20 J. O. Goodchild — Motion of Land-ice. 



Professor Prestwich, in summarizing the results of these obser- 

 vations, points out the importance of their bearing upon theories of 

 glacier motion ; but in doing so he takes into account the effects 

 of changes of temperature of ice arising only from what one may 

 term subaerial causes. These causes, diurnal, or seasonal, etc., 

 variations of temperature must, of course, play a very important 

 part ; but their effects are most marked at or near the surface, and 

 cannot directly affect the movements of the lower parts of the ice to 

 any appreciable extent. 



But differences in temperature of large bodies of land-ice are 

 not by any means necessarily connected with the effects of solar 

 heat, or of other causes acting from the upper surface of the ice 

 downwards. However low the surface temperature of a thick 

 mass of ice may fall, its internal temperature will not every- 

 where be the same as at the surface. Observation and experiment 

 alike have proved that the cold- waves (those below 0° Centigrade) 

 affect the surface layers most, and that their effects become less 

 and less marked as the cold-waves are propagated further into 

 the ice. Eventually, the cold-waves travel downwards to a zone 

 where they hardly lower the temperature of the ice at all. The 

 position of this zone in relation to the surface varies, of course, 

 with the degree of cold prevailing at the particular place under 

 observation. There is thus within every thick mass of land-ice 

 a zone below which diurnal or seasonal variations of temperature 

 may be said to produce no appreciable effect. Above that zone, 

 when the temperature falls below the freezing-point, contraction 

 takes place. Where the cold is most felt, i.e. at the surface, the 

 contraction is greatest, and there the ice snaps, unless the downhill 

 flow of the adjoining ice at a higher level is sufficient to relieve the 

 tension. From the surface such cracks extend downward, gradually 

 diminishing in width as the downward effects of the cold-waves 

 diminish, until they die out altogether and terminate downward in 

 unbroken ice. Water flows into the cracks, freezes, and the expansive 

 force thus exerted helps to thrust forward the ice on the downhill 

 side, or wherever else may happen at that spot to be the direction of 

 least resistance ; and in this way the ice is made to flow. A rise of 

 temperature brings about a general expansion of the upper part 

 of the ice, and, consequently, also forces it to flow in the direction 

 of least resistance from this cause as well as from the others. 



But a flow of the surface layers of ice — extending downward at 

 the most perhaps not more than a few hundred feet — will hardly 

 satisfy the requirements of those geologists who are familiar with 

 the effects of ice-action in well-glaciated regions. Every field- 

 geologist who has spent many years in, say, North Wales, the North 

 West of England, or in Scotland, must feel that, somehow or other, 

 the bottom of the Ice-sheet must have flowed, and must have moved 

 steadily, and with nearly-uniform directions of movement, long 

 enough to admit of the stone-shod sole of the Ice-sheet scoring deep 

 and wide grooves upon the surfaces of even the toughest of rocks. 

 M. Brunner did not follow up his researches ; but his conclusions 



