19^1- No. II. THE STRANDFLAT AND ISOSTASY. 21 



investigations will be given in a later paper. It is difficult to estimate 

 the total erosive power of glaciers, as it will greatly depend on the tectonic 

 structure of the underlying rock. Where the rock surface is relatively 

 easily smoothed, and few edges and projecting stones are left for the 

 flowing ice to attack, the erosive work will to a great extent be reduced 

 to grinding, the effect of which will depend on the quantity of stones and 

 grit transported by the glacier along its under side. 



The erosive effect will in that case be very much less than in places 

 where the rock surface remains rough, and where there are always pro- 

 jecting rocks to be attacked and broken loose by the flowing ice. 



I have come to the conclusion that, on the wdiole, the erosive power 

 of a moving glacier may increase opproximately with the third power of 

 the velocity of the motion at its under side. 



Hence we may expect that if this velocity be increased ten times, 

 the erosive power of the glacier may be increased about a thousand times. 

 A glacier which, with a velocity of ten centimetres in 24 hours, was able 

 to denude the underlying ground to an average depth of ten centimetres 

 during a certain long period, would then be able to excavate a hundred 

 metres of rock during the same period, if its velocity be increased to one 

 metre in 24 hours, and provided that the glacier were working onlv 

 vertically. 



The observed velocities of the moving glaciers of Norway and the 

 Alps are generally between o.i and 0.4 metre in 24 hours, while on the 

 big glaciers of the Himalaya velocities of between 2 and 3.7 metres in 24 

 hours have been observed during the summer. At the end of the big 

 glaciers of northern Greenland, however, velocities of as much as 20 to 

 30 metres in 24 hours have occasionally been measured by Amund Hei- 

 land, Stenstrup, Hammer, and Ryder, during the summer. The normal 

 movement during the whole year is less, but still very considerable. There 

 must consequently be an enormous difference between the erosive powers 

 of these various glaciers. 



If we take a European glacier, moving with a velocity as great as 

 0.5 metre in 24 hours, and a Greenland glacier, moving with an average 

 velocity of 10 metres in 24 hours, we find that, while the former is able 

 to erode 10 centimetres of rock, the latter will erode 800 metres. 



This explains why the inland ice has had so remarkably little erosive 

 effect in its more central areas, in Norway and Sweden [cf. Høgbom, 19 10], 

 while its outflowing glaciers have had such enormous effect in the narrow 

 valleys and fjords of the coast land, just outside these regions. 



We also understand why there are so sudden and striking differences 

 between, on the one hand, the great deepening effect of the glaciers in the 

 narrow fjords where they may have eroded a thousand metres of rock or 

 even more, and, on the other hand, the quite insignificant erosive effeci 

 of the inland ice which covered the high land on the sides of these deep 



