GROUXD WATER 



395 



The connection between the existence of fissures and the formation of 

 flints is further rendered clear b}^ the fact that the middle and lower 

 chalk, which consists mainly of much more compact, often sandy, giau- 

 conitic, or marly beds, has fewer or no flints, though it contains phos- 

 phatic concretionary nodules. 



Owing to the free passage which the formation ofl'ers to rainfall, the 

 ground-water level in the chaUv hills is very low, often little if at all abo^e 

 the neighboring valleys. The writer was familiar with one village well 

 on an upland which was 67 meters in depth." For the same reason tun- 

 neling through the chalk hills presents very little difficulty in the matter 

 of drainage, though the presence of the gas already mentioned is some- 

 times troublesome and the abundance of joints and cracks renders the 

 roof somewhat unsafe and timbering necessary in the Upper Chalk. 

 On account of the extensive tunneling operations carried out during the 

 war, this item of expense must have been a large one. The softness of 

 the rock, on the other hand, facilitates such work to an extent which 

 probably more than counterbalances this expense as compared with rocks 

 strong enough to require no timbering. 



UXDEEGEOUXD StREAZMS 



Another phenomenon due to the permeability and solubility of the 

 chalk consists in well defined dry valleys which intersect the chalk hills. 

 Some of these open on the sea and others upon the valleys of the larger 

 rivers. Their general plan and pattern is that of valleys of erosional 



Old Scarp Covered by Soil r''-^ o" "(/•"o'o 



Fresh Scarps^^o .<^^i q'^. 



W ^-^': \ 0*\"^ "^^^^^ f^c^l 1 en I n roof, JT- V/'Jv/"^^ ^ '<' f ^ 



p-^7Ti p. II, I stream 



—^ '-naiK ^_g^g, of stream- cutting and of groundwater 



^^^^ Soil (orwind-blown-sand) 



Figure 2. — Diac/ramviotic Section of Chalk Yalleij and suliterranean Stream 



and not structural origin. They have in general flat floors, at least \n 

 the lower part of their courses, and the plan of the valley presents the 

 usual sinuous shapes, with branching tributaries common to stream-cut 

 valleys, a difference being that their heads are rounded depressions in 

 the chalk uplands rather than Y-shaped ravines. While destitute of sur- 

 face streams, their floors are marked by oblong "graben"-like depressions 



