UNDERGROUND MOVEMENTS 23 



physical geography of that district has been consider- 

 ably altered. The valleys by which the downs are 

 there trenched were formerly filled with tidal waters, 

 so that the ancient camps, perched so conspicuously 

 on the crest of the heights, could not communicate 

 directly with each other except by boat. Instead of 

 being a connected chain of fortifications, as was once 

 supposed, they must have been independent strong- 

 holds, surrounded by water on three sides, and on 

 the north by dense forest and impassable morasses. 



The subterranean forces which throughout geological 

 time have been so potent in the area of the British 

 Isles, appear to have on the whole remained nearly 

 quiescent during the centuries of authentic history. 

 Volcanic energy, which played so notable a part through- 

 out most of the geological past of our region, has 

 never awakened here since the remote Tertiary ages, 

 but has shifted its site northwards to Iceland, where 

 it continued its activity even during the Ice-Age and 

 has remained vigorous down to the present time. But 

 two forms of underground movement have affected 

 Britain since man appeared here. In the first place, 

 there have been important changes in the relative 

 levels of sea and land, and in the second place, the 

 islands have frequently been shaken more or less per- 

 ceptibly by earthquakes. 



In southern Scotland, the north of England and of 

 Ireland, the land has been upraised some twenty feet 

 or more. As a consequence of this elevation a selvage 

 of flat alluvial land has been added to a number 

 of the estuaries, particularly to those of the Forth, 

 Tay and Clyde. Canoes and other remains of human 



