218 DE. WILLS AXD ME. SMITH OS THE LOWEE [vol. Ixxviii, 



proj)ose to criticize or discuss the nature of the Cae-mor Fault as 

 described by Dr. Groom & Mr. Lake, beyond claiming that it does 

 not pass northwards of Llwyn-mawr, for the mapping of the rocks 

 on the valley-sides noi-th of Dol-y-wern precludes the possibility 

 of this. 



South of the Glyn-Ceiriog Fault the same simple structm-e noted 

 bet\yeen it and the Llangollen Fault persists. The gentle north- 

 ward dip steepens somewhat, it is true, and a greater number of 

 adjustment-faults can be proved in the older rocks, which are 

 possessed of more distingiiishing qualities than is the Lower Ludlow 

 Series. The faults rano-e from almost north-and-south in the 

 west to north-west and even west-north-west in the east. 



The simplicity of the structm-e appears to be disturbed in the 

 eastern part of the area by the Dolhir strike-fault at the base of 

 the Ashgillian. This fault has ah-eady been discussed on p. 189. 

 It may be noted, howeyer. that it has the same general strike as 

 the Llangollen and Glyn-Ceiriog Faults, and is a thi-ust. 



Probably other strike-faults occur in this area, but none of any 

 magnitude have been definitely proved. The thinness of the 

 Yalentian at the outcrop near Glyn-Ceii'iog perhaps may be due to 

 such a fault (p. 199). 



Caledonoid Structures. 



All the structures so far considered are related to folding on 

 almost east-and-west axes. There are, however, two south-west 

 and north-east (' Caledonoid,' as the late Prof. Charles Lapwoi*th 

 would have called them) features of prime importance to the 

 district. 



(a) Central Wales or Llandderfel Syncline. — The fii'st 

 may be called the 'Llandderfel,' or, better, the 'Central Wales 

 Syncline,' for it is a continuation of the downfold to which Prof. 

 0. T. Jones 1 gave the latter name farther south. The syncline 

 lies for the most part outside the area under description. The 

 Dee Yalley follows it from Llandderfel to Corwen. Its influence 

 is seen in the westward pitch of the western end of the Berwyn 

 Dome. The downfold cannot be definitely traced north-east of 

 Corwen. Two explanations of its disappearance seem possible : 

 either the fold has been bent on the fulcrum of the Corwen 

 Buttress into an east-and-west structure (the Llangollen Syncli- 

 norium) ; or the synclinal tendency has found expression in the 

 westward downthrow of the Bryneglwys Fault between Corwen 

 and Llandegla, and in the synclinal an-angement of the strata 

 between Mynydd-Cricor and Cyrn-y-Brain (pp. 219, 220). 



(h) Bryneglwys Fault. — The second great 'Caledonoid' 



^ ' The Geological Structure of Central "Wales & the Adjoining Eegions ' 

 Q. J. G. S. vol. IxTiii (1912) pp. 328-44. 



