C— GEOLOGY. 69 



sediments (probably Upper Cretaceous) over the Palaeozoic rocks in order to 

 account for the origin of the South Wales drainage system. His argument 

 rests on the complete disregard of the structures of the Palseozoic rocks 

 shown by the streams which traverse them, and he therefore assumed that 

 the drainage must have originated on a cover of newer rocks and have 

 been superimposed upon the older strata as the cover was stripped. Since 

 these conditions apply more particularly to the elevated region of the 

 coalfield which stands in general considerably higher than the surrounding 

 plateau of Central Wales, he implied the extension of the sedimentary 

 cover over areas which, according to Jukes-Browne, remained above the 

 level of the Cretaceous sea. If, in fact, Strahan's explanation is correct, 

 then either the plateau tract of Central Wales must have been covered by 

 such a depth of Mesozoic sediments as would allow them to extend also 

 over the coalfield region and the Brecon Beacons, or the whole plateau 

 of Central Wales and some of its present residual elevations, particularly 

 the escarpment of the Brecon Beacons, must have been developed by 

 erosion subsequent to the Cretaceous period. There are, however, many 

 difficulties in accepting the latter view. 



Once it is granted that the sea may have spread over the Palaeozoic 

 area not once but possibly several times during the Mesozoic period, we 

 have to recognise the possibility that the great plateau of Wales may have 

 been eroded, as Ramsay believed, by marine erosion, and that the margins 

 of the elevations which rise above its surface, including among these the 

 escarpment of the Brecon Beacons, may be the sites of ancient cliff lines. 



We have thus an alternative explanation to that previously suggested 

 for this feature, viz., by marine planation as opposed to desert planation. 

 Whichever vievf is adopted, however, it seems to be clear that these 

 features do not now stand at the same relative levels as they once did. 

 The evidence for this conclusion is dealt with in the next portion of my 

 address. 



The Post-Cretaceous Movements. 



Various authors have called attention to the important part played 

 by the Miocene earth-movements in determining the physical features of 

 the south-east of England. In that region these movements have affected 

 not only the Mesozoic and early Cainozoic strata, but also the underlying 

 Palseozoic rocks, which, having been folded in pre-Triassic times and 

 eroded in subsequent periods, form a more or less even floor upon which 

 rest the gently inclined Mesozoic strata. The Miocene folding was in the 

 main responsible for the anticlines of the Weald and of the Isle of Wight, 

 and the complementary synclines of the London and Hampshire Basins. 

 Lamplugh has shown, however, that the anticlinal structure exhibited by 

 the Cretaceous rocks of the Weald changes gradually in depth, so that the 

 structure of the underlying Jurassic rocks is that of a syncline. This 

 strange superposition of an anticline in the upper layers upon a syncline 

 in the lower layers is explained by the warping of the area during the deposi- 

 tion of the Mesozoic sediments. Had it not been for deformation during 

 sedimentation, the anticlinal arrangement evident in the surface rocks 

 would have persisted downwards, and the Palaeozoic floor would probably 

 have been folded in much the same way as the superficial strata. I 



