Vol. 58. ] THE RIVER-SYSTEM OF SOUTH WALES. 213 
felds. The existence of the Coalfield is, in fact, due to a broad 
synclinal structure produced by that movement. The period during 
which the Armorican movement was active in this district is well 
defined. It affected equally all rocks from the Silurian to the 
youngest Coal-Measures, and was therefore post-Carboniferous. 
On the other hand, it affected neither the Keuper in South Wales 
nor the New Red rocks (including the so-called Permian) in Devon, 
and was therefore pre-Triassic. 
The movement is characterized by folding and overthrusting, and 
was essentially of an elevatory type. It marked the commencement 
of a prolonged continental epoch, and was the direct cause of the 
enormous denudation which took place between Carboniferous and 
Triassic times. A river-system must have existed; and Sir Andrew 
Ramsay even attributed the Welsh valleys to erosion continuously 
proceeding ‘ever since the close of that great continental epoch that 
ended with the influx of the Rhetic and Liassic sea.’! I should 
suggest that the erosion more probably took place during the conti- 
nental epoch than after itsclose; but, so far as J am aware, no part 
of the valley-system now in use can be proved to be of this early 
age, except some breaks in the escarpment already mentioned, and 
certain broad low-lying tracts, such as the Vale of Glamorgan and 
the Bristol Channel, which are, or have recently been, deeply over- 
spread by Keuper Marl. That the existing rivers make for these 
tracts is true; but it is to be remembered that in so doing they ignore 
the Armorican folding which determined the positions of those tracts, 
and were therefore themselves determined by another cause. 
(2) The north-north-westerly, or Charnian, faults show 
a remarkable persistence through England, Wales, and probably 
Scotland.” So far as South Wales is concerned, it can be proved 
that the movement was in progress after the deposition of the Lower 
Lias, but that it had-commenced at an earlier date ; for the faults, 
though they affect strata of all ages, are more considerable on the 
average in the Carboniferous than in the Secondary rocks. In other 
parts of the kingdom it can be shown that the movement was in 
full activity in post-Cretaceous times, for its effects are shown in 
the Chaik both of Devon and Yorkshire. If the injection of the 
innumerable dykes of the North-west of the British Isles, which 
have this direction, was due to the same movement, the period of 
activity is extended into Hocene times, and, as a fact, one of the 
north-north-wester!y fractures in Staffordshire has been injected 
with basalt of a Tertiary type. Whatever their age may be, 
however, the north-north-westerly faults had no share in determ- 
ining the courses taken by the rivers; and this I believe te be 
true not only of South Wales, but of other districts. The Vale of 
Clwyd would seem to be an exception: the existence of the Vale, 
+ « Physical Geology & Geography of Great Britain’ 6th ed. (1894) p. 563. 
* Geol. Mag. 1899, p. 111. 
3 W. W. Watts, Proc. Geol. Assoc. vol. xv (1898) p. 397. 
