part 2] PALEOZOIC eocks of the Llangollen district. 209 



The Lower Palaeozoic rocks are folded on approximately east- 

 and-west axes into a series of major and minor synclines and anti- 

 clines, which are cut through by a few mighty master-faults and a 

 multitude of smaller adjustment-faults. The earth-stresses were 

 so intense that all save the most resistant rocks were severely 

 cleaved. 



Practically all the folding was effected in Devonian or earl}'- 

 Carboniferous times, while the major faults were also initiated 

 then. Proof of these statements can be seen in the transgressive 

 unconformity of the Dihunopliyllum Zone of the Carboniferous 

 Limestone, which strikes north and south at right angles to the 

 earlier structures. All the Carboniferous is subject, on the one 

 hand, to wide gentle warping that did not modify to any appreci- 

 able degree the Devonian folds below ; and, on the other hand, to 

 influential faulting that produced considerable displacement of the 

 fault-blocks already defined by the earlier movements. 



The problems of the tectonics of the region fall, therefore, 

 naturally under two headings, namely: — {a) Pre- Carboniferous, 

 (5) Post- Carboniferous. This is true only in a general way, 

 however ; for the earth-movements went on intermittently 

 throughout the Devonian, Carboniferous, Permian, and Triassic 

 Periods, and possibly in even more recent times. In a paper dealing 

 with the Lower Palaeozoic rocks, a full account of the Devonian 

 movements is appropriate and will be attempted here ; while the 

 post- Carboniferous modifications can only be touched upon. 



In the following description of the structures, our views have 

 in part been influenced by the fact that one of us (B. S.) has 

 collaborated with Mr. C. B. Wedd, of H.M. Greological Survey, 

 in a study of the tectonics of North Wales. This study has led 

 us to regard the structure of the Llangollen district as produced 

 by torsion, set up by stresses originating in the Caledonian move- 

 ments of Devonian times and continuing to act in later epochs. 

 In this paper the data are set forth, without emphasizing their 

 bearing on the torsional hypothesis. 



Pre-Carboniferous Movements. 



Viewed in the broadest outline, the area is embraced by the 

 following essentially pre- Carboniferous structures (fig. 1, p. 178) : — 



Clwydian Anticline — the north-eastward extension of the Harlech Dome. 



Llanelidan Syncline — expressed by the Llanelidan Fault. 



Mynydd-Cricor Nodal ^ Anticline. 



Bryneglwys Synclinorium. 



The Central Wales or Llandderfel Syncline. 



Brjoieglwys Fault. 



Cyrn-y-Brain Nodal Anticline. 



Llangollen Synclinorium with the Corwen Buttress. 



^ The anticlines defined as nodal bring to the surface Ordovician rocks, 

 which, by their superior toughness, appear to have acted as knots, or nodal 

 points, in the general earth-movement (see p. 212). 



Q. J. G. S. No. 310. p 



