The work described below was carried out as a subsidiary part of a glaciological programme the main work of which was to continue observations made the previous year by the Leeds University Greenland Expedition 1948. In 1949 the aim was to study the movement, over as long a period as possible, of the Fraya Gletscher. Plans were made to observe the movement every half-hour for a period of twenty-four hours, in order to gain more information on the short period variation of glacier flow in polar regions. Washburn and GoldthwaitReference Washburn and Goldthwait ¹ working in Alaska had found that glaciers do not flow at a constant speed even over short periods. Similar results had been obtained about the same time by R. FinsterwalderReference Finsterwalder and Pillewizer ^{2a ,} Reference Finsterwalder ^b in the Himalaya. It was also hoped to find out whether there were any slight differences in surface flow which might be correlated with the thrust planes. The Fraya Gletscher was considered most suitable for velocity observations as AhlmannReference Ahlmann ³ had already studied the regime of this glacier in 1939–40.

A Wild theodolite was used to measure the linear movement of the glacier. In order to measure the velocity over a cross section of the glacier stakes were set out across it and their angular movement from a fixed point was observed. The distance moved by each point was then computed. The stakes were driven into holes in the glacier bored to a depth of 80 cm. Over twenty-four-and forty-eight-hour periods the stakes remained quite firm, but sights were always taken on the junction between the stake and the ice. When observations over longer periods were required, the holes were bored to 120 cm. and deepened every third or fourth day.

This work was only a part of the glaciological programme and the two observers were not able to devote more than two twenty-four-hour periods to continuous readings. These were confined to two stations. Station 1 lay at an altitude of 518 m., 2.4 km. from the snout. Station 2 was at an altitude of 396 m., 1.2 km. from the snout, see sketch-map in Fig. 5 (p. 561).

Adverse weather conditions made the twenty-four-hour readings at Station 1 too discontinuous to be included, but the observations from this station over three weeks are presented. The only short-term readings to be discussed are those from Station 2. Although such observations can only be really valuable when extended over at least a month (a year would be ideal), they correspond sufficiently with those of Goldthwait to be worth recording. Also, so far as is known, they are the first hourly records of movement ever attempted in Greenland.

Observations at Station 1 (Fig. 1, p. 561) taken at irregular intervals over a period of three weeks show:

Fig. 1 Station 1. Movement in centimetres (ordinates) during the periods indicated. Abscissae show distance across glacier

1. That the Fröya Gletscher does not, like most glaciers, flow fastest in the centre. Instead, if the movement of the line of stakes is plotted, one finds that the glacier has two zones of greatest speed separated by a zone of slower moving ice. The zone of greatest speed was 600 m. across the 760 m. broad glacier, and in three weeks the stake here moved 209 cm. down-stream. Between this and the other zone of faster moving ice was a slow zone at 470 m. where the stake only moved 80 cm. The second zone of fast moving ice moved 203 cm., its position being approximately half-way across the glacier. It is not easy to explain these zones of differing velocities. They would be understandable if the Fröya Glacier were fed by two accumulation areas separated by a rock ridge, but this is not the case. However, there may be a rock ridge running up the glacier which, though causing thinning, is still invisible.

2. The variation of speed over the three-week period is shown in the other two graphs of flow at Station 1 (Figs. 2 and 3, p. 561). In Fig. 2 the distance travelled by each stake is plotted against the time, and the minimum overnight temperatures at the camp are also shown. The steep drop in velocity from 1 to 10 August is more readily distinguished in Fig. 3, where the average daily velocity has been plotted against time. The curve of maximum night temperatures in Figs. 2 and 3 gives some indication of changing conditions over the period. The steady drop in temperature was also noticed in 1948, and it may account for the diminution in glacier speed, if allowance is made for a slight time lag. Forbes,Reference Forbes ⁴ Goldthwait, Finsterwalder and others correlate temperature changes with glacier movement over short periods, although CarlsonReference Carlson ⁵ working in West Greenland, could not find any such relationship.

Fig. 2 Station 1 Showing (ordinates) distance moved in centimetres plotted against time in days (abscissae)

Fig. 3 Station 1. Showing (ordinates) average velocity in centimetres per day plotted against time intervals

The negative movement of the farthest stakes so clearly marked in Fig. 3 is difficult to explain. PfaffReference Pfaff ⁶ obtained somewhat similar results in 1875 on the Great Aletsch Glacier, where he found a backward movement in the firn area.

