These chalk cliffs at Etretat. Normandy, are 

 slowly being cut away by wave action. It is 

 in sedimentary rocks like these, which have 

 been uplifted from the sea floor, that 

 fossils of sea creatures are found. Below 

 Is a fossil of a sea lily which grew on the 

 sea floor in Mesozoic times. 



The distribution of water over the Earth has changed through- 

 out geological time. Today 70.8 per cent of our planet's surface is 

 covered by oceans that average two to three males deep, the total 

 water area being 139,480,000 square miles. The rocks composing 

 the floors on which the deep water rests are quite different from 

 the rocks making up the continental gardens that rise out of the 

 seas. The floors of the large, deep oceans appear to be permanent 

 and not to change places with the continents by buckling and 

 rising up to the surface, but this is not to say that the ocean floors 

 are flat, featureless expanses. Wrinkling of the deep-ocean crustal 

 rocks has formed a network of great mountains and valleys ranging 

 over the floors of the oceans. Valleys like the Marianas Trench in 

 the Pacific could accommodate Mount Everest with room to spare. 

 But the crustal rocks under these deep oceans are only a few miles 

 thick compared with the twenty miles or so of the continental 

 gardens. 



When geologists examine rocks they try to picture what forces 

 were at work inside our planet when the rocks were formed. 

 SoUdified lavas are the result of great outpourings from ancient 

 fissures or from volcanoes. Other rocks, such as limestones and 

 sandstones, are shallow water deposits compressed by their own 

 weight in the course of time, and they often contain abundant 

 remains of animal life that existed when the fine deposits began to 

 rain dawn. These animal remains provide wonderful markers for 

 the geologist. While some plants or animals continue to evolve 

 gradually, others suddenly die out and may be replaced by still 

 others, so the existence of certain characteristic fossil remains of 

 past life gives the geologist proof that a rock was formed during a 

 certain period in geological history. 



By examining rocks from different parts of the world we can 

 tell which parts of the continents were covered by water at any 



10 



