6 SECRETS OF EARTH AND SEA 



feet above the level of the present rivers. The land 

 surface has risen and the rivers have simultaneously- 

 excavated deep and wide valleys leaving terraces of gravel 

 high up on their sides. These show where the rivers once 

 flowed. The vastness of the excavation of the valley from 

 the level of the old river bed 600 ft. up on the sloping 

 hill-side to its present low-lying bed in the floor of the 

 valley — gives us some measure of the time which has 

 elapsed in the process. 



No one can tell, at present, the limit in the past of 

 Palaeolithic man. The period of time over which his 

 existence extended, as indicated by the trimmed flints 

 undoubtedly made by human workmanship, is a matter of 

 hundreds of thousands of years. In Western Europe races 

 came and went, succeeded one another and disappeared, 

 either migrating or absorbed or more rarely destroyed by 

 the later invaders. Naturally enough, in the later deposits 

 of rivers and in the higher layers of earth and limestone 

 cake which fill many caves to the depth of 30 or 40 ft. we 

 find the remains of man's workmanship more abundantly 

 than in the older deposits. 



We can broadly distinguish in the Palaeolithic epoch 

 three (perhaps four) periods, separated by the occurrence 

 of great extensions of the northern or arctic ice cap of 

 such a volume as to cover North Europe and North 

 America, and the simultaneous extension of the glaciers 

 of the mountains of Europe. This period of the alternat- 

 ing extension and retreat of the great northern glaciers 

 is known as the Glacial period, or Ice Age. The latest 

 Palaeolithic men are subsequent to it — that is, post-Glacial. 

 We can distinguish several successive ages of these post- 

 Glacial Palaeolithic men, altogether distinct from and 

 anterior to the Neolithic men. In the earlier of these 

 ages many of the great animals of the Glacial period — 

 now extinct or withdrawn to other regions — still survived 



