528 Rivers of Scotland. 



of its course. South of Strath Spey, the rivers find 

 their way east and south-east to the German Ocean ; 

 the Tay and the Forth from a high watershed that 

 crosses Scotland from the neighbourhood of Fraserburgh 

 on the east to Crinan on the west coast. To a great 

 extent it is formed of hard granitic rocks and associated 

 gneiss, and on this account it is high because of its 

 power to resist denudation. 



Like so many other rivers, the Tay has cut its way 

 in old times over, and now through, a high belt of 

 ground, that of the Sidlaw Hills just above the estuary ; 

 and the Forth, the Teith, and the Allan have in like 

 manner breached that long range of Trappean Hills, 

 known as the Ochils and the hills of Campsie. ^ 



The whole of the estuary of the Forth and the 

 greater part of the valley of the Clyde lie in an exceed- 

 ingly ancient area of depression. That country is also 

 covered more or less with Boulder-clay, and with later 

 stratified detritus of sand and gravel which were 

 formed in part by the remodelling of the Glacial drifts. 

 These rivers ran in that area before the commence- 

 ment of these deposits, and indeed for unknown 

 ages before that period. But we have no distinct 

 traces of those earlier epochs when we try to trace 

 them as regards the history of the rivers of Scotland ; 

 and we know little besides this, that the Forth 

 and the Clyde ran in their valleys long before the 

 deposition of the Boulder-clay, and with other rivers 

 resumed to some extent their old courses after the emer- 

 gence of the country. 



As of the rivers already mentioned, this may also 

 be said of the Tweed, that we know nothing for certain 

 of its history, except that its valley is of later age 

 than the Old Eed Sandstone and Carboniferous rocks. 



