70 SECTIONAL ADDRESSES, 



cannot conceive of the Mesozoic and Cainozoic cover being folded unless 

 at tlie same time the surface upon which it reposes was affected, and there 

 is little doubt that the relative movements of that floor during the Miocene 

 folding were of the same type as in the superficial layers, but owing to 

 warping during the Jurassic there is now no close correspondence between 

 the form of the Palaeozoic floor and the dominant surface features already 

 enumerated. 



Before proceeding to the inquiry which is the main object of this 

 portion of my address, namely the extent to which these movements may 

 have influenced the physical features of the Bristol Channel region, I 

 wish to refer briefly to some of the characteristics of the Miocene folding 

 as exhibited in the South of England and in Northern France. 



Although we habitually speak of the anticline of the Weald and the 

 syncline of the London Basin, each of these folds when considered in detail 

 is not a simple structure. 



The folding in the Weald was described in some detail by Topley," 

 who recognised that the so-called Wealden anticline is a compound 

 structure which is made up of ' a number of minor folds traversing the 

 district in lines more or less parallel to its general axis of elevation.' 

 These axes are described in detail, and the more important ones are 

 represented on the map accompanying the volume. No individual fold 

 can be traced more than a few miles ; it is well defined at some point on 

 its axis, but both to the east and to the west the structure becomes less 

 definite and ultimately imperceptible. Where a particular axis ceases to 

 be traceable, its place is usually taken by another either to the north or 

 to the south. Each anticlinal fold is thus an elongated pericline, the 

 structure being most clearly defined in the central region of its traverse, 

 near where the change of pitch from an easterly direction to a westerly 

 direction takes place. 



The axes of the numerous minor anticlines that go to make up the 

 Wealden elevation are thus arranged en echelon. The place of any given 

 anticline, say in the east, may be taken farther west by a pair of anticlines, 

 one lying to the north and the other to the south, while the actual prolonga- 

 tion of the anticlinal axis may coincide with a syncline. 



This is precisely the character of the folding worked out by Mr. W. B. R. 

 King in the surface of the marls of the Middle Chalk in Northern France. ^'- 

 That author took advantage of the information afforded by a large number 

 of bore holes that were put down in the war area in search of water, to 

 construct contours representing the surface of the Middle Chalk marls, and 

 thus obtained the form to which these originally horizontal strata were 

 warped by Post-Cretaceous (probably Miocene) folding. The contoured 

 map so obtained is most instructive. To the eastward of Amiens an anti- 

 clinal area near Rosieres occupies ihe territory included between the Upper 

 Somme and its tributary the River Avre. This pitches eastward on the 

 east and westward on the west. If its axis is prolonged, however, for some 

 miles to the west it almost coincides with a syncline, the axis of which runs^ 

 parallel to the Lower Somme Valley. The anticline has therefore disap- 

 peared completely in a westerly direction. 



" Geology of the Weald, pp. 216-42. Mem. Geol. Survey, 



12 Quart. Journ. Oeol. Soc, vol. Ixxvii. (1921), p. 135, and PL III. 



