected for study was the south-easterni concave flank of the Rhone Glacier, adjacent to the termination of the path which leads to the Fort and the road to the Furlka Pass, and at a distance of about 500 metres north-east of the Belvedere Hotel (see accompanying map, fig. 1). The reason for regarding this area as a zone of compression is as follows :

The ice to the north of it, for a considerable distance, showed a regular banded structure, with a normal trend more or less parallel to the side of the

glacier (Plate 19, A). But inside the area itself the ice was broken (by fissures) suggesting planes of movement, and within each of the separate units thus severed the banding was independent and without relationship to that of its neighbours (Plate 19, B and C).

In addition, the surface of the ice showed special features, four longitudinal idges being separated from one another by elliptical basins, each having

a specialised series of crevasses (Plate 20, A, B, C, D). The well-marked series of marginal crevasses were, on the other hand, clearly associated with the movement of the glacier as a whole. Some of the crevasses extended into the eastern elliptical basini. Vertical sections in the lateral walls of the ridges showed highly inclined thrust-planes, associated with contorted ice (fig. 3).

Similar ridges have been described by Forbes (1). At a point of the Mer de Glace named l'Angle, where the glacier was about to turn at a considerable angle with its former direction, Forbes noted in 1844 compact ridges parallel to one another, and separated by localised systems of crevasses. These ridges were characteristic of this part of the glacier. " The crevasses may be nearly transverse to the glacier, whilst the systems of crevasses form an angle of perhaps 30' with the transverse line. The veiniecl strueture again cuts the crevasses at right angles." (p. 205) . . . "I have found a repetition of this phenomenon of a series of discontinuouXs but parallel fissures ranged along a line or axis oblique to their direc-tion, to recur at several points wlhere the strain is very violent " (p. 205). . . "where the violence of the pressure opens a system of such fissures to relieve it, the bands, or systems of surfaces of molecular discontinuLity, disappear, or are less well developed " (p. 205).

Before describing the structure of the compressed area the levidence of the movement of the glacier as a whole will be considered in its bearing on the present problem.



206 G. Slater.

zz,/t''K \ C -

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ezvedere Af2 \Q\ Scale of Metres r _/; f/ % Q ( \ \ \ N ( _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

2 /= r\kt s ' [ \22724 100 50 0 100

Contours. Successive margins of advancing ice.

e Direction of movement of bench-marks ((reptres).

FIG. 1 1, 2, 3, 4 indicates ridges of ice. The thick square shows area described. This ma,p is a portion of Plan No. 3 (with omissions) of the Swiss Glacier Commission [10]1 Reproduced bky permission of the Glacier Committee of the Soci6te Helvetique des Sciences Naturelles and Prof. Dr. P. L. Mercanton.




11. Trends of Movement on the Rhone Glacier.

The lines of mnovement of the Rhone Glacier are approximately showNTn oil the rnap, fig. 2, which emnbodies the results of the Swiss Giacier Commission (1874-1900) (10). The lines of movement may be divided into three groups as follows: -

1. Those in the upper part which converge. 2. Those in the intermediate part which tend to parallelism with one another

and the side. 3. Those in the lower part which diverge.

It is the intermediate part, 2, on which the present paper has a direct bearing. These lines for convenience may be sub-divided into three groups (a), (b) and (c) (see fig. 2) as follows:

(a) A central group of curved liines which pass completely down the glacier and ultimately reach its termination.

(b) A marginal group having an unusual orientation. These lines bend gradually towards the nearest flank and terminate there at an angle more or less acute.

(c) A third group, on the eastern side of the glacier, intermediate between (a) and (b). These lines are inarked only partially, and terminate in the south-eastern area as shown in figs. 1 and 2.

Speaking of the loss of reperes, MIercanton writes as follows (10, pp. 154-5)

" Malheureusement la disparition des reperes marginaux, les plus illneres- sants, nous a prive trop tot d'un element d'information de tres grand prix. Ce que nous savons suflit encore 'a confirmer les vuies de Ainster- waider: dans le collecteur, les lignes de mouvement se rapprochent de l'axe d'ecoulement du glacier en tendant au parallelisme."

The reason for the loss of the reperes will be dealt with at a later stage in3 this paper.

