THE BRITISH MAMMALS PAST AND PRESENT. 79 



east, through which they both subsequently made their way, so as 

 to leave it as a promontory extending from England into Belgium. 



Elevation followed, and again separated the seas and began the 

 Oligocene period, in which all the British Isles were part of a land 

 surface extending in a broad isthmus to the Continent, with the 

 eastern sea over Holland and the western driven back and reduced 

 to a shallow gulf between Cornwall arid Brittany, ending in 

 estuaries and lagoons in the Hampshire and Paris basins, in which 

 the deposits thickened during the ensuing period of subsidence. 

 Thus Britain, during the Oligocene epoch, was mainly dry land, 

 and as such it remained during the formation of the Miocene rocks 

 of southern and western France and elsewhere. 



In the subsidence that occurred during early Pliocene times, the 

 conspicuous events, as far as England is concerned, were the 

 deposition of the Coralline Crag in the Suffolk gulf, which was 

 the northernmost extension of the eastern sea, and the sinking of 

 the southern portion of the country below the waves, causing the 

 formation of the plane of denudation across the north and south 

 downs, which was the first stage in the cutting down of the Valley 

 of the Weald. 



In Newer Pliocene days the movement of the land shut off the 

 communication between the eastern sea and the Mediterranean, and 

 opened up that between the Atlantic and the Arctic Ocean. In the 

 south-east this uplift took the Coralline Crag up to water-level : 

 the depression that closely followed taking it down again, throwing 

 open communication between the Arctic Ocean, Atlantic, and North 

 Sea, and leaving the British Isles as a triangular extension of 

 France and Belgium, with a western boundary stretching from the 

 Bay of Biscay beyond Ireland and the Hebrides up to the Orkneys. 

 This subsidence was, however, not continuous, and the minor 

 oscillations of level on the east coast are traceable in the formation 

 of the Crags and Forest Bed. Even in those days Norfolk and 

 Suffolk had their chain of " broads," evidently fed by a large 

 river which, to judge by its gravels, received the drainage of the 

 Ardennes and was in fact a branch of what is now the Rhine. 

 There is good reason for supposing that during the deposition of 

 the Walton Crag the southern subsidence was so general as to 

 sever the peninsula for a time from the Continent : but whether 

 this be so or not, it is clear that it was in Pliocene times that the 

 Eocenes and Chalk were mainly eroded, and the direction given to 

 many of the existing rivers, including the Thames and those of the 

 H umber and the Wash. 



Meanwhile a colder climate had been steadily advancing from 

 the north, and in the next epoch, that of the Pleistocene, we find 

 Britain under glacial conditions, with occasional warmer intervals 

 as the ice receded to return again, until it left us almost entirely. 

 To begin with, the land rose enough to join Scotland to Greenland, 

 thus cutting off the extension of the Gulf Stream, and a vast amount 

 of denudation took place, resulting in the deep excavation of many 

 of our river valleys and the opening up of many of our limestone 

 caves, while the glaciers were forming on the northern and western 

 mountains. Then the land sank beneath the sea north of the line 

 between the mouths of the Severn and the Thames; and Wales, 



