particular time. About a hundred million years ago the whole of 

 southeast England was a shallow sea in which thick deposits of 

 chalk were being laid down. This sea extended across the present 

 site of the English Channel into Europe. In a similar way large 

 parts of the North American continent have been submerged by 

 shallow water at one time or another. The continental areas have 

 been subjected to a continuous series of warpings and tiltings. 

 Great mountain ranges such as the Rockies and the Himalayas were 

 formed of shallow water deposits together with volcanic lavas 

 which forced their way to the surface when these deposits were 

 being folded and pushed upward. These mountains are already 

 being worn away by wind, frost, and rain to form new sedimentary 

 rocks. And so the process of reworking the continental rocks goes 

 on inexorably with the world distribution of shallow seas varying 

 from one geological period to another. 



Some of the geological reconstructions of the shallow seas of 

 the past do not make sense, and there is a large body of evidence 

 suggesting that entire blocks of continental rock have moved as a 

 whole relative to each other. A glance at the map here will show, 

 for example, that Africa and South America would fit together very 

 well if we could move them toward each other across the Atlantic. 

 And the fit is even better if the continents are joined at the edges of 

 their continental shelves, rather than at the present-day coast lines. 



We can carry this exercise even further by grouping all the 

 continents to form one large land mass, which has been called 

 Pangaea. Such a reconstruction fits India neatly between south 

 Africa and Antarctica, with Madagascar and Ceylon forming useful 

 space-fillers. The German geologist Alfred Wegener and his as- 

 sociates worked very hard on this type of jigsaw puzzle, and 

 Pangaea is only one of the super-continent possibilities that exist. 

 Their arguments do not rest merely on the fitting together of 

 shapes. Rock formations of some of these separate land masses 

 show a strong correspondence both in the continuity of mountain 

 ranges and in the composition of the rocks themselves. 



This theory of continental drift has attracted strong criticism 

 because it is difficult to visualize solid granitic continents moving 

 about through the dense, basaltic rock floor of the oceans. However, 

 the most recent evidence shows that time seems to be on the side of 

 some kind of slow drifting. Possibly there have been heat changes 

 in the past which permitted large-scale continental movement. 



Although the Earth has been cooling since it was first formed, 

 there is a built-in heat generator in the crust and mantle rocks — 

 the inexorable decay of radioactive atoms. Rock is a poor conductor 

 of heat and it forms a blanket which keeps heat inside the Earth. 

 There may, therefore, be periods when the upper part of the mantle 

 is warming up. Earthquake studies have shown that there is a layer 

 of rock about a hundred miles within the mantle which appears to 

 be softer than the normal mantle rock above and below. This could 

 be the effect of a zone of excessive heating, and it may be that from 

 time to time this zone slowly moves toward the surface of the 

 mantle. When it reaches the crust it provides a soft layer on which 

 the continents can move about. There would then be enormous 

 volcanic activity, and heat would be released to such an extent that 

 the mantle would become completely solid, and once more another 

 steady regime of slow accumulation of heat would begin. 



Quaternary 



According to Wegener's theory of continental 

 drift, ttie continents t<nown to us today 

 were once a unified land mass ttiat separated 

 into segments, whicli drifted to their present 

 positions. Pale blue indicates shallow seas; 

 deep blue, deeper oceans. Present coast lines 

 are shown for identification. 