The ice between that traversed by the central group of lines (a) and the eastern margin of the glacier, forms a segment of a circle, in which the movement lines are either directed towards the sides, or lost as stated above. How does this ice ultimately reach the end of the glacier ? Only two alternatives seem possible

1. The lines or movement mnay converge: or 2. The mnovement of the ice within the segm-ent may be of a specialised

type.



208 G. Slater.

North

-7000 720 0 -73500

j$~j

__ 770 _ _ _ i:,_ __ iz 1

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/ , ~~Ga/enht)tten : 0 0 X Dbin10 (

4 _2000- C - -- _ _ -

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v</~~~~~~~

Gletsch,> .000 Scale of Metres

-~~~~~~ ~ ~ -t - - t-- = -- - 2 1000 500 O 1000

FiG. 2.-Rhone G-lacier, 1916. BaJsed on plans No. I and No. 3 of the Swiss Gla ier Comnmission 11101. Reproduced by permission.




On this problem it is well to recall the pioneer observations of Forbes, recorded in his 'Theory of Glaciers' (1). Describing, with a diagram, an embayment of the glacier of La Brenva, 1842, he writes

"it seems clear that all the ice within the line FED [that is, the ice in the more central part of the glacier], or between it and the shore is embayed, as it were, and has bnt little motion in consequence of the intense pressure with which the whole mass of the glacier is urged against the side of the valley " (pp. 182-3).

Forbes's trigonometrical measurements confirmed this view, for whereas the outer zone of ice FED moved 14 * 2 inches in 24 hours, the embayed ice was practically stationary relative to the gen-eral mnovement of the glacier, but had moved outwards and upwards against the margin of the glacier by 2 inches. Forbes compared the behaviour of the embayed ice to the rotatory motion of water under similar circumstances (pp. 182-6).

The structure of the south-eastern embayment of the Rhone Glacier confirmed the view that movement in that area was of a specialised type. The ice in the segment of the glacier moving downwards, collected in the south- eastern em-lbayment, and produced a zone of ice under concentrated pressure.

IV. Metlhod of Study.

The trigonoinetrical survey of glaciers has resulted in the accumulation o a mass of data onI the differential movement and structure of the ice. In spite of this excellent work, however, the scale of such maps is inadequate to show the detailed structure, and further, in no case, to my knowledge, has a map of a glacier been published which shows the thrust-planes developed more especially in compressed areas.

Results previously obtained by detailed mapping of disturbed drift deposits of the Ipswich district (13), encouraged the autlhor to adopt the same method on the Rhone Glacier. The mnethod adopted was the usual onte employed by geologists, viz., the detailed mapping of the structures displayed, and the recording of vertical sections whnere visible. The surface of the ice was marked out in squares, 50 feet on the side, the sides being orientated N.-S., E.-W. (magnetic).

The square was outlined by two 100-feet tape-lines, and the structure seen on the ice plotted to a scale of 50 feet to an inch. On the comlpletion of one square an adjoining one was marked out and the structure recorded as before.

Q 2



210 G. Slater.

The total number of squares coimpleted was about 70, representing a total area

of about 4 acres. The results are plotted on the plan (Plate 21).

V. Structure of the Ice (Plate 21, map).

The two most striking features of the structure of the south-eastern embay-

melit of the Rhone Glacier were (1) the presence of compressed blocks of ice,

and (2) the association of these blocks with thrust-planes.

(1) Compzressed Block of Iee (Plate 19, cf., A and B).--The banded structure of

the ice had suffered extreme dislocation, and was orientated in many directions

in the separate blocks. The ice was frequently contorted, and the bands of

the ice in each block usually curved near the mnargins. The blocks of ice were

of various shapes but were usually triangtular in form. Towards the southern

part of the map (Plate 21) it will be n-oticed that the ice was compressed and

contorted into narrow strips. Such a structure must inevitably mean mass-

movement due to pressure, and indicates a type of movement unlike that

already described by Deeley. (2) Thrust-planes (Plate 19, C).-The planes separating the compressed

