Note on the Franz Josef Glacier, December, 1940.
[Read before the Canterbury Branch, December 16, 1940; received by the Editor, December 24, 1940; issued separately, September, 1941.]
Table of Contents.
Form of the Ice-face.
Features in Front of the Ice-face.
Wastage of Ice, and Comparison with the Fox and Tasman Glaciers.
The present (December, 1940) condition of the lower parts of the Franz Josef Glacier, Westland, is so interesting that it merits some record. In 1934 the author (Speight, 1935) noted the position of the front as recorded at various dates from 1894 to 1934, showing that the ice after having an almost stationary front from 1894 to 1909, retreated till 1921, and then advanced till 1934. According to Messrs. Peter and Alec Graham, of the Franz Josef Hotel, Waiho, it again retreated to some extent during the years 1934 to 1938, and a fall in level was noted in the higher parts of the glacier, e.g. near the Almer Hut, but since that date the recession and lowering of the frontal portion have been most striking. There is now evidence of a rise in the upper portions, notably at Roberts Point and Cape Defiance, which will probably produce a reversal of movement at the face within a reasonable period. It is possible that the glacier is near a definite minimum, whose date should be determined as accurately as possible, and its condition at that time noted for future reference.
Park Rock, a commanding roche moutonnée lying in the middle of the valley, the furthest upstream of those on which Dr. Mackintosh Bell caused an iron peg to be placed (No. 4) in 1909 (Bell, 1910) to mark the edge of the ice, will be used as a point of reference for localities mentioned in the text. The rock is of schist, the prevailing country rock of the lower portion of the Franz Josef Valley.
B. Form of the Ice-Face.
(See Plates 11, 12, and 13.)
While one cannot always regard the position of the face of a glacier as indicating the true border of the ice, seeing that it may extend further forward at a lower level, it is a dominant feature and its position should be noted.
The face of the Franz Josef as it exists now has a somewhat acute point directed north towards Park Rock, from the base of which it is distant 12 chains. It extends S.S.E. for forty chains on the eastern side and S.S.W. for thirty chains on the western side, till it ends in both cases in close proximity to the valley-wall, its present front indicating an average retreat of about 25 chains (500 metres) from its 1934 position. The existence of the point in the middle of the valley can be attributed largely to the attacks of the issuing river on each margin in turn, for while it formerly flowed from the western side, it now runs out from the eastern side, and has not issued from the middle of the ice-front for a considerable period, so the outstanding point has been left in a central position. Although the river now flows along the eastern side of the valley till it is clear of the roches moutonées lying furthest downstream, some few months ago it crossed over in front of the ice-face, ran into the lake as it existed then, and discharged therefrom between Park Rock and Wilson Rock.
According to Mr. Peter Graham, the outflow of the river on the east side was, five months ago, as far back as the mouth of Rope Creek, whereas now it is three chains below it, * and the abandoned bed is occupied by gravel and rock debris. The front of the ice is nowhere high, and does not exceed forty feet at most, while south of the face the surface of the glacier is fairly flat till it rises to the broken ice resulting from the constriction of the valley at Roberts Point and the consequent disturbance of regular movement. Only on the extreme point of the ice and near its western margin is there any cover of moraine, and this is a thin veneer consisting always of small, rounded, and sub-angular pebbles of schist, rarely more than six inches in diameter, which have been brought from beneath the ice by the differential upward movement of the lower layers, a direction indicated by the angle of veining which shows plainly near the western edge of the ice-front. The rounding of the pebbles no doubt took place in the bed of the sub-glacial stream, but the deposit now lies at too high a level on the ice to have been laid down by river action; it must have been raised to this position by the ice.
C. Features in Front of the Ice-Face.
(a) River-bed. (Plate 12.)
All over the ground not occupied by water, whether river or lake, immediately in front of the face, lies a veneer of river gravel or of rounded moraine deposited by the river, and between Park Rock and the ice-face this rests on solid ice, for sink-holes and ice covered with gravel occur widely distributed over the flat ground, and three mounds composed of ice with a veneer of moraine rise from it. Two of these form a low ridge abutting against the lower slopes of Park Rock, so it is clear that ice extends from the front of the glacier right to the base of the rock.
