Sir,

Mr. Fisher’s very interesting observations on the Arolla Glacier, the Trift Glacier and the Mer de Glace, contending that the white bands of the “ogives” consist of ice containing air and that the dark bands consist of clear ice, are undoubtedly an important contribution to the problem of ogive formation. These views must certainly be considered when attempting a solution. Whether the pressure wave hypothesis can be reconciled with Fisher’s basic observations only the future can show. Nevertheless the following reasons make it appear quite probable.

The causes of the sudden increases in speed in the summer, as may be observed at the Mt. Collon Glacier, are, in my opinion, not due to the changes in viscosity of the surface ice layers since shallow sections of the glacier are influenced by the annual temperature variations—as Fisher rightly observes. The sudden increase in speed seems much more likely to be due to the melt water which on reaching the glacier bed gives a sudden added impulse to the ice by various influences. Measurement of the speed in the tunnel of the Mt. Collon Glacier yielded very small values in winter (for instance 3 cm. a day). Calculations according to Somigliana indicated that during the winter the glacier does not slide upon its bottom layer in the lower, flat reaches of the tongue. This result has since been confirmed by observations in the lower tunnel of the Mt. Colon Glacier and by further investigations in other glaciers. The sudden increase in speed at the surface of the Mt. Colon Glacier in July, which is a multiple of the winter speed, can therefore only be explained by the additional impulse caused by gliding upon the glacier bed. This implies, however, that extensive variations in pressure must occur which will influence the entire cross section of the glacier with varying intensities in different places and at different times. This might cause the effect of the pressure waves to penetrate deeply into the glacier, even causing change in the lowest ice layers which only come to the surface at the end of the glacier.

A glacier phenomenon can never be due to one single cause, many influences are usually brought to bear. It seems to me that more research will be necessary before we can answer the question whether the different textures of the ice established by Fisher have a primary influence upon the formation of Forbes’s Bands, for instance because bubbly ice has a different viscosity from that of clear ice, or whether, on the contrary, the difference in texture is a result of pressure variations or even simply a secondary phenomenon of the glacier surface. Unfortunately technical difficulties due to rapid plastic distortion did not permit crystallographic investigations to be made of the upper Mt. Colton Glacier. It is remarkable that the wave formation of the glacier surface below the ice fall, similar to that of the water surface immediately below a water fall, is by far the most intensive at that point and flattens out in the direction of the advance. Ablation reached the following values according to measurements made by E.O.S. in the summer of 1950: at 2390 m. about 7.3 m., at 2270 m. about 7.8 m. and at 2220 m. about 8.2 m. These were exceptionally high amounts. On the other hand, it is equally remarkable that important variations of the Newtonian viscosity may occur in the interior of the glacier as was proved later in the Z’mutt tunnel (E.O.S.) which, with a length of 1060 m., had a maximum ice cover of 80 m.Footnote *

Finally, I must thank Mr. Fisher for his remarks which will help in the further study of the formation of Forbes’s Bands. The point he has raised regarding the different conditions obtaining in different glaciers deserves particular attention.