﻿546 DK. T. GEOOM AND ME. P. LAKE ON THE [IS'OV. I908, 



29. Tlie Bala and Llandoveey Eocks of Glyn Ceieioo (I^oeth 

 Wales). By Dr. Theodoee Geoom, M.A., P.G.S., and Philip 

 Lake, M.A., P.G.S. (Read May 20th, 1908.) 



[Plate LIII— Map.] 



Contents. Page 



I. Introduction 54G 



IL Literature 547 



III. Section in the Glyn Yalley 551 



IV. Detailed Description of the Subdivisions 555 



V. Structure of the Area 580 



VI. Comparison with other Areas 589 



I. Inteodtjction. 



It was in continuation of the work which we had begun at Corwen 

 that in 1892 we paid our first visit to Glyn Ceiriog.^ At Corwen 

 we had been unable to detect any decisive proof of unconformity at 

 the base of the Corwen Grit ; but in the Bala Beds below we did not 

 find the characteristic fossils of the Sholeshook Limestone or of 

 other Upper Bala deposits, and were therefore led to believe that 

 the highest beds of the Bala Series were absent, and that there was 

 probably an unconformity which was too slight to be visible in so 

 small an area. 



The section at Glyn Ceiriog encouraged the hope that higher 

 Bala Beds might there be found, and it seemed, moreover, to afford 

 excellent opportunities for the examination of the whole Bala Series. 

 To judge from the map of the Geological Survey, or from a cursory 

 inspection of the ground, it would be difficult to imagine a locality 

 more favourably situated for the purpose. The valley of the Ceiriog 

 appears to cut through the whole of the Bala Series, from the lowest 

 ash-band up to the Glyn Grit ; and, owing to the steepness of its 

 banks, the rocks are admirably exposed, while most of the beds are 

 fossiliferous almost to excess. 



It was not long, however, before we found unexpected difficulties. 

 The same succession, or rather the upper portion of it, should 

 appear in the nearly parallel valleys of Tyn-y-twmpath and IS^antyr ; 

 but when we examined those valleys we found beds that were 

 quite unlike any in the Ceiriog Yalley, and yet it seemed improbable 

 that the character of the deposits and of their fossil contents should 

 change completely in so short a distance. It was not until we 

 had paid several further visits to the district that we found the 

 explanation. 



Although the section in the Ceiriog Yalley appears complete, both 

 on the map and in the field, this appearance is really deceptive. 

 The valley is crossed by a long fault that runs nearly parallel to 

 the strike of the beds and dips at nearly the same angle as the beds 



^ This village is often locally called Glyn. 



