42 GLACIERS 



fragments. A special surface-layer is produced. Now 

 when the glacier again, after some thousands of years, 

 extends and deposits a new moraine over the old one, and 

 again retreats, it is found to be possible to distinguish the 

 later from the earlier moraine by the "special surface- 

 layer " of the old moraine, which marks it off from the 

 new material piled over it. Thus three extensions have 

 been traced in Bavaria, and in other regions within the 

 area of the great Swiss glaciers of the glacial period. 

 These " extensions " and " retreats " are not small variations 

 of two or three miles, such as we see occurring in Switzer- 

 land under our eyes within recent years. They refer to 

 differences of hundreds of miles in length, and to incalculable 

 differences in the volume of solid ice concerned, due to periods 

 of long-continued climatic differences separated by many 

 thousand years. It is not possible to induce any cautious 

 geologist to state how many thousand years separate us 

 from the last great glacial extension, nor how many 

 thousands of years separated it and those which occurred 

 earlier in the Pleistocene epoch and at the end of the 

 Pliocene from one another. If the astronomical cause 

 were really the determining one, we might conclude that 

 intervals of about 26,000 years were what occurred 

 between the full seventy of each glacial period. But on 

 other grounds such intervals are considered to be too short, 

 and doubt and speculation surround the attempt to put the 

 period into figures of so many thousand years. For 

 instance, if 26,000 years is all that should separate a 

 future glacial period from the last (which seems to have 

 been the greatest and most severe), we do not get enough 

 time (even supposing that we are within a couple of 

 thousand years of another glacial period) to account for 

 all the changes in the surface of Western Europe, and in 

 the animals and men upon it. The Neandermen, the 

 mammoth, the hyaena, the lion, the rhinoceros, bison, and 



