part 2] CHALK MAELS IN THE SOMME VALLEY. 141 



practical importance, and it was largely with this aspect of the 

 subject in view that the work of tracing the folds of the Chalk was 

 begun. 



Since the diameter of the boreholes made by the British Army 

 was either 6 or 8 inches, the results derived from a study of the 

 yields of these bores may be valid only for small-diameter borings 

 yielding a maximam flow of 12,000 gallons hourly. 



The majority of the borings were sunk in areas where the clayey 

 nature of the Middle Chalk marls was well developed. In districts 

 where the marls actually cropped out at the surface, it was obvi- 

 ously of little use to bore for water, for the chance of striking a 

 good water-yielding fissure was small. The same conditions, from 

 £i practical point of view, hold in those areas where the main Chalk 

 water-table surface falls below the Senonian and Holaster-pla iius 

 Ohalk and enters the marls. 



As evidence that this is actuall}^ the case, several borings which 

 were situated in these areas, where the surface of the marls was 

 above the water-table, may be quoted : namely, the boring on 

 the Vimy ridge near Roclincourt, the Saulty boring, and some of 

 the borings in the neighbourhood of Candas, etc. These borings 

 yielded less than 1000 gallons hourly. The map of the marl- 

 surface, together with details of the depth at which water is found, 

 naturally gave useful information on this subject. 



The converse generally, but not alwaj'^s, held good — namely, that 

 in the synclinal areas, where the water-table was well up in the 

 Senonian, all the borings yielded good supplies, from 6000 to 

 12,000 gallons hourly. This variation in yield could frequently be 

 accounted for by the change in mechanical efficienc}^ of the air-lift 

 pump with the different ratios of lift to submergence in the 

 various borings. The exceptions, however, are worthy of note, since 

 they corroborate the results obtained in the areas where there Avere 

 •only a few feet (up to 50 feet) of water-bearing Chalk below the 

 water-table. 



In both these areas it was found that bores on the summits 

 ■of hills (Gurlu Wood) or on the highest points of the plateau 

 {Pozieres No. 1, etc.) yielded very poor supplies; while borings 

 within a few hundred yards in the heads of dry valleys yielded 

 ^ood supplies (Pozieres No. 2, Orvillers, etc-). Perhaps the most 

 noticeable example of a good yield in this zone, where the sui-face 

 of the marls was almost coinciding with the water-table, was that 

 of Bernaville ; but this bore Avas situated at the bottom of a dry 

 -valley, situated some 400 yards back from the highest point of the 

 plateau. Similar results Avere found at Fressonneville (west of 

 Abbeville), where the boring on the top of the plateau proved a 

 failure ; but one in a dry valley, about half a mile from the crest 

 of the plateau, yielded an excellent supply. 



It was the exception to find that borings on the actual summit 

 of the plateau yielded as Avell as borings situated a little down the 

 dry valleys, regardless of the fact Avhether the area Avas synclinal 

 or anticlinal in structure. 



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