222 THE LOCH LOMOND AND OTHER VALLEYS. 



may be accounted for as follows : the lowest portion 

 is gradually filling up by sea-brought detritus ; the 

 centre portion is also slightly silting up ; while the 

 sea has very little power in the upper portion. 

 Furthermore, ice may have existed longer in the 

 upper than in the middle portion, in consequence of 

 which, the former would be more .worn and hol- 

 lowed out thereby, than the latter, while most of the 

 detritus brought down would have been caught on, 

 and carried away by, the ice, so that little or none 

 of it could now remain in the upper part of the basin 

 of the fiord. 



