jmrt 2] PALEOZOIC eocks of the llangollen district. 215 



exception of the Caer-Drewyn area in the angle between the 

 Brjneglwys and Llangollen Faults. 



Caer-Drewyn Hill forms part of an area shaped like an isosceles 

 triangle, lying between the converging Bryneglwys and Llangollen 

 Faults which form its two long sides, and the Rhagatt-Hall Fault, 

 which forms its base (fig. 4, p. 216). It occupies a position 

 analogous to that of the patch of Tarannon shale lying between 

 the converging Llanelidan and Bryneglwys Faults, south-east of 

 Llandegla (p. 219) ; but, owing to the different rocks and 

 movements involved, the resulting structure is somewhat different. 



The rocks in the apex of the triangle and along the southern 

 (or Llangollen Fault) side are concealed by drift and alluvium, the 

 exposed rocks consisting of A^alentian Slates and Pen-y-glog Slates 

 and Grrits. It seems probable that the rocks thrown down by the 

 Bryneglwys Fault on its northern side also had a horizontal move- 

 ment south-westwards; while those south of the Llangollen Fault 

 (also throwing down north) travelled eastwards. The stresses 

 involved squeezed the rocks in the triangle, and set up a movement 

 of revolution in an anti-clockwise direction. The resistance of the 

 tough Wenlock grits to the pressure apparently caused them to 

 buckle into a syncline, and finally to break and to be overridden, 

 except along the base of the triangle, by the underlying shales. 

 The boundary between the grit and the shales is everywhere shown 

 by the mapping to be a fault. ^ 



Simultaneously with the squeeze came the attempt at revolution 

 which caused the rocks to fracture by transverse faults. The 

 thrusting appears to have taken place to a greater or smaller 

 degree in the strips between these faults, at some points bringing 

 the Lower Wenlock Shales, at others different horizons of the 

 Tarannon Shales, against the Grrits. 



South of the Llangollen Fault, the adjustment-faults and the 

 simple pitching synclinal structure have already been noted ; but 

 the western termination of the fold is also of interest, and may 

 now be described. 



Cor wen Buttress. — The syncline passes westwards into a 

 projection of the Berwyn anticlinorium that may be appropriately 

 termed the Cor wen Buttress. It occupies the high ground 

 west of the Bonwm Fault and south-east of the Bryneglwys Fault, 

 while the Llangollen Fault forms a convenient northern limit. It 

 is an anticlinal spur separating the Central Wales (Llandderfel) 

 Syncline from the Llangollen Synclinorium. 



The whole of the ground has not been thoroughly examined ; 

 but it appears probable that it is occupied by shallow rippling 

 folds which give a very broad outcrop to the Ashgillian and Lower 

 Valentian beds that form it. The outcrop of the Corwen Grit on 



^ The interpretation of this fault as a thrust furnishes the only reasonable 

 explanation of the geological map. 