blocks (1) were thrust-planes. Their orientation was significant, for they were

arranged in fan-shaped groups, each group radiating fromn a point usually,

but not always, adjacent to the easterly margin of the glacier. The orientation

of the thrust-planes in each group varied from approximnately north-west to

south-east, to nearly east and west when traced down the glacier. The latter

direction, for example, was domninant in the southern part, as sho-wn in the

map (Plate 21). The association of the thrust-planes with the compressed blocks showed that

the latter occurred between the " jaws " of thrust-planes. Pressure of ice

between the thrust-planes had apparently squeezed the blocks away from the

lateral margin of the glacier and towards the adjacent elliptical basin, which,

as will be shown later, was a zone of tensioni. The arrangement of the thrust-planes clearly suggested differential move-

ment of the ice, that near the margini mov7ing more slowly than the ice farther

away from the side. The orientation of the thrust-planes also supported the

view of a progressive change in direction of miiovement down the glacier. The

movement towards the south-west, for example, had been slowly deflected to

a mo-vement more nzearly southwards. Vertical sections in the walls of thle elliptical basins were extremely instructive.

These sections showed a series of highly inclined thrust-planes associated with



Studies on the JRhone (7lacie%. 211

contorted ice. The sections proved that the ridges of ice were dlute to an upward movement along thruLst-planes (fig. 3).

S ___N

T P TF p r R Fie.. 3.-Vertical section in the side of one of the elliptical basins. Contorted ice

associated with thrust-planes (T.P.). Height of section about 40 feet.

The amount of detailed work which .had to be done only allowed the structure to be recorded in the area adjacent to the most easterly ridge, blut the principles formulated from this area will apply to the adjacenit ridges.

VI. Relationship between General Moveeinet of Rhone Glacier and that of the Com3pressed Area.

The lines of movemenit of th-ie lower part of the Rlhonie Glacier (fig. 2) inldicate a general direction of movement towards the south-west. The tlhrust-planes in the compressed area, on the other hand, showed this trenid to have been deflected to a mxovement to the south. S-utch a deflection was evidently due to the retarding by friction of the lce adjacent to the flank, against the side of the glacier.

The point wher-e deflection of nmovement becamne apparent was a tensional area, for the ice at this point was drawn apart. This area was marked by the easterly elliptical basin, the floor of which gradually rose towards the south where the southerly movement aloXne became operative (Plate 20, A).

The generat effect of these movements was reflected in the structure. The compressed ice had obtained relief in two ways, (1) by an upward rise, th-us forming the ridge (section, fig. 3); (2) -by lateral squeezing towards the ellipticat basin (see mnap, -Plate 2 1).



212 G. Slater.

The zone of greatest compression occurred adjacent to the projecting spur of the side of the glacier south of the map. These observations suggested that the loss of bench-marks mentioned by Mereanton was due to specialised types of movement of the ice. These movements would raise the bench-marks to higher levels and deflect them towards the more central part of the glacier. Hence parallelism of the lines of novement would, as a natural consequence, be destroyed.

The sequence of evenits which led to the formnation of ridge I and its elliptical basin (Plate 20, A) was repeated at a somewhat later stage in the more central parts of the glacier, resullting in the progressive fornation of ridges 2, 3 and L,

and their associated basins (map, fig. 1).

VII. Application of the Principles of the Structure of the Compressed Zone of the Rhone Glacier to Areas of Disturbed Pleistocene Drift Deposits.

As already mentioned in the Introduaction, the value to geologists of work on the structure of compressed areas of " living "' glaciers lies in the fact that similar structures are preserved in disturbed drift deposits, owing to the

removal of the interstitial ice associated with englacial material. Amnongst the structures preserved, flow-curves and thrust-planes are dominant.

The groups of radiating thr-ust-planes seen on the surface of the Rhone Glacier are the same in principle as those also seen, in plan, in the disturbed deposits of the Hadleigh Road Area, Ipswich (13). In the PRiigen area the lines of trend of the thrust-planes have been mapped by Keilhack (8). As a result of the present investigation it occurred to the anthor to extend these coastal lip-es seawards by nmeans of dotted lines. The result of so doing was to show that the thrust-planes arranged themselves into a series of groups. The

thrust-planes of each group converged towards a common point, whilst there

was a progressive deflection of the dominant trend of each group, when compared with its neighbour to the south (taf. IV, fig. 1 (16)). Th-e total distance in which this phenomenon could be traced in the Rilgen area was 5 miles.

