CHAPTER IV. 



PHYSICAL GEOGRAPHY. 



Summits of Drainage. We designate by the term { summit of 

 drainage' the line on the surface from which rain, falling vertically, 

 does run or might run on one side to one valley, on the other side 

 to another. * Watershed' is a word of equivalent meaning, and 

 in the north of England it is often the limit of extensive property, 

 and is sometimes called the ' heaven-water' boundary. 



The valleys of the Thames thus considered present some curious 

 points for remark. 



And first with reference to the valleys of other river systems, as 

 the Avon of Wilts ; the Severn ; the Avon of Warwickshire ; the 

 Nen of Northamptonshire ; the Ouse of Bedfordshire. 



Towards the Wiltshire Avon that western branch of the Thames 

 called Swill Brook is directed, and on its course the low summit 

 of drainage may be traversed at points not exceeding 300 feet 

 above the sea. 



The 'true source of Thames' in Trewsbury Mead is in the de- 

 scending course of a valley which extends beyond it a few miles, 

 and allows of an easy passage by a short tunnel to the Water of 

 Stroud, which descends to the Severn near Newnham. The summit 

 of drainage is here about 500 feet above the sea. 



Beyond each of the springs of the Churn, under Leckhampton 

 Hill, a dry dale extends to a low ( col/ from which a rapid descent 

 conducts by the channels of the Chelt to the plain of Severn. These 

 summits are about 750 feet above the sea. 



More curious even than this is the long, straight line of the 

 Upper Coin from Andover's Ford meeting the similar straight line 

 of water-course which descends from Charlton-Abbots to Winch- 

 combe. This summit of drainage, between high hills of oolite, 



