part 2] PALEOZOIC hocks of the llangollen distkect. 213 



which, with a large downthrow northwards, strikes east 10° south 

 roughly parallel to the axis. Throughout the synclinorium the 

 cleavage has a general strike of east 10° to 20° south. 



Before we enter upon the details of the folding, it will be well 

 to describe the major faults. 



Parallel to the Llangollen Fault and about 2 miles south of it, 

 runs another master-fault, the Gljn-Ceiriog Fault, which also 

 throws down northwards. The parallelism of these two faults 

 with the direction of folding and cleavage emphasizes their con- 

 nexion with the stresses that caused both. It is thought that in 

 both cases the initiation of the fault may have been connected 

 with the bending-down of the Llangollen Basin, and that possibly 

 they may be in the nature of 'undertow faults.' But, at any rate 

 in the Llangollen Fault, such movement appears to have been 

 accompanied and succeeded by lateral movement Avestwards of the 

 block on the northern side of the fault. 



This movement appears to have induced a series of adjustment- 

 faults with a general northwestward trend springing from the 

 master-faults. In some of these an apparent eastward downthrow 

 may in reality represent a northward lateral displacement of the 

 block on the east of the fault. Other of the adjustment-faults 

 counteract the effect of the pitch of the synclinorium by throwing 

 up eastwards, thus allowing its great length to be compatible with 

 a steep pitch throughout. 



Of the adjustment-faults springing from the Llangollen Fault, 

 the most important are the Aqueduct Fault (separating Carboni- 

 ferous from Silurian north-east and north of Llangollen), the 

 Llandynan Fault, and the Khagatt-Hall Fault ; while the following 

 are connected with the Grlyn-Ceiriog Fault: — The Castle-Mills 

 (near Chirk Castle), the Pont-fadog, the Nant-Fridd-isel (near 

 Carrog), andtheBonwm (near Corwen) Faults, as well as a number 

 of less conspicuous ones (see fig. 1 & PI. V). 



The complex folding of the northern limb of the synclinorium 

 may now be dealt with in somewhat greater detail. Abandoning 

 the consistently southward dips observed on the flanks of Cyrn-y- 

 Brain, the rocks, from the Tarannon slates upwards, become 

 involved in a type of structure best described as ' concertina- 

 folding' (see sections, PL IV). 



Upfold and downfold follow each other in rapid succession, 

 with their axial planes nearly vertical and sometimes reversed. 

 The packing of the folds has gone so far in places that an isoclinal 

 structure has been produced. The height of the folds does not 

 seem to be great, and sometimes the bottom of one fold and some- 

 times the top of another are found at the surface, dissected for us 

 by the deep valleys. 



As the axial area of the sjaiclinorium is approached, flatter dips 

 become the rule, although in places sharp buckling disturbs the 

 gentler folds. This is well seen near the gorge of the Dee at 