In explanation of this extensive zone of compression, it muLst be remembered that the structure displayed in the clifis represents ice-action on a regional scale. The compressed zone is therefore quite a local feature comnpared with the probable area of the Baltic Glacier. The repetition seen in the Rilgen cliffs demonstrates in a convincing manner imbricate-structure (taf. AV (16) ). The lower boulder clay, which is divided repeatedly into isolated strips, forms an agreed horizon, all workers agreeing that it once formed a connected stratum



Studies on the Rhone Glcaeier. 213

of boulder-clay. In the absence of englacial material in the Rhone Glacier, it is difficult, perhaps impossible, to prove a similar repetition of structures.

VIII. Sumimary.

The investiga-tioni of the Rhone Glacier by the author in 1927 was confined to two lines of enquiry: (1) the structure of the ice forming the concave side of the south-eastern flank of the glacier ; (2) the relationship between the air- temperature and rate of the surface melting of the ice.

(1) The surface of the mnarginal ice was ularked into 50-foot squares, and the structure plotted on a map to a scale of 50 feet to an inch. The ice formed a mound' dissected into ridges by three longitudinal, basin-shaped trenches, which were heavily crevassed laterally and bounded by crevasse-like walls of ice longitudinally. Thrust-planes dipping at high angles formned a characteristic feature of the ridges. In plan they formed radiating groups, the fulcrumni of each group being near the margin of the lateral moraine. The trend of these thrust-planes varied progressively southwards, from NN. W.-S.E. to E.-W. approximately, and adjacent pairs formed the jaws of squeezed wedges of ice showing displaced ribbon-structure. Crevasses radiated 'from the lateral morainie and in places dissected the junctions. of the thrust-planes. The structure as a whole suggests pivotal mnovement of the compressed marginal ice, the "' trenches " representing tensional areas due to the deviation in direction southwards of the movement of the ice, from the naormal south- westerly trend. Relief from pressure was obtained both laterally and longitudinally by the squeezing inwards of the ice towards the tensional areas on the one hand, and by the upward rising of the ice along thrust-planes on the other.

The second line of enquiry (2) forms the subject of aniother paper published by the Royal Meteorological Society, London (17).

TX. Conclusion.

In conclusion the author expresses his thaniks to Prof. P. L. Melrcanton anld the Commission of Glaciers (S.H.S.N.) for permiission to reproduce thlze maps shown in figs. I and 2. To Prof. W. 'W. Watts, F.R.., he is indebted for granting facilities for the working out of the results in the Geological Depart- ment of the Imperial College of Science and Technology, London. The expenses of this investigation were defrayed by a Government grant from the Royal Society, for wlhich- the author tenders his tlanlks.



214 (. Slater.

X. B fBLIOGRAPHY.

(1) 1859. Forbes, Jaines D., "Occasional Papers on the Trheory of Glaciers," Edinburgh. (2) 1882. Reid, Clement, " The Geology of the Country around Cromer," ' Geol. Survey

Memoir.' (3) 1895. Chamberlin, T. C., " Recent Glacial Studies in Creenland," ' Bull. Geol. Soc.

America,' vol. 6, pp. 199-220, Plates 3-10. (4) 1895. Deeley, R. M., " A Viscous Flow of G$lacier Ice," ' Geol. ildag.,' decade IV,

vol. 2, pp. 408-415. (5) 1898. Garwood, E. J., and Gregory, J. W., " Contributions to the Glacial Geology

of Spitsbergen," ' Q.J.G. Soc.,' vol. 54, pp. 197-225. (6) 1903. Larmplugh, G. W.Y, " The Geology of the Isle of Man," ' eol. Survey Memoir.' (7) 1907. Philippi, E., "43Die Stsrungen der Kreide ulnd des DilutiviuLms anf Jasmnund und

Arkona (Rilgen)) 'Zeitschr. f. Gletscherkunde,' vol. 1, pp. 81-112 and 199-244. (8) 1912. Keilhack, K., "Die Lagerungsverhdltnisse des Diluviums in der Steilkiiste von

Jasmund auf Rilgen," ' Jahrbuch K. Pireuss. Geolog. Landesanstalt,' vol. 33, pt. 1. (9) 1913. Evans, J. 'W., " The WAVearing Down1 of the Rocks," ' Proc. Geol. Assoc.,'

vol. 24, pp. 241-300, Presidential Address, pt. 1. (10) 1916. Mercanton, P. L., ancd others, " Vermessutngen am Rhonegletscher-Mensura-

tions an Glacier dn Rhone, 1874-191.5," ' Dirigees et publiSes par la Coommission des Glaciers de la Societe' Helvetique des Sciences Naturelles,' Ziirich.

