116 A. Strahan — The Age of the Vale of Chvtjd. 



Trias we see at once that it is quite as unconformable to the 

 Carboniferous rocks in the Vale of Clwyd as it is everywhere else. 

 It certainly overlaps much of the limestone in places, and it 

 certainly reposes upon strata some hundreds of feet above the top 

 of that formation elsewhere. We see further that the greatest 

 thickness of Carboniferous rooks lies in the synclinal trough under 

 the Trias, and that along the margin of the Vale the purple beds 

 are usually on the point of being overlapped. These facts can only 

 be explained on the supposition that the Carboniferous rocks were 

 bent into a syncline, and preserved from denudation thereby, in 

 pre-Triassic times. From the intimate connection between the 

 syncline and the faulting it would seem almost certain that the 

 latter also must have been partly pre-Triassic, and that the post- 

 Triassic disturbances only instituted fresh movements along old 

 lines. 



In this, as in other respects, the Vale of Clwyd repeats on 

 a small scale the features of the Vale of Eden. Mr. Goodchild, 

 writing on this subject in 1897,^ states of the Vale of Eden that 

 " there can be no doubt that, so far as the faults are concerned, they 

 acted as planes of weakness again and again during several 

 geological epochs. Some, probably the majority, are certainly of 

 pre-Carboniferous age ; but it can easily be shown that, once a fault 

 has arisen, newer rocks at first deposited continuously across its 

 outcrop, have been dislocated over the older fractures, sometimes in 

 directions of movement the opposite of the original." In the same 

 chapter he informs us that post-Carboniferous (pre-Permo-Triassic) 

 faults followed the direction of pre-Carboniferous line of fracture, 

 and that " the differential movements accompanying the upheaval 

 [of the Permo-Triassic strata] were locally repeated yet again over 

 the older lines of fault." 



The overlap of the Carboniferous by the Triassic rocks gives the 

 best proof I know of the pre-Triassic movements in the Vale of 

 Clwyd, but I may add that a strong impression was left in my mind 

 after seeing all the sections that the fracturing and displacement of 

 the Trias was on a small scale compared with that which threw the 

 Carboniferous in a fragmentary condition against the foot of the 

 Silurian ridge. 



W^ith the great pre-Carboniferous movements of North Wales the 

 Vale seems to have no connection whatever. The Upper Silurian 

 strata were bent into a multitude of small folds, roughly cleaved, 

 and subjected to vast denudation before the Carboniferous epoch 

 commenced. It is true that they assume a broad synclinal ai-range- 

 ment between the Dee and the Conway, but, as pointed out by 

 Earasay, the synclinal axis does not coincide with the Vale of Clwyd, 

 but lies considerably to the west of it.^ The dip of the Carboniferous 

 Limestone, moreover, both in East Flintshire and Denbighshire, is 

 sufficient to carry it clear of the tops of the Silurian hills wherever 



1 Geological Survey Memoir on Appleby, etc., p. 33. 



2 Memoirs of the Geological Survey, vol. iii (1881), p. 307. 



