A. Strahan — The Age of the Yale of Clwijd. 115 



Thence to the south end of the Vale we get repeated exposures 

 of the purple strata, and no positive proof of their being anywhere 

 wholly overlapped. Even here, however, there would have been no 

 reason to doubt the existence of a great unconformity, for I noticed 

 that the lower beds of the Trias, in a little escarpment at Garth- 

 gynan, contained bands made up apparently of the resorted material 

 of the purple beds with what seemed to be small fragments of 

 Silurian shale. The occurrence of such fragments would at once 

 prove that the Trias overlapped the whole of the Carboniferous 

 system, and would demonstrate the existence of a pre - Triassic 

 syncline, but I was unable to determine them with certainty. 



At the extreme southern end of the Yale the synclinal arrangement 

 of the Carboniferous strata is fully revealed. The limestone dipping 

 north-west at Tyn-y-berllan swings round to an easterly dip near 

 Bryn-ffynnon, and in the trough thus formed encloses a small thickness 

 of the purple beds. The Trias comes on close by in the line of the 

 syncline, but appears to be nearly horizontal, while the Carboniferous 

 rocks are more or less tilted. The same remark holds good of the 

 two formations east of Llanfair Dyffryn Clwyd, but the observations 

 are too scanty to be conclusive. 



More importance attaches to the relations of the Trias to the 

 limestone along the flanks of the hills west of Llanfair, for in 

 following the base of the Trias along these hills we recede from the 

 axis of the syncline, and at the same time find the Trias creeping 

 across the edges of zone after zone of the limestone, until near 

 Ruthin half that formation has been overlapped. This overlap 

 shows that, whatever synclinal structure the New Eed Sandstone 

 may possess, there was a still more pronounced syncline in the 

 Carboniferous rocks, and that it coincided with the existing syncline 

 of the Vale of Clwyd. Between Euthin and Denbigh the Trias is 

 believed to rest generally on the limestone, but in four places the 

 purple beds peep out between the two. At Llanfwrog they are 

 thrown against the northern termination of the Coed Marchon 

 limestone-range by an east-and-west fault, which is possibly of 

 pre-Triassic age. At Bachymbyd they come out again for a short 

 distance, and with the limestone below them and the Trias above 

 are thrown against Silurian rocks by the Llanrhaiadr fault. The 

 Trias, however, extends to no great depth near the fault, for the 

 stream at Llanrhaiadr, below the waterfall caused by the fault, has 

 cut down to the purple beds, giving the third of the four exposures 

 referred to. The fourth exposure occurs at Pont Ystrad, and 

 occupies exactly the same position relatively to the Denbigh fault 

 as the Bachymbyd exposure occupies to the Llanrhaiadr fault. 



Lastly, we have the large area of purple sandstones and shales 

 which were found by Mr. Tiddeman to extend out into the Vale as 

 far as St. Asaph.^ There, again, the purple beds survived pre-Triassio 

 denudation in considerable thickness. 



From this rapid traverse of the sections along the base of the 



1 " Geology of Ehyl," etc., p. 25. 



