20 HILLS AND VALES. CHAP. 



rocks, in others hardly a trace of iron-sand or lower green-sand; 

 so that the true series of the strata is only to be discovered by 

 comparing" many sections. 



A considerable part of the vales of Ock and Thame is not more 

 than 250 feet above the sea. 



The strata may be thus stated, as observed near Swindon, that 

 hill being regarded as belonging to the vale : 



Gault. 



Lower green-saiid. 

 Purbeck beds (traces). 

 Portland oolite and sands. 

 Kimineridge clay. 



Looking southward, a range of smoothly-outlined hills forms 

 the horizon line, from all the country round Oxford. These are 

 composed of chalk, resting on feebly-pronounced upper green-sand, 

 which in its turn reposes on a continuous band of gault. Dry 

 valleys, branching over all the surface, descend with uniform slopes 

 and graduated sinuosities, such as to indicate the former action 

 of water. At some point in the downward course springs arise, 

 more or less intermittent in their outflow, and occasionally appear- 

 ing at points higher than usual up the valley. In the open 

 downs of Wiltshire and Berkshire these valleys are usually called 

 ' bourns/ 



The chalk hills between these valleys absorb all the rain which 

 falls, and conduct it to subterranean fissures which form a perennial 

 reservoir of clear water. Wells sunk to the requisite depth never 

 fail to obtain supplies from this reservoir. Thus London draws 

 water from the chalk hills of Herts on the north and of Surrey 

 on the south. 



Naturally the chalk hills were covered, as some parts still are, 

 with green herbage suited to sheep farms. In this state they were 

 called ' downs ' in Wiltshire, and ' wolds ' in Yorkshire. The names 

 remain, though the condition of the surface is greatly altered by 

 the invasion of the plough. 



The elevation of the highest summits of the downs is on the 

 whole very nearly equal, from Marlborough Downs to Wendover. 



Great waste of the chalk hills is manifest in every part ; long- 

 continued watery action has smoothed whatever of inequality once 

 existed, and completed a system of according slopes and windings 



