part 2] PALEOZOIC eocks of the llaxgollen disteict. 211 



followed on the east and south by a broad stretch of Valentian 

 Slates, dipping outwards, and broken by numerous faults. 



On the south the dip is mainly southwards, but traced eastwards 

 the strike swings round through north-east to north until it is 

 almost north-north-west near the Llanelidan Fault. The change 

 in strike suggests the dragging-round of the beds by the westward 

 displacement of the block of country north of the Llanelidan 

 Fault. 



The numerous faults affecting the area appear to be adjustment- 

 faults ancillary to this anti-clockwise twist. They slice across the 

 dome with strikes varying from almost north-west in the east to 

 north in the west. They throw down sometimes one way, some- 

 times the other. At the western end of the dome the arrangement 

 of the strata is more complicated. A triangular strip of country 

 is found there having the dip inwards towards the dome, from 

 which it is separated by a fault. As a result of this arrangement 

 the Wenlock is brought to lie against Ordovician a quarter of a 

 mile west of Cricor Farm ; while farther south, the Tarannon and 

 the Wenlock Series are in juxta^Dosition. 



The rocks in this strip of country are affected on its south side 

 by two curved thrust-faults, convex towards the south and south- 

 west. The more southerly thrust forces Pen-y-glog Grit over the 

 Ludlow Series ; while the second, which may be termed the 

 Ffynnon-Tudur Thrust, carries Lower and U^^per Valentian over 

 the Grit. 



Bryneglwys Synclinorium. — The general anticlinal struc- 

 ture of the Cricor dome gradually merges southwards into a 

 complex synclinorium, the axis of which runs east and west in the 

 neighbourhood of Moel-Truan, 2 miles west by south of Bryn- 

 eglwys. The structure hereabouts is similar to that of the 

 northern half of the Llangollen Synclinorium (see p. 213), and 

 the two regions occupy analogous positions with regard to the 

 Mynydd-Cricor and Cyrn-y-Brain domes respectiveh\ The simi- 

 larity of structure suggests that the two synclinoria are parts of 

 one structure, subsequently displaced laterally by the Bryneglwys 

 Fault. 



Cyrn-y-Brain NodalAnticlin e. — The great anticlinal dome 

 of Cyrn-y-Brain constitutes a node of Ordovician rocks similar to, 

 though larger than, that of Mynydd-Cricor. Both nodes have 

 similar effects on the rocks to the south of them. 



Cyrn-y-Brain mountain is covered to such an extent by peat and 

 heather, except around its periphery, that little can be made of its 

 internal structure ; but on its northern side there are patches of 

 Valentian involved in the faulting, which show that, as a whole, 

 the rocks are arranged as a periclinal dome. The large area 

 covered by the Ashgillian Beds makes it almost certain that the 

 anticline is complex in structure. The western end of the dome 

 has been faulted out of sight by the Bryneglwys Fault or fault- 



p2 



