IN THE GRIP OF AN ICE AGE 51 



word: the long perpetual summer of the early earth 

 ended in a great Ice Age. That is to say, about four 

 million square miles of the earth's surface were 

 covered with a permanent sheet of snow and ice, as 

 our Arctic and Alpine regions are to-day. We can 

 detect traces of glaciers, or rivers of ice, millions of 

 years after they have disappeared. Their weight is 

 such that they grind pebbles deep into granite rocks, 

 and the scratches remain almost for ever, in suitable 

 places. There had been two earlier Ice Ages in the 

 story of the earth; but, as we saw, there was then no 

 life on land, and we have not considered them. This 

 new ice sheet stretched from India to Australia and 

 Africa. A great continent spread over that area at 

 the time, and it must have been one vast Arctic 

 region. I have seen the marks of glaciers of, say, 

 nine or ten million years ago — the periods we have 

 reached — in Australia. 



Four million square miles are by no means the 

 entire land surface of the earth, but the climate of the 

 whole earth seems to have changed. No doubt there 

 were smaller sheets of ice in other regions. Wherever 

 there were mountains there would be at least a frozen 

 winter-time. And there were now, for the first time 

 in millions of years, many mountains. That is, in 

 fact, probably the great cause of the Ice Age. I have 

 carefully studied Ice Ages in my Story of Evolution 

 and End of the World, and am convinced that, as 

 many geologists think, the cause of Ice Ages is a great 



