Art. VI.—Recent Changes in the Terminal Face of the Franz Josef Glacier.
[Read before the Philosophical Institute of Canterbury, 6th October, 1920; received by Editor, 31st December, 1920; issued separately, 27th June, 1921.]
In 1909 Dr. J. Mackintosh Bell, then Director of the New Zealand Geological Survey, placed a number of pegs along the face of the Franz Josef Glacier in order to enable its subsequent advance and retreat to be definitely determined. Their position, and other particulars about the glacier, were recorded in a publication issued by the Survey in 1910, entitled “A Geographical Report on the Franz Josef Glacier.” Since then Mr. A. Graham, who is guide at the glacier and takes the keenest interest in its varying moods, has from time to time recorded the movements of the face, and a summary of his observations was published by the present author in 1914 under the title, “Recent Changes in the Position of the Terminal Face of the Franz Josef Glacier.”* Since the appearance of this record the glacier has rapidly retreated, as will be clear from the observations detailed below; but it is approaching a stage when an advance may be expected, and it is therefore most important that its present features should be placed on record as definitely as possible in order to afford a sound basis for future comparisons. Mr. Graham has most kindly assisted with observations, and a recent visit of the author to the locality (February, 1921) enabled these observations to be confirmed and brought up to date, Mr. Graham rendering most willing and valuable assistance. It is somewhat difficult, however, to get precise records at present, since ponds of water of varied width up to some 100 to 120 metres lie in front of the greater part of the face and prevent close approach to it except by means of a boat, which was not available; and, further,
[Footnote] * Trans. N.Z. Inst., vol. 47, pp. 353–54, 1915.
these ponds cover extensive areas of submerged ice lying in position, so that the precise location of the end of the ice is almost impossible. Nevertheless, the observations conclusively prove that there has been a marked retreat of the ice since 1914, and still more since 1909. In this account reference will be made to each of the pegs in turn, and the characteristics of the face in its vicinity recorded as accurately as possible; and for the purposes of ready comparison all measurements will be recorded in metres. As the general trend of the front of the glacier is approximately
east and west, the line in which measurements were made from the pegs was north and south, unless special reasons occurred for deviating from this direction. It should also be mentioned at this stage that the principal part of the Waiho River now runs from the eastern side of the glacier, and that lying in front of its western edge is a complex of roches moutonnées, evidently the remains of a spur of the pre-glacial valley, destroyed as described in a paper published elsewhere in this volume (see p. 47). The solid barrier presented by these rocks has no doubt caused the stream to discharge near the eastern side where the lip of the obstruction is lower.
Fig. 1.—Ice-front viewed from Park Rock, looking south-west, showing pond with ice continuing down below water-level. Freshly exposed roche moutonnée on light.
Fig. 2.—General view of glacier, looking south from Park Rock, showing overthrust upper layers in foreground and advancing pulse in background; Roberts Point on the extreme left top corner.
Fig. 1.—View looking east from Park Rock, showing part of pond fronting ice in the foreground, with collapsed glacier ice to the right. Peg No. 7 is situated on the rock-edge to the left of the picture, at a height of 200 ft. above the river. The Waiho River runs along the foot of the slope over ground from which the ice has retired since 1909.
Fig. 2.—View from peg No. 7, looking south, showing source of the Waiho River, and slightly advancing ice to the left; advancing pulse in the background.
The circumstances of the ice in the front of each peg will now be taken in turn, the chief features and points of interest being recorded on the map.
Peg No. 1.—This was placed on solid rock on the western side of the valley, but it is now covered with moraine, and its precise location is impossible without detailed survey. In 1909 the ice was 1 metre from the peg; it is now 279 metres distant, the measurement being made approximately parallel with the valley-wall to the point where the ice meets it. The face is here quite low, but immediately to the east the pool of water fronting the glacier commences, and the face is higher, sometimes overhanging; farther east the face again becomes low. The pool is about 60 metres broad on its western margin. The rapid retreat of this part of the face was mentioned in the records issued in 1914, as Mr. Graham then noted that the river had cut a wide gap between the ice and the western wall of the valley. The movement has apparently been much accelerated since the last observation.
