part 2] CHALK MAELS IN THE SOMME VALLEY. 137 



arranged in card- catalogue form, under place-names, so that any 

 record could be easily accessible when needed.^ 



Unfortunately, the method of drilling and the nature of the 

 rock made it extremely difficult to determine with any degree of 

 accuracy the exact position of the top of the ' Marls ' in any bore. 

 Percussion -drilling was the method alwa3^s used, so that the 

 change from Upper White Chalk to the bluish marls of the Middle 

 Chalk could only be detected by the colour of the sludge or by 

 small fragments adhering to the end of the chisel. The driller, 

 also, had some indication of the change to the marls by the way 

 in which the chisel was gripped more firmly by the stiffer marls 

 than by the more friable chalk. It was, therefore, often a matter 

 of considerable doubt as to where, within 5 or even 10 metres 

 (say, 16| or 33 feet), the line of the top of the marls should be 

 drawn. The presence or absence of flints was a good indication ; 

 but frequently fragments of flint were knocked oft' the side of the 

 bore where it was in the Senonian, and these flint-fragments were 

 pounded into the marls by the chisel, and tluis were brought up 

 mixed with the marl. This gave rise to the frequent entry of 

 * Marl with flint ' on the part of the driller. 



Another and important source of error in the figures on the map 

 arose from the inaccuracy of the contour-lines on some of the 

 maps, and also from the uncertainty of the exact position of the 

 boring where the map-location had not been given in sufficient detail 



It will be seen from the foregoing observations that frequent 

 and often considerable errors are to be expected in a map con- 

 structed from such data, but the great number of borings helps to 

 reduce the errors in the curves, and the map thus made brings 

 out several points of interest. 



If we study now the form of the surface of the marls as shown 

 on PI. Ill, the general lines are seen to be very similar to those 

 of the surface of the Chalk given by M. G. F. Dollfus.2 The 

 chief differences are that, Avhile this map shows all post-Turoniaii 

 deformations, M. Dollfus's map indicates only the post-basal 

 Eocene folding. 



The general scheme of folding shov/s, in the west, a series of 

 anticlines and synclines, somewhat irregular in their amplitude, 

 but with a definite north-west and south-east strike ; while in the 

 eastern part there is a central broad anticlinal ridge, with a stiike 

 changing from north-west and south-east to east-north-east and 

 west- south-west on the extreme eastern margin of the map. In 

 the north-eastern corner is a well-marked basin, and in the south 

 a low irregular dome. 



If we examine the map in detail from south to north, the first 

 feature that leaps to our ej^es is the syncline of the Somme. 



^ A catalogue of the bores, together with tabulated details of the thickness 

 of the various strata, has been published by the Societe Geologique da Nord, 

 Ann. Soc. Geol. Nord, vol. xlv (1920) pp. 9-34 & map pi. i. 



2 Bull. Serv. Carte Geol. France, No. 14, vol. ii (1890-91) pp. 1-68 & map ; 

 map reproduced in Proc. Geol. Assoc, vol. xxi (1910) pi. v. 



