part 3] 



METAMORPHISM IN THE MOKA COMPLEX. 



339- 



not, however, by the Padrig slide, but by thrusting at a lower- 

 angle than the major axis. This phenomenon is seen on a small 

 scale, on the same northern limb, along the coast, where the 

 Fydlyn-Gwna zone of passage is partly cut out several times by 

 just such thrusting, which drives the Gwna Beds over it — thus 

 restoring, locally, the chronological order of succession. 



(2) Wylfa. — The Fydlyn Beds, readily decomposing, have 

 weathered into a grassy curving hollow, running westwards across 

 the neck of the headland. Of the fact that the succession is 

 inverted, we have (p. 341) independent evidence. Here again, 

 therefore, a major isoclinal infold is revealed, its core being 

 occupied by Fydlyn Beds. I am now, however, convinced that it 

 is ruptured on, at any rate, its southern side. The Fydlyn Beds 

 of the core fail to emerge in Porth-wnol, where they come on 

 to the Wylfa thrust-plane ('G. of A.' pp. 218-19 & fig. 96) at 

 a lower angle than the isoclinal axis. Further, as there is a 

 considerable angular divergence between the strike of the infold 

 and that of the thi'ust, it would appear that they are tectonically 

 unconnected, and that the infold is the older structure of the two. 



(3) Bull Bay. — A few paces inland, Gwna Beds are onlj r 

 17 yards away across the strike, but they just fail to reach the 

 cliff, with the result that Fydlyn Beds are there brought against 

 the Church Ba}>- Tuffs. The difficulty of postulating a rupture 

 too large to be credible at this place can now be removed. The 

 cliff-section, which is 50 to 60 feet high, is represented in fig. 3. 



Fig. S.—Clif-sectiou at Bull Bay. 

 MF 





MF = Fydlyn Beds. 



MS = Church Bay Tuffs. 



Let us interpret it also on the hypothesis that throughout the 

 region the succession is inverted, and that the Gwna Beds have 

 been reduced in thickness by the local waste whereof the pebbles 

 in the Skerries Group are evidence. Suppose, then, that a major 

 isoclinal infold containing Gwna Beds is pitching eastwards, so as 

 to be just bringing in Fydlyn Beds l at the meridian of the present 



1 Revision of the 6-inch map favours contact of the two series for a short 

 distance inland. 