Peg No. 2.—It was not found possible to determine the distance of the face from this peg in a satisfactory manner—first, on account of the pool, about 50 metres wide, fronting the glacier, and, secondly, because the ice in its retreat has exposed a large rock about 30 metres in height above the level of the water. The pool now washes the southern face of this rock. This rock was not exposed in 1914, so that its appearance and situation give some idea of the great distance the ice has retreated and the change in the condition of the face. (See Plate XII, fig. 1.)
Peg No. 3.—This is situated on Harper Rock. When originally placed the peg was at the ice-face. In 1912 it was 15 metres away, in 1914 it was 37 metres, and now it is 160 metres distant. The ice is fronted here by water 50 metres wide. (See Plate XII, fig. 1.)
The trend of the ice-front along the stretch just dealt with is slightly east of north, and running in a line with Strauchon Rock. Between Harper Rock and Park Rock another smaller rock has been exposed, and all three present a face towards the glacier not suggested by the map attached to Bell's account. The southern faces of all three are in approximate alignment, the direction running E. 30° S., and being determined by the dominant joint-planes traversing the schist of which the rocks are entirely composed. They all present a steep face to the south, and do not exhibit the effects of glacier erosion to a marked degree, there being a tendency to split both along the foliation-planes and also the joint-planes, so that any glacial smoothing originally existing has disappeared as the slabs have flaked off.
A low tongue of ice runs from the glacier into the pool (Plate XII, fig. 1), between the large new exposed rock in front of peg No. 2 and Park Rock, but ice occurs in position under the water of the pool, so that it extends farther forward at this part of the face than elsewhere. The end of this tongue is almost due west of peg No. 4, on Park Rock. The edge of the pool reaches the south-west side of the rock, but the pool narrows to a point, and there is a small stream issuing from it immediately to the west of Park Rock. The southern face of Park Rock is reached by the ice, but the rock has a much greater extent to the south-south-east and south-west than is suggested by Bell's map.
Peg No. 4.—This is on Park Rock. When originally placed it was surrounded by ice except to the northward. In 1912 the ice was 23 metres away, in 1914 it was 58 metres, whereas it is now 100 metres distant. The face is also low, but the upper layers show signs of being pushed over differentially. (See Plate XII, fig. 2.)
Opposite Strauchon and Barron Rocks there is a good expanse of water, and the edge of the ice reaching down into it is low and irregular, presenting embayments such as occur on a drowned coast-line, and no doubt the ice extends forward below water-level. For these reasons it was not considered advisable to measure the distance of the face from pegs Nos. 5 and 6, but the retreat from the line of the ice-front indicated on Bell's map certainly exceeds 160 metres, since the farthest exposed ice is at present almost due east of peg No. 4, on Park Rock. The whole of this portion of the face affords evidence of collapse, and the upper layers of ice show shear-planes and have evidently been pushed over the lower layers, an effect certainly due to differential movements; but whether this is to be attributed to the collapse of the glacier or to a definite thrust forward of the upper layers of ice is quite uncertain. This phenomenon seems to be more pronounced as Park Rock is approached. (See Plate XIII, fig. 1.)
By far the greatest volume of water issuing from the glacier runs out, of the north-east side of the pool which fronts the ice east of Park Rock, but a very considerable stream issues from close to the eastern side of the glacier and runs along between the ice and the wall of the valley for over 400 metres. In this part of the face the retreat has been most marked of all, as the measurements clearly show.
Peg No. 7 was initially placed 2 metres from the ice; by 1912 it was 14 metres away, by 1914 it was 24 metres, and now it is as much as 456 metres distant from the peg to where the ice abuts against the eastern valley-wall near river-level. The front is very high, over 20 metres in this section, and there is evidence of a small advance, since the ice is crowding over lichen-covered rock at the side. This advance may be of local character and therefore of little importance, but it may be symptomatic of a pronounced forward movement which is impending (See Plate XIII, fig. 2.)
