part 2] MIDDLE CHALK MAELS IX THE SOMME YALLEY. 135 



6. The SuEEACE of tlie Marls of tlie Middle Chalk in the 

 SoMME Valley and the Neighboueing Disteicts of 

 NoETHEEisr Feance, and the Efeect on the Hydeology.. 

 By William Beenaed Robinsois^ King, O.B.E., M.A., 

 F.G.S. (Read March 9tli, 1921.) 



[Plate III— Map.] 



The folds of the Mesozoic and Tertiary rocks of Northern Franco 

 have been studied by various geologists in the past, notably 

 E. Hebert, M. Bertrand, J. Grosselet, L. Cayeux, (>. F. Dollfus,. 

 and H. Parent. 



Theoretical considerations regarding the relationship of the later 

 folds to the older pre-existing folding have led some of these 

 geologists to stud}^ this problem, while others have endeavoured to 

 trace some connexion between the tectonic structure and tho 

 development of the river-systems. In these pages, however, 

 I ^^ropose to set forth certain information derived from the records 

 of the boreholes which were made bv the British armies during- 

 the War, and to draw some conclusions regarding the relationship- 

 between the tectonic features and the hydrology. 



During the War the geological work in connexion with water- 

 supply fell into two main groups — (1) the endeavour to furnish 

 details of the strata in the area occupied by the British armies, 

 together with notes on their w^ater-bearing capacity ; and (2) the 

 preparation of notes on areas over which an advance might take 

 place. 



In connexion with the first of these requirements, much useful 

 information was obtained from the late Prof. J. Grosselet's works,, 

 particularly his maps of the contours of various horizons, ^ which 

 proved of great use in estimating the dej^th at which various 

 strata occur. 



The area under consideration (see map, PL III) consists funda- 

 mentally of Cretaceous deposits, but frequently has a superficial 

 covering of Quaternary loams, Clay-with-Flints, and small outliers, 

 of the basal Tertiary clays and sands. From the point of view of 

 water-supply from boreholes only two members of the Cretaceous. 

 System need be considered : namely, the Turonian and the- 

 Senonian. 



In the eastern parts of the area there are two markedly-different 

 lithological types. The Lower and Middle Turonian (zones of 

 Inoceranms lahiatiis and Terehratulina gracilis^ are grej^-blue- 



^ 'Etudes des Gites Mineraux de la France — Les Assises Cretaciques & 

 Tertiaires dans les Fosses et les Bondages du Nord de la France ' Paris,., 

 1904-1913. 



