i8 4 



THE BOOK OF NATURE STUDY 



through which its journey passes. If the travel is fast, variations 

 in the spring will follow close on variations in rainfall, and the 

 spring may run dry soon after rain ceases. But if the travel is slow 

 the yield of the spring will be more steady and it will only run dry 

 during seasons of exceptional drought (Fig. 71). Thus surface 

 springs are often intermittent, the dry periods being ultimately 

 dependent on variations in rainfall. 



One of the most remarkable of the intermittent springs are 

 those known in Chalk districts as " bournes/' These are surface 

 springs issuing generally at the junction of the Chalk with the 

 beds above it. Some such springs are permanent, and water 

 may issue in sufficient quantity to form good sized streams, and 



Drainage Area 



/mperrious Stocks. The rest are porous. 



FlG. 72. Surface Springs, S. Deep-seated Spring, D. 



by them the saturation level of the Chalk is kept down to a certain 

 point. If the rainfall of any season is exceptionally heavy the 

 saturation level rises higher, and this produces two effects. In 

 the first place, the regular springs discharge an unusual quantity 

 of water ; and, in the second place, new springs burst out along 

 the same rock junction, but at a higher level. These latter are 

 the bournes, which only flow after exceptionally rainy seasons, 

 occurring at fairly regular intervals. After flowing for some 

 weeks or months they gradually dry up and disappear till the 

 next excess of rainfall. The amount of water now pumped from 

 wells in the Chalk has lowered its saturation level so much that 

 outbreaks of the bournes occur at longer intervals than in former 

 years. 



If the disposition of the strata is such that a great mass of 