It will be evident from the foregoing records that the minimum retreat of the face since 1909 has been 100 metres, and the maximum 456 metres, and after making all due allowance for the form of the face the average retreat of the front of the glacier is found to be approximately 180 metres.
As noted previously, there are evidences of approaching advance. A pulse indicating a marked rise in the ice is strongly developed about half a mile (800 metres) up the glacier, and the ice is pushing over the moss-covered glaciated rock-surfaces of the valley-walls at Roberts Point and Cape Defiance, still farther up. (Plate XIII, figs. 1, 2.) If the rate of movements of the glacier be that determined by Bell—viz., from 1 ft. to 2 ft. (0.3 to 0.6 metres) per day—this pulse should reach the terminal face in from three to five years. If the rate of movement is faster, as it probably is, the space of time will be correspondingly reduced, and it may be reduced still more as the oncoming wave affects the ice immediately in advance of it. A similar pulse is observed in the neighbouring Fox Glacier, and Mr. Graham intends to place a mark in a good position on the Chancellor Ridge near the glacier so that the rise of the ice-level may be correctly determined.
Mr. Graham has also made observations to arrive at the rate of flow. Selected morainic blocks lying on the surface of the ice below Roberts Point have shown an average movement of 3 ft. (1 metre approximately) per day during a period of 200 days, and it is likely that at the base of the first ice-fall the rate is much faster. Observations have been made since November, 1920, but the results are not yet available.
An interesting point to consider is the possibility of periodicity in advance and retreat. My first experience of the glacier was in the year 1905, when it was advancing. It was also advancing in 1909 when Bell made his observations, and was retreating in 1912. I cannot determine the precise year when this retreat commenced, but it had probably set in during 1910, and has continued since that date, so that it has been falling back for approximately eleven years. One cannot predict at present when this retreat will end, or what the total length of the cycle is likely to be.
There are one or two other points to which brief allusion may be made. First, the angle of the shear-planes near the present terminal face, especially those near the eastern front of the glacier, suggests that a great thickness of ice, probably to be measured in hundreds of feet, exists behind the rock bar which stretches from the western wall of the valley towards the present mouth of the. Waiho between Barron Rock and peg No. 7. If, therefore, the glacier should retreat farther, the lake along its face will probably increase in size, and it will furnish a suggestion of what usually happens as the ice retreats from a rock bar across a valley. Such conditions must have occurred in the Rakaia, Wilberforce, and Waimakariri Valleys when the ice commenced to retreat towards the heads of the valleys from the barrier near the plains in late Pleistocene times.
An examination of the rock-surface recently exposed does not suggest that glaciers have any marked power of erosion near their ends even when advancing, slight abrasion being all that was noted on the roches moutonnées recently exposed before the terminal face; but, of course, this does not negative their power to erode their beds where the ice is thicker.
The presence of an apparent wave of high ice might have been credited to the influence of an irregular bottom during a period of ice-decline, analogous to the effect of obstructions in the bed of a river, masked as they frequently are at high water, were there not definite proof that ice is actually rising relative to the rocks at the side. In any case, the thickness of the ice is very great even in times of lower level; all the same, there is some suggestion in the alternating stretches of ice-fall with more gently inclined surface, shown not only in this glacier but in the Fox as well, that if the ice were removed the valley-floor would exhibit in a perfect form the characteristic stairway developed in glaciated regions—as, for example, those in Deep Cove and other valleys at the heads of the sounds of the west coast of Otago.
In conclusion, I have to express my indebtedness to Mr. Graham for much valuable information and for ready help. He has promised to continue observations and to take photographs of the face from already-selected positions on Park Rock at the same time each year, so that changes in the character and position of the terminal face can be accurately recorded. It is important that they be taken during the same month of the year, so as to eliminate any error due to variation between the summer and winter heat.