1 8 THE MIGRATIOX OF BRITISH BIRDS 



the surrounding sea, and eventuall}- exterminated in the 

 final disappearance of this area beneath the waves. 

 How long the contour remained at some 20 fathoms we 

 have no means of ascertaining, but the final severance of 

 the British Isles from continental land occurred at no 

 very remote geological period, when the waters of the 

 English Channel and the German Ocean encroached 

 upon the narrow isthmus between France and England 

 and finally met at the Straits of Dover. At first the 

 newly-formed Strait was exceedingly narrow, perhaps 

 less than a mile. For some time after the final sever- 

 ance of Great Britain from continental Europe nearly 

 all the coast-line of England, part of Ireland, and part 

 of Western Europe remained apparently at the 15-fathom 

 contour, as is suggested by the phenomenon of submerged 

 forests. At the 15-fathom contour the coast-line of the 

 cast of England from the Humbcr to the north of 

 Norfolk would extend many miles further out to sea 

 than is now the case ; elsewhere it would only reach a 

 few miles beyond existing limits, with the exception of 

 a considerable distance outside the present coasts of 

 Norfolk, Suffolk, and Essex. The coasts of the Nether- 

 lands and Belgium also probably extended further out 

 to sea during this period. The significance of these 

 facts will be made apparent in a later chapter. 



Such, broadly speaking, are the principal physical 

 changes which the British area has undergone from the 

 close of the Pliocene epoch down to prehistoric time. 

 In order to render the subject more complete and to 

 enable the reader better to grasp these mighty changes 

 in their natural sequence, I append a brief resume of 

 the events recorded, associating with them the various 



