Based on the observations and velocity measurements on the Mt. Collon glacier, it was shown that during the time of ablation large local increases in velocity can arise, which were taken to be the cause for the formation of pressure wavesReference Haefeli 1 . In the report quoted below, the reason for the astonishing local acceleration of the movement in July, i.e., at the time of strongest ablation, was thought to lie in the increased amount of melt water and in the resulting reduction of friction on the glacier faceReference Haefeli 2 .
Since the measurements of J. D. Forbes (1842–43) on the Mer de Glace and of L. Agassiz on the Unteraar Glacier (1845–46) it has been known that the flow velocity in the ablation area is often very much higher in summer than in winter. For example, Agassiz found in a profile of the Unteraar Glacier, at that time 6800 m. from the glacier snout, that the minimum winter velocity (January) amounted to only 36 per cent of the highest summer velocity. On the other hand, R. Blümke and S. Finsterwalder ascertained that on the Hintereisferner conditions in the firn area were exactly the _reverse, in that there the velocity measured in summer was smaller than the average yearly velocityReference Blümke and Finsterwalder 3 , Reference Haefeli and Bendel 4 . These observations were confirmed by the more recent velocity measurements on the Claridenfirn (Streiff-Becker) as well as on the JungfraufirnReference Haefeli and Kasser 5 .
The yearly velocity variation in the ablation area may well be brought about almost solely through the change of slip velocity on the glacier bed. Lack of melt water in winter is doubtless the reason for a decrease in the component of slip.
In judging the specific friction between ice and rock influence of intergranular water pressure at the surface of contact must be considered, where the following equation as advanced by K. Terzaghi can logically be appliedReference Terzaghi and Peck 6 :
where
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s = shear resistance or specific friction in the sliding surface
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p = total normal pressure at the sliding surface
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u = intergranular water pressure
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ϕ = effective angle of friction. (Internal friction or friction in contact zone)
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If u = p
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s = 0 which, as is well known, leads to the outbreaks of the glacial lakes which are so much the cause of apprehension.
Experiments on ogives should therefore, if possible, run parallel with water run-off measurements at the glacier snout. A large amount of melt water in the summer causes an increase in intergranular water pressure at some of the less permeable places in the slip plane and therewith a reduction in friction between the ice and rock. The lubricating effect of the water may thus become specially noticeable on steep steps. The difference in velocity between the steep slope and its base therefore increases during the time of ablation and causes a pressure wave on account of the plastic transverse expansion of the ice mass, which gives way upwards. If the magnitude and rhythm of the differences in velocity, or of the specific deformation (strain) due to lengthwise compression are known, then the transverse expansion, or the wave height and wave length can be formulated. Furthermore, the longitudinal pressures causing the wave formation may be estimated if the rheological properties of the ice are known. Thus for example longitudinal pressures in the range of 10 kg./cm.2 were calculated, based on the maximum shrinkage of 1.4 per cent per day measured in the upper ice tunnels of the Mt. Collon glacier (see also J. W. GlenReference Glen 8 ).
Dr. Cuchlaine King and J. D. Ives have observed in Iceland not only annual ogives but also ogives of a similar type with smaller featuresReference King and Ives 7 . From this it can be deduced that variations in longitudinal pressure below steep, slopes can have different causes with quite different rhythms. The attached photograph, Fig. 1 (p. 29), of the steep step of the Mayangdi Glacier (1953) made available by the Swiss Dhaulagiri Expedition of the Akademische Alpen Klub, Zürich, may serve as a further example.
Steep fall on the Mayangdi Glacier (Dhaulagiri) below the north-east col, 1953
This shows that smaller waves, which are evidently connected with crevasse formation, lie above the long pressure waves, the regularity of which points to an annual rhythm. It will be noticed that these secondary waves tend to decrease more and more the greater the distance from the steep slope. Another beautiful example of annual ogives is to be found in the Trift GlacierReference Streiff-Becker 9 , Reference Haefeli 10 .
