140 ME. W. B. E. KIXG OIS^ THE MIDDLE [vol. Ixxvii, 



tectonic structure and the hydrology, and consult the account 

 given by Prof. L. Cayeux {op. jam cit.) of the country around 

 Cambrai : in this he correlates practically every stream or dry 

 stream-course with a tectonic fold. M. Gr. P. Dollfus,^ however, 

 argues that the numerous folds at right angles to the main axis 

 are difficult of explanation, and he does not agree that there is 

 likely to be any close connexion between the tectonic lines of the 

 Cretaceous and the present-day river-system, since the latter 

 originated on a Tertiary covering and is superimposed on the Chalk 

 as at present exposed. 



This may be the case, but it is probable that in this area the 

 folding of the Tertiary beds was along the lines of the post- 

 Cretaceous and pre-Tertiary folds, and that even if the folds did not 

 absolutely coincide in the position of their axes, yet the general 

 trend was probably the same. This may be the explanation of the 

 lower valley of the Somme. From the map it is clear that the 

 river flows about 4 miles north of the lowest point of the syncline, 

 as indicated by the Cretaceous dejDosits. It may be that the river 

 is on the axis of the S3aicline of the Tertiary rocks, which was 

 situated north of, but parallel to, the old pre-Tertiary axis on the 

 south. 



Similarly'', the upper Ternoise and upper Scarpe valleys do not 

 appear to coincide with the sj^ncline which runs from Hesdin to 

 Arras, but the rivers are situated about 3 miles north of the 

 Cretaceous tectonic axis. 



The Somme from Amiens to Peronne is clearly influenced by 

 the dome which splits the syncline of the Lower Somme into two 

 parts, one being occupied by the Avre and the other by the 

 Somme. 



The sudden change of direction of the Somme at Peronne is 

 more difficult to explain, unless it be due to a north-and-south 

 fold ; and in this connexion the fact that this stretch of the 

 Somme is in line with the Tortille and that part of the Hirondelle 

 stream which is followed by the Canal du Nord, must be of 

 significance. It has been pointed out before that along this line 

 the main axis from Le Cateau to Nurlu is lost in the broad, slighlly 

 undulating, low-lying area Avhich is indicated by the altitudes of 

 the marls. 



Perhaps, also, the Ancre-Sensee line and Tortille-Hirondelle line 

 are complementary one to the other at each side of the low area of 

 the Somme battlefields. 



The correspondence between the folds and the general parallel 

 alignment of the rivers in their lower reaches is too well 

 established to need comment here. 



The relationship of the tectonics of the area to the capacity of 

 the strata for 3delding water in boreholes is a point of considerable 



1 Bull. Serv. Carte Geol. France, No. 14, vol. ii (1890-91) p. 54. 



