138 ME. W. B. R. KllS'a OlS" THE MIDDLE [vol. Ixxvil, 



The centre of the fold, however, does not appear to coincide with 

 the present line of the River Somme, but to lie a few miles south 

 of that river. The bifurcation of the S3nicline at Amiens is clearly 

 seen, the main axis following the line of the Avre ; and a distinct 

 synclinal line is seen coinciding with the Somme river from 

 Amiens to Peronne. The southern side of this syncline is formed 

 by the dome-like elevation indicated b}^ the borings of Assevillers, 

 Rosieres, etc. (over 40 metres above sea-level) ; while all borings 

 in this reach of the Somme Valley find the marls below sea-level. 

 This dome, then, is the slight secondary anticlinal axis suspected 

 by M. Dollfus.i 



North of the river the contour-lines rise to a crest, which runs 

 in a sweeping curve from Le Cateau through Le Catelet and Nurlu 

 to Flers. Before the line of the Ancre this crest appears to become 

 somewhat indistinct ; but a strong anticlinal ridge replaces it 

 en echelon on the line Bilmcourt-Saulty, while farther west a 

 second dome on the line Valheureux-Bernaville appears between 

 the syncline of the Somme and that of the Authie. 



North of the Le Cateau-Flers and Bihucourt-Saulty ridges the 

 contours fall to the great depression of Douai and the basin of 

 Orchies, and to the synclinal axis which runs somewhat south of the 

 main road from Arras to St. Pol. Still farther north the curves 

 rise again to the high ground of the Vimy and Notre Dame-de- 

 Lorette ridges, which coincide with the maximum elevation of the 

 marls (over 17o metres above sea-level) before they are brought 

 down about 100 metres (328 feet) by the great fault and fold of 

 Marqueffles, to fall gradually under the Tertiary covering of the 

 plain of Flanders. 



This series of contour-lines indicates, therefore, in a general wa}^ 

 the amount and position of the deformation which has taken place 

 since the Turonian marls were deposited : the assumption being 

 that the sea-bottom at the time of deposition was horizontal. 



The folds are seen to trend in a north-west and a south-east 

 direction in the Avest of the area ; but they graduallj'' swing round 

 to an east-and-west line, and finalh% on the eastern edge of the 

 map, the}'^ are trending east-north-east and west-south-west. The 

 reason for this change in strike is discussed later. 



Another point of considerable interest is that, when the line of 

 maximum elevation of the marls is traced from east to west, 

 we find that it is not a continuous curving line, but a series of 

 short curves arranged en echelon, resulting in a stepping of the 

 crest-line gradually farther and farther northw^ards. 



Thus, in the east, the main crest-line is on the line Le Cateau— 

 and Le Catelet-Nurlu, gradually falling from over 100 metres 

 (328 feet) at Le Cateau to less than 75 metres (216 feet) on the 

 line of the Tortille River, with some slight indication of the con- 

 tinuation of this line w^estwards, as proved by the borings at Flers. 



^ Op. cit. p. 45. 



