CHAP. XIII.] PLEISTOCENE EPOCH. 273 



could not excavate its channel below the level of low-water 

 mark. 



The submerged forests which occur at so many points 

 round our coasts point to the same conclusion. A terres- 

 trial surface with rooted trees was found in Portsmouth 

 29 feet below high-water mark, and overlain by clay with 

 marine shells. In Cornwall a similar bed occurred at 67 

 feet below high- water mark ; but at varying levels above 

 this are other peat and forest beds indicating progressive 

 subsidence, and in southern England there is no evidence 

 of any subsequent elevation. 



Ireland. 



Glacial Deposits. These resemble the Glacial deposits of 

 Wales and western England, and there are probably two 

 groups or series of Boulder-clays, though these have not 

 yet been completely differentiated. In the mountain dis- 

 tricts there are tough Boulder-clays like those of northern 

 England and Scotland, and over the central plain there is 

 a wide spread of stony clays and gravels which consist 

 largely of the debris of the Carboniferous limestone. 

 Where this Limestone Drift reaches to mountain dis- 

 tricts it seems to have completely filled up the pre- 

 glacial valleys, and the aspect now presented by such 

 valleys is thus described by Professor Jukes : " The 

 steeper hills, as they descend into the valleys, are met by 

 gently sloping plateaus of Drift, forming inclined planes 

 from the heads of the valleys towards their mouths ; these 

 inclined planes seeming once to have stretched continu- 

 ously across the valleys, but being now deeply trenched 

 by the ravines at the bottom of which the present brooks 

 run. These have excavated channels for themselves either 

 through the Drift, or between it and the solid rock, leaving 

 the gently sloping surface of the Drift often most dis- 



