C— GEOLOGY. 79 



movement on certain ' lines of disturbance ' along which the Carboniferous 

 and older rocks have been jorofoundly affected. As a result the streams 

 along these lines have exerted a trapping influence upon the south- 

 easterly streams. 



Not only in the region discussed in detail by Strahan which extends 

 from the Vale of Glamorgan to the Vale of Towy, but in other parts of 

 South Wales, especially north of the Towy, we find many examples of 

 south-easterly streams which have been trapped by others flowing to the 

 south-west. 



Apart from these, however, if we look at any map of South Wales we 

 shall find that the dominant trend of the main streams such as the 

 Loughor, the Lowerj Towy and the Eastern and Western Cleddau is 

 either south-westerly or southerly, but so far as is known it does not 

 coincide with disturbances, as does that of the Neath, Tawe and others. 

 This is also the trend of the Ograore, which is exceptional among the set 

 of south-easterly streams in the eastern part of the coalfield. 



In the Towy Valley there are many gaps in the escarpment on the 

 south side of the valley that are situated in the direct line of tributaries 

 flowing into the Towy from the north-west, and apparently indicate 

 diversion of the streams south-westward into the Towy. Still more 

 remarkable instances occur along the Cothi, a tributary which enters the 

 Towy at Abergwili. The lower part of the Cothi flows nearly due south 

 from Brechfa for about eight miles. Above this place its valley trends 

 north-eastward for over 20 miles and along its course numerous tributaries 

 entering from the north-west have prominent wind-gaps corresponding 

 to them, on the south side of the Cothi valley. There can hardly be any 

 doubt that these indicate former diversions of the south-easterly flowing 

 streams into the Cothi. 



This has proceeded to such an extent that a tributary formerly flowing 

 south-east into the Towy 25 miles above where the Cothi now enters has 

 been diverted into the valley of the latter. ^^ It is true that the courses of 

 both the Towy and the Cothi coincide in large part with belts of dis- 

 turbance in the Palaeozoic rocks. It is remarkable, however, in the case 

 of the Cothi, that the extension of the drainage along this belt to the 

 south-west of Brechfa has been insignificant in comparison with that to the 

 north-east. Only one such stream, Nant Pib, two miles west of Brechfa, 

 appears to have been diverted. 



While agreeing with many of Strahan's deductions regarding the origin 

 of the South Wales drainage system, I am of the opinion that we may be 

 dealing with the effects of two distinct movements, one, the earlier, 

 which gave a prevailing south-easterly trend to the streams, and the other, 

 a later one, to which was due the southerly or south-westerly tilt ; the 

 effect of the former predominating in the easterly region of South Wales 

 and that of the earlier becoming more marked in the west. The earlier 

 movement was in all probability that which occurred between the 

 Cretaceous and the Eocene. 



Strahan {op. cit., p. 218) calls attention to the evidence that uplift in 

 the west had already commenced when the Eocene was deposited ; he 



"^ 0. T. Jones, The Upper Towy Drainage System. Quart. Journ. Oeol. Soc, 

 vol. Ixxx., pp. 568-609. 



