78 SECTIONAL ADDRESSES. 



with the well-marked Glastonbury syncline which is continued westward 

 to the shores of the Bristol Channel itself,. and we may probably regard it 

 as the master structure which has determined the existence of the Channel. 

 The London Basin may be prolonged into the tectonic depression between 

 North Hill and the Mendips, but I believe that its representative in the 

 west is the syncline that extends in a west-north-westerly direction nearly 

 along the valley of the Avon west of Bath, and it is not improbable that 

 the course of that river was determined, like that of the Thames, by this 

 downfold. In the western region the effects of the folding are more 

 prominent in the south than in the north, so that the syn^ine of the 

 London Basin becomes insignificant westwards and its placeTs taken by 

 the Glastonbury syncline, which itself appears to die away eastwards. 

 The same thing is found in the reverse direction in comparing the folds 

 at the west end of the Weald with those near the centre. The maximum 

 uplift on the west was along the Kingsclere fold near the northern margin, 

 whereas farther east it was nearer the southern margin along the Battle 

 axis of elevation. 



The correspondences between the post-Liassic folding in the south-west 

 of England and the Miocene folding of the south-east of England are so 

 striking that it appears we may safely accept as a working hypothesis 

 that the post-Liassic folding in the former district is in its main character- 

 istics also of Miocene age. The precise effects of the folding would, 

 however, depend upon the condition of that area at the time when the 

 movement occurred. 



I have already given reasons for supposing that the Palfeozoic areas of 

 the west were covered by the Cretaceous and possibly by earlier Mesozoic 

 formations. It can hardly be supposed, however, that the thickness of 

 the Mesozoic cover was as great as in the south-east of England. The 

 Palaeozoic floor would, therefore, be nearer the surface of the folded strata 

 and its deformation would reflect much more closely that of the strata 

 of the cover than does the Palaeozoic floor of the south-east of England. 

 Accepting as a provisional hypothesis that the Bristol Channel was 

 determined like the London Basin by movements, in the main of Miocene 

 age, it is of interest to examine the possible consequences of such movements 

 within the Palaeozoic regions that adjoin the Channel to the north and 

 south. 



The area to the north has been dealt with exhaustively by Strahan. 

 to whose conclusions reference has already been made.^* That author 

 believed that the rivers of South and Central Wales originated upon a 

 surface of Upper Cretaceous sediments which completely blanketed the 

 Palaeozoic rocks beneath, and that the direction of the main streams such 

 as the Wye, Usk, Rhjraney, Taff, etc., was determined by a tilt of the 

 Cretaceous cover to the south-east, i.e. in the same direction as the dip 

 of the Chalk outcrop from Wiltshire to the border of the Fens. He called 

 attention, however, to aaother system of valleys such as the Neath, Tawe, 

 Towy and others which had a south-westerly trend almost parallel to the 

 strike of the chalk escarpment. Both systems were assigned to the 

 Miocene period. The latter were attributed to movement or renewal of 



i" The Origin of the River Svstem of South Wales. Quart. Journ. Geol. Soc, 

 vol. Iviii., p. 207. 



