COLD AND ICE ON THE EARTH 



may have no glacier now, it must have once been occupied 

 by one. 



These erratic blocks, now found all over Europe, tell 

 us a good deal about the ice on which they were trans- 

 ported. The blocks that fall on the edges of the glacier 

 remain on the side where they descend. Hence, if there 

 is any notable difference in the composition of the rocks 

 on either side of the valley, the existence of this differ- 

 ence will be preserved in the moraines. If, therefore, in a 

 country where the glaciers have disappeared we can trace 

 the scattered blocks up to their sources among the 

 mountains, we can say what was the track followed by 

 the prehistoric glaciers. In Europe there are several 

 examples of the uses of this detective evidence. Thus 

 the peculiar blocks of the Valais mountains can be 

 traced right on to Lyons ; and this shows us that the 

 glacier from which the River Rhone sprang extended 

 once right across the east of France to Lyons, and 

 probably farther. It was therefore once at least 170 

 miles longer. Similarly, blocks which are exactly like 

 the characteristic rocks peculiar to Southern Scandinavia 

 are found in Northern Germany, Belgium, and East 

 Anglia; and we therefore believe that a great sheet of 

 ice once filled up the Baltic and the German Ocean, 

 carrying with it immense numbers of northern " erratics."" 

 In our own country, in fact, glacier boulders are found in 

 nearly every county, and show that once the greater part 

 of the country was buried under ice. 



But, as we have said, it is not only on its shoulders, but 

 in its interior and beneath its base that a glacier rolls 



72 



