114 A. Strahan — The Age of the Vale of Chcyd. 



with basalt of a Tertiary type.'- In South Wales recent work has 

 shown that the N.N.W. faults which intersect the coalfield with 

 curious regularity are of later date than the east-and-west folds, 

 and are in part, at least, post-Liassic. Lastly, in Somerset and 

 Devon a similar set of faults is known to be of post-Cretaceous age, 

 while the great disturbances and overthrusts in the Carboniferous 

 rocks of those counties are pre-Triassic. Assuming that all these 

 N.N.W. dislocations were contemporaneous, so far as regards their 

 post-Triassic effects, we still can prove no more than that they are 

 post-Cretaceous. It is probable, however, that they ai'e principally 

 of Miocene age, and due to the same movements which led to the 

 fracturing of the north-west part of the British Isles and the injection 

 into the cracks of an almost infinite number of basaltic dykes. 



That the Vale of Clwyd Fault is also, in part at least, of post- 

 Triassic age, is certain from the fact that the Trias is repeatedly 

 shifted by both it and its branches. It is obvious, moreover, that 

 that rock must have been tilted along the line of fault, for the 

 current-bedding planes dip at 50° or 60° from the fault, an 

 impossibly steep angle to be due to deposition in either air or 

 water. The existence of pre-Triassic disturbance can therefore only 

 be determined by eliminating the post-Triassic movements and 

 ascertaining the position which had been assumed by the Carboni- 

 ferous rocks in pre-Triassio times. This can be done by observation 

 of the overlap of the Carboniferous by the Triassic strata. 



In the north and north-eastern part of the Vale we know nothing 

 of the relations of the two formations beyond the fact that Coal- 

 measui'es, dipping steeply to the north-west, are thrown by the Vale 

 of Clwyd Fault against the limestone at Dyserth and Meliden, 

 while two boreholes out in the plain at Rhyl and Prestatyn proved 

 red sandstone believed to be partly of Triassic and partly of Carboni- 

 ferous age.^ Though we have no clue to the exact origin of the 

 measures there proved, it is obvious that they are not likely to be 

 low down in the Millstone Grit, while they may well belong even, 

 to the Middle Coal-measures. 



At Bodfari, on the other hand, it appears that the limestone must 

 have been directly exposed to the Triassic waters, for it displays the 

 partial conversion into heematite which is so commonly developed 

 under such circumstances. The limestone, moreover, belongs to the 

 lower part of the formation, which suggests that not only the Coal- 

 measm'es but nearly all the limestone itself was hei'e overlapped. 

 The evidence, however, is more suggestive than conclusive. 



Further south we find a narrow strip of the pui-ple beds faulted 

 in between the Trias and the limestone, while at Plas Draw there 

 was in 1881 a section showing Trias unconformably banked against 

 a little cliff cut out of purple beds. The Triassic overlap diminishes 

 therefore from Bodfari southwards, and it seems to be in consequence 

 of this that there is so considerable an exposure of the purple 

 strata in the Llanbedr Farm Dingle. 



1 W. W. Watts, Proc. Geol. Assoc, 1898, p. 399. 



2 " Geology of Rhyl," etc., p. 27. 



