34 THE FLOOR OF THE OCEAN 



stand this indirect method of diagnosing the foundations of 

 the deep sea, we shall have to go ashore. 



We go ashore to watch powerful Nature test the idea of 

 true crust and quasi-liquid substratum by the most trusted 

 criterion of science, namely experiment. Any convincing ex- 

 periment has to be on a scale far beyond human contrivance. 



During the last Glacial Period enormous volumes of water 

 were evaporated from the ocean and, in the form of a dozen 

 thick ice-caps, piled on the continents. Because of their wide 

 spans, these great weights bent down the earth's crust and 

 naturally bent it down most where the ice was thickest. In 

 other words, the crust was basined under each of the broad 

 ice-caps. With their melting and consequent removal of load, 

 the crust began to bend up again. In two cases this recoil of 

 the crust is still going on, and there we find new proof of 

 extreme weakness for the subcrustal material. 8 



The most complete evidence comes from the part of north- 

 western Europe that was overwhelmed by the last of the ice- 

 caps that waxed and waned during the Pleistocene Period. 

 The limits of this glaciated tract are shown in Figure 15. The 

 ice was thickest over the western shore of the Gulf of Bothnia, 

 the northern branch of the Baltic Sea. From that center the 

 ice flowed out and spread over Finland and Scandinavia, as 

 indicated in Figure 16. An appropriate name for the piled-up 

 mass is, therefore, the Fennoscandian ice-cap. It covered nearly 

 one and a half million square miles, had central thickness not 

 far from three miles, and had average thickness of about one 

 mile. 



Forty thousand years ago the Fennoscandian ice began to 

 melt. With occasional short pauses in the melting, the mass 

 grew thinner and lost area, leaving moraines at intervals from 

 northern Germany and northwestern Russia to the Gulf of 

 Bothnia. Eighty-seven hundred years ago the melting was so 