Observations at Station 2 (Figs. 4 and 5, p. 561) show that the surface movement of the ice downstream seems to be unaffected by the position of thrust planes. In Fig. 5 (left) the positions of the major thrust planes are marked as dotted lines. The distance between the stakes was only one metre, and it was hoped that there might be some detectable difference in movement between either side of the thrust planes. This was not so and over the three-week period it seems that the speed of flow throughout the glacier was not materially affected by any discontinuities resulting from the position of thrust planes. There was some slight differential movement between the group of stakes F, G, H, I and the others. These differences will be noticed on Fig. 4 in the readings just before noon. The whole of this group was situated on one side of a thrust plane and the movement may be significant on this account.

Fig. 4 Station 2. Showing (ordinates) movement over 24 hours of each stake against time in hours. One small marginal division = 1 cm.

Fig. 5 Station 2. Distance moved in centimetres (ordinates) during the periods indicated. Abscissae show distance between each stake in centimetres. The sketch map gives the positions of Stations 1 and 2

It will also be seen from Fig. 5 that at Station 2 there is a noticeable reduction in the surface speed of the Fröya Gletscher towards the end of the summer, just as at Station 1. Thus the line T₁ represents the movement of stakes between 7–10 August and varies from 40–50 cm. The line T₂ represents the additional movement between 10–16 August. This is only 20 cm. or less, even though it is over a longer period. If the number of days represented by the lines T₁ to T₅ are divided by the intervals between them, one finds a diminution from 17 cm. a day to 1 cm. a day. The decrease in velocity in Fig. 5 appears earlier in August at the upper station, Station 1. One is tempted to suggest that they mark a general slowing down of velocity as the winter advances, noticeable first in the upper part of the glacier, and later in the lower section.

3. The discontinuous movement of the glacier over a period of twenty-four hours is clear from Fig. 4, the dotted line and breaks indicating the time during which visibility was obliterated by snow. The movement of all the stakes A—J is shown, and their position relative to the glacier as a whole is given in the sketch-map, Fig. 5. This jerky movement appears similar to that recorded by Goldthwait and Washburn, although, as would be expected, the variation seems to be of greater magnitude in Greenland. In the South Crillon Glacier variations of between 0 and 5 min. were recorded in the half-hour, whilst on the Fröya Gletscher variations of up to 1–2 cm. in the same period were noted. The posts used in Greenland were probably not as accurate as the indicator used by Goldthwait, but the experimental error in measuring the distance moved by the posts was probably never more than 50 mm. and usually much less. This error is not sufficient to nullify the erratic motion observed. The Fröya Gletscher was crevassed very little, and there were no crevasses between the theodolite station and any of the posts at Station 2.

The observations show that the glacier moved about 1–2 cm. more by day than by night. Similar conclusions were reached by Goldthwait and also by Krasser,Reference Krasser ⁷ although the results of the latter were questioned by MercantonReference Mercanton ⁸ The greatest movement at all but one stake took place between 16.00 and 17.00 hr. There also appears to be a decrease followed by a slight forward surge about two hours before midnight. On the South Crillon Glacier, however, the peak periods were 08.00, 12.00 and 16.00 hr. It is noteworthy that both these series of observations show a steady forward surge of the glacier during the day, which appeared to start earlier on the Fröya Gletscher than on the South Crillon Glacier. The jerky movements also seem to coincide in the late evenings.

This irregular movement over the twenty-four-hour period on the Fraya Gletscher and noted by Goldthwait in Alaska in 1937, may perhaps be attributed to some form of Block-Schollen motion, if one agrees with Finsterwalder’s views.Reference Finsterwalder ⁹ The speeds do not quite come within the range found by Finsterwalder in the Himalayan glaciers, 800 cm. a year, i.e. 7 cm. an hour, but these are not necessarily the minimum for Block-Schollen flow. Undoubtedly the glacier motion recorded by Carlson in West Greenland, between 18–80 cm. an hour, would come well within the suggested range. Much more work over continuous twenty-four-hour periods must be carried out before one can be certain whether all glacier motion is jerky or whether this motion only applies to glaciers moving faster than a certain minimum.