26 THE THAMES AND ITS TRIBUTARY RIVERS. CHAP. 



to be always dry, rose from immemorial time the clear, full, bright 

 source of Thames, till, in the latter part of the last century, the 

 Thames and Severn Canal drank up the river, to feed its thirsty 

 navigation on the summit level ; drank it up by the violent efforts 

 of a steam-engine, which lowered the level of the ' water-bed' in 



Diagram X. ' The very head of Isis.' LELAND. 



all the adjoining country, so that the natural efflux now takes 

 place half a mile below its former opening. Though diminished 

 and lowered, it still delivers a strong current of sparkling water 

 to a channel almost choked with water-flowers. 



The road from Cirencester to Tetbury is part of the famous 

 straight Cotswold track, called the ' Foss.' It crosses the Thames 

 valley near the source, the latest source perhaps we may say, for 

 the upper part of the valley, now dry, has been in earlier times 

 traversed by descending streams. A large solitary stone stands 

 by the roadside in the valley ; and there is a mark of ancient 

 occupation in the circular earthwork called Trewsbury Castle, in 

 close proximity to the once famous source of Father Thames. The 

 source thus named is at a height of 330 feet above the sea. To 

 this low situation the water finds its way from the surrounding 

 uplands of oolite, and a tract of branching dry valleys, from which 

 the transit is easy to the beautiful and deep Vale of Stroud, 