(b) Lake. (Plates 11, 13, and 14.)
On the western and north-western side of the glacier lies a lake of roughly quadrangular form, 42 chains in length, and 17 in maximum breadth. It is covered in parts, specially in the middle,
[Footnote] * According to Mr. A. Graham, the ice had receded a further twenty chains on the east side of the glacier by April, 1941.
with floating and stranded icebergs. All the eastern side as far as the base of Park Rock is margined by ice in position, either clear or capped with moraine, while the western side is bordered mostly with debris with a medial section of solid rock, and in the north-west corner with ice capped by moraine. On the north-east it is bounded by the solid mass of Park Rock, off which depths of fifty feet were registered. Although one cannot be certain, it is reasonable to consider its floor to be formed of ice in position; and, if the ice under the river-bed were to waste away, a lake would extend right across the valley, ponded back behind the barrier of solid rock which stretches across on the line of Wilson and Park Rocks. This barrier probably continues to the margin of the valley above Barron Rock, the line being determined by a fault which has a downthrow to the south (Speight, op. cit., p. 319), and the roches moutonnées are remnants of the ridge left across the valley on the upthrow side. A lake so formed would occupy the lowest tread of the glacial stairway. The present lake may be regarded as an ephemeral phenomenon; but it is probable that it will persist for a considerable period, and, if wastage of the ice continues and there is no accession of supply from the upper parts of the glacier, it may extend as just suggested. If accession of supply does take place the lake will disappear and its site will be occupied by ice once more.
(c) Morainic Deposits. (Plate 12.)
The small amount of terminal moraine lying immediately in front of the glacier occurs first of all in a triangular shaped area to the east of Park Rock, and secondly to the south of Wilson Rock, where between the rock and the lake at least two low but definite ridges occur. They bend round and take the form of lateral moraine south-west of the rock between the edge of the lake and the valley wall. These deposits consist almost entirely of schist pebbles of small size; large masses of greywacke, derived from the upper levels of the valley, where greywacke overlies the schist, occur only rarely.
This paucity of moraine is peculiar, for the glacier at one time formed a large typical latero-terminal moraine from half a mile to over a mile downstream from Park Rock as noted on Bell's map (op. cit., opp. p. 6), and the forward part of this is covered with bush containing large, mature trees, whereas the upstream side of the moraine is covered with scrub, the sequence being analogous to that showing on the sides of the valley above the glacier. Next to the ice lies bare rock, above that scrub-covered rock, and above that again bush with large trees. If one considers how quickly the roches moutonnées in front of the present glacier have been covered with scrub within the last thirty years, it seems reasonable to suggest that the growth of scrub on the valley sides may have taken a comparatively short time, seeing that the rock is the same, and that therefore at a period, say 100 years ago, the glacier may have reached up to the lower margin of the large bush on the valley walls and the line of similar bush on the moraine downstream. In support of this possibility there is a statement by Harper (1896, p. 306) that according to a photograph taken in 1867, the ice was 80 to 100 yards further downstream at that date than it was in 1894. A similar succession
View of the frontal portion of the Franz Josef Glacier taken from Park Rock, a part of which just shows in the lower left corner of the picture. The flat ground in the foreground is formed of gravel overlying ice. The front of the glacier has fallen by wastage at least 100 feet, but there are signs of an advance at Roberts Point in the middle distance. Only the front margin of the glacier is covered with moraine. The lake lies on the extreme right.
View of the frontal part of the Fox Glacier taken from Cairn B on Cone Rock, the spot identical with that from which the picture shown in Plate 6l (Trans. N.Z. Inst., vol. 64) was taken. The foreground consists of the abandoned bed of the issuing river: in the middle distance is the down-wasted moraine-covered Ice. In the earther picture the ice was higher than the cairn, whereas now the river-bed and the moraine-covered ice is far below it.
of bare rock, scrub, and bush with large trees occurs near the Fox Glacier, concerning which a conclusion may be reached similar to that in connection with the Franz Josef Glacier.