(11) 1925. Slater, G., (Oxford Unliversity Expedition to Spitsbergern, 1921) " Observa- tions on the Nordenski6ld anid Neighbouring Glaciers of Spitsbergeni," also in " Spitsbergen Papers," Oxford Univ. Press, 'Journ. of Geol.,' vol. 33, No. 4, pp. 408-446.

(12) 1927. Tutton, A. E. H., " The Natural History of Ice and Snow " (Kegani, Pand, Trench, Triibner & Co., Ltd.) [Illustrated froin the Alps.]

(13) 1927. Sater, G., "The Structure of the Disturbed Deposits of the Hadleigh Road Area, Ipswich," 'Proc. Geol. Assoc.,' vol. 38, pp. 183-216.

(14) 1927. " The Structure of the Disturbed Deposits of MIoens K-lint, Denmark," ' Trans. Roy. Soc. Edinb.,' vol. 55, pt. 2 (No. 12), pp. 259--302.

(15) 1927. " The Disturbed lacial Deposits in the Neighbourhood of Lonstrup, near HEjrrhig, North Denmark," 'Trans. Roy. Soc. Ediab,,' vol. 55, pt. 2 (No. 13), pp. 303-315.

(16) 1929. Slater, George, " Die Strukturverh5iltnisse der gestdrten Kreide und Diluvial- Ablagerungen der Ostkiiste Rilgens (Jasmund Distrikt)," ' Neuen Jahrbuch ffir Mineral., etc.,' ' Beilageband,' vol. 63, abt. B, 1929, pp. 123-136.

(17) 1929. Slater, Ceorge, "Studies oni the Rhone Glacier, 1927. The Relationship between the Average Air Temperature and the PRate of Melting of the Surface of the Glacier." (Communicated by Sir Gilbert Walker, C.S.I., F.R.S.), 'Q.J.R. Meteor. Soc.,' vol. 55, No. 232, pp. 385-393, October, 1929.



Slater. Roy. Soc. Proc., B, vol. 106, Pl. 19.

(Faci,ng p. 214.)




CfCP




Plan of I (

South-Eastern Portion of //

RHONE GLACIER i

1927.

/ ~~ ~~~ ~~~~~~0 50 tOO ISO 200 SCALE OF FEET

'~ROCKI (Facing p. 215)



Studies on the Rhone (Glacier. 215

DESCR1PTION OF PLATES 19-21.

PLATE 19.

Strueture of fce in S.E. portion of Rhone Glacier, 1927.

A.-Normal a'rrangerient of ribbon-structure in the ice, dissected by a crevasse. B-Abnormual arrangement of ribbon-structure in the ice. One of the compressecl blocks of ice associated with thriust-planies. C.-Middle portion of the Rhone Glacier. The compressed zone occurs in the foreground a u(l displays thruist-planes, with ice raised to higher levels.

PLATE 20.

Ridges aad Basins in S.E. part of Rlhonie Glacier, 1.927.

A.-Easterly ridge of ice (ridge 1 of map, fig. 1), with associated elliptical basin anCd portion of ridge 2. B.-Ridge 2 of map, fig. 1. C.-Upper portion of crevassed basin of A. D.-Portion of the same basini as C, showing the steep easterly wall.

P,LATE 21.

Plan of S.E. portion of Glacier.

Firm lines = thrust-plan-es. Open spaces between firm lines c crevasses. Narrow parallel lines _ ribbon structures. Blank squares in southern part. sno-v- covered areas. Ifn upper part of m-ap blank squares represent UroiLappedl areas.

I Ridge 1 of map, fig. 1, p. 206, and Plate 20, A. 2 Ridge 2 of map, fig. 1, p. 206, and Plate 20, B.

A crevassed basin occurs between 1 and 2.



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