RATE OF RECESSION 85 



When the details of the ice retreat are known, it will be possi- 

 ble to discriminate between peculiarities dependent upon the 

 character of the mother rock and those due to the rate of reces- 

 sion or to the climate. 



Conditions Controlling Recession 



The factors causing and influencing the disappearance of the 

 ice sheets were many and different: temperature, precipitation 

 in form of snow and in form of warm rains, topographic condi- 

 tions, conditions favoring discharge of icebergs, etc. The ice 

 itself moved forward, and the retreat of the ice edge was the 

 excess of the melting over the advance. The combinations of 

 the factors were complicated and changing, and so in many 

 cases they cannot yet be analyzed; but detailed and exact 

 chronological studies promise to shed light upon these funda- 

 mental questions. 



TEMPERATURE 



The importance of the summer temperature is probably best 

 shown by De Geer's studies in Sweden (1912, 1912a, 1914). 

 The most significant evidence is perhaps the great difference in 

 the rate of the recession of the ice on the Swedish west coast 

 and that in the Baltic region. In western Sweden the ice edge 

 stood practically still for one or two thousand years, while it 

 receded at great speed in the eastern part of the country. De 

 Geer's explanation is that on the west coast the air was very 

 foggy and the temperature low and constant, while in the Baltic 

 region the sky was clear and warm and sunny days were frequent. 

 The intensely dense fog along the Gulf Stream in the Arctic is 

 well known. On Bear Island, halfway between Norway and 

 Spitsbergen, the writer from the beginning of August to the 

 middle of October in 191 6 saw the sun in all for but a few 

 hours. The summer temperature on this island is, on the average, 

 a few degrees above the freezing point and exceedingly even. 

 In other Arctic regions, as Spitsbergen, the insolation on bright 