D. Wastage of Ice. (Plates 12, 13, and 15.)
In 1934 the ice-front of the Franz Josef was just level with the top of Park Rock, and the surface rose gradually southward, whereas now for an average distance of 25 chains it has sunk approximately 125 feet, and, over the projecting tongue of the glacier, for more than 100 feet. To this must be added the depth of the lake in order to estimate the total amount of wastage since 1934. The ice lying between Park Rock and the face of the glacier, and some of the glacier front also, is probably in a stagnant condition, but it was not found possible to make any precise observations to demonstrate this definitely. It is perhaps not entirely correct to attribute all this deficiency to down-wasting in a strict sense, for the wash of the river as it occupied different courses has contributed not only to the destruction but also to the removal of the ice.
In this connection it is instructive to consider the condition of the lower portion of the Fox Glacier. This is now strikingly similar to that of the lower part of the Tasman Glacier, since, for at least half a mile from the front, it is completely masked with moraine furnished by rock-falls from the valley walls. Heavy falls have recently come from Cone Rock, from the eastern side of the valley just opposite, and from the Passchendaele Slip higher up the glacier, which will maintain the morainic cover for a long time to come. Although the actual front of the glacier has changed but little in condition and position from what it was in 1934, the lower part of the glacier has collapsed in a remarkable manner (Plate 15). Behind the actual face inroads have been made on the main body of the ice by the glacial stream, wide areas have been channelled by abandoned river courses, have been honeycombed with tunnels, and pitted with sinkholes, some of the last of large size, while further up the glacier near the south-eastern end of Cone Rock the level of the ice has fallen about 150 feet since 1934.
It is probable that the collapsed ice is in a stagnant condition or has a very slow rate of movement, and it differs from the Franz Josef only in being more heavily masked with debris, while its general condition corresponds exactly with that of the Tasman. This indicates that some declining glaciers do not retreat at right angles to the face, which may exhibit little change, also that down-wasting occurs behind a relatively stable front and may extend far up the glacier.
This condition suggests a reference to the Sierra (U.S.A.) glaciers as mentioned by Matthes in his report as Chairman of the American Committee of Glaciers (1940, p. 399). He notes the disproportionately large size of the terminal moraines lying in front of these glaciers, and that the feature was recently explained in a somewhat surprising manner as the result of a sudden and accidental draining of a lake ponded behind the moraine of the Conness Glacier, the outlet of which gave a clear-cut section through the barrier. It was formerly supposed that this was constructed entirely of morainic material, but it proved to be mainly of glacier-ice covered with a thin veneer of moraine, that this ice must have persisted from the
time when the glacier reached the barrier, and that the persistence was due to the protection against sun-heat afforded by the rock veneer. Thus the general circumstances indicate that the retreat of the glacier took place not hundreds of years ago, as formerly supposed, but only decades ago. This explanation may fit the case of the moraine-covered ice-fronts of the Fox and Tasman Glaciers, which are practically stationary, since in both cases the moraine would screen the ice from direct sun-heat.
The absence of this protection may be an important factor in the down-wasting of the frontal part of the Franz Josef Glacier and the clear-ice portion of the Fox near the upper end of Cone Rock; but, if so, the pronounced collapse of the moraine-covered area lying immediately behind the front of the latter merits further consideration, since the cover of debris has not protected this part of the glacier against rapid down-wasting, and the same may be said concerning the lower reaches of the Tasman upstream from the terminal face. Basing the opinion on my own experience, and not on accurate meteorological observations, as the latter are not available, I conclude that summer sun-heat is not the only important melting agent in these two cases. The conclusion rests partly on the comparative rarity of ice-tables on New Zealand glaciers, and the low altitude above the general surface of the ice of such as do occur. The highest I have noted occur on the Ashburton Glacier, which lies 10 or 12 miles east of the main divide under the shelter of Mount Arrowsmith (9171 feet), and their development in this case may be explained by the fact that this glacier is more exposed to the direct action of the sun than those in the west, and it does not experience the full effect of the heavy rains which fall on the western slopes and just east of the crest of the main divide. This rainfall, where heaviest, is approximately 200 inches a year, it falls mostly in summer, and makes its way down the mountain slopes to the floor of a valley, and where this is occupied by a glacier, it runs under the ice to emerge as a full-bodied stream, increased in size by what it has collected from the ice itself. It not only melts but erodes actively, undermines the glacier, and thus causes collapse, an effect entirely independent of the morainic cover. To this effect must be added that of the rain falling directly on the surface of the glacier itself whether it be clear or covered with debris. In regions where summer rains are exceptional, as in the European Alps, direct sun-heat is a very effective-agent, but in regions like those of the Southern Alps, where summer rains are the rule, it is of relatively less importance. The action of the sub-glacial stream explains the collapsed condition of the lower moraine-covered reaches of the Fox Glacier with its abandoned tunnels and river channels, and the persistence of the front in this case is due to its not having been exposed to the action of the sub-glacial stream through some accident of position. The Franz Josef, being relatively free from moraine, is exposed to direct sun-heat as well—hence the pronounced down-wasting of its lower reaches. Although these statements may well be true, they do not explain the recent abnormal retreat or collapse of the front of these glaciers.
The similarity of the Fox and Franz Josef in this respect suggests a cause which has effected them both equally, and specially so since the general circumstances of both glaciers are analogous. Their feeding grounds lie near the summit of the main divide of the Southern Alps, are sub-equal in area, formed under similar climatic conditions, and the length, gradient, and size of the valleys occupied by the glaciers are analogous. Thus some cause operating equally on their upper levels should make its effect felt in the lower reaches at about the same time. Therefore one is prompted to suggest that the prime cause may be a failure in the supply of snow during a succession of seasons in the past, and this effect is only now apparent at the terminal face. But this explanation cannot account for similar and synchronous behaviour in the case of the Tasman. Although the snow conditions on the divide are closely analogous for all the glaciers, the greater length of the Tasman, its gentler grade, and consequent slower speed would require that the effect should appear in its lower reaches long after a like effect had appeared in the lower reaches of the Fox and Franz Josef.
It will no doubt be suggested that the down-wasting is due to greater sun-heat, but recent records of solar radiation do not support such an hypothesis, and it is uncertain how a substantial increase in sun-heat, granted that it did occur, would affect the glacier. Sir George Simpson suggests that, in its initial stages, it would promote an increase in the supply of snow for such a country as New Zealand, and owing to increased melting it would cause a retreat of the glaciers in its later stages. The result of variability of sun-heat therefore remains uncertain.
Finally, there is a possibility that the retreat might be due to heavier rainfall operating in some such way as I have suggested. By the kindness of Dr. Barnett, Government Meteorologist, I have been allowed to inspect the rainfall records of several stations near the area under consideration. He says in a personal communication, “The records of the last five summers do not show any very pronounced increase in precipitation, although the months December, 1938, and February, 1940, were very wet. The latter was a record for any month in many western districts.” He also points out that the summer of 1934–5 was unprecedentedly warm over the Dominion, while the autumn of 1939 was very mild after a very cold summer.
It will therefore be apparent that the real cause for the abnormal retreat of the glaciers within the last few years cannot be definitely assigned.
In conclusion, I have to acknowledge my indebtedness to Messrs. Peter and Alec Graham for much valuable information and for substantial help in making observations.
Bell, J. Mackintosh, 1910. A Geographical Report on the Franss Josef Glacier, N.Z. Geol. Surv, Wellington.
Harper, A. P., 1896. Pioneer Work in the Alps of New Zealand.
Matthes, François E., 1940. American Committee on Glaciers, Report for 1939–40.
Speight, R., 1935. Notes on the Franz Josef Glacier, February, 1934, Trans. N.Z. Inst., vol. 64, pp. 315–322.