210 DE. WILLS AND ME. SMITH 02s' THE LOTVEE [vol. Ixxviii, 



Clwydian Anticline. — At the present time, the graben of 

 the Yale of Clwyd, with its stupendous eastern boundary-fault, 

 makes it difficult to appreciate the pre-Carboniferous structure. 

 However, the Lower Ludlow rocks of the Clwydian range and of 

 the country on the west of the Vale appear to form part of the 

 extension north-eastwards of the great Harlech Dome. The axis 

 of the anticline probably strikes east and west in this region, and 

 appears to lie somewhere in the latitude of Moel-Famau, where 

 Wenlock Beds are found. 



Although the initiation of the depression of the Yale of Clwyd 

 probably took place in pre-Carboniferous times, it did not obliterate 

 the effects of the folding. The rocks on each side of the Yale 

 appear to have formed part of a local dome on the axis of the 

 great Harlech upfold: for in the Nant- Clwyd area, west of the 

 Yale, the folds pitch westwards, whereas in the Clwydian range 

 they pitch eastwards. Further, in conformity with this general 

 doming, we find in the Nant-Clwyd area that the leintioardinensis- 

 beds cover most of the ground, while they also probably occupy 

 the easternmost part of the southern end of the Clwydian range. 



Llanelidan-Fault Syncline. — In view of the facts just 

 related, it is held that in pre-Carboniferous times a synclinal 

 tendency existed between the Cl'W'ydian anticline and the dome of 

 Mynydd-Cricor. This tendency found its expression chiefly in the 

 Llanelidan Fault, which strikes almost due east and west between 

 the present trough of the Yale of Clwyd and the surviving half of 

 the Cricor dome, and ultimately passes between the Millstone Grit 

 of Moel-G-aregog and the Ordovician of Cyrn-3^-Brain. Con--, 

 siderable movement has taken place in post- Carboniferous ages 

 along this fault ; but it was one of the great master-faults in this 

 region in Devonian times. The displacement produced by it has 

 probably always been lateral rather than vertical, the block of 

 countiy south of it travelling eastwards relative to that on the 

 north. At the same time, it is probable that it conformed to the 

 synclinal tendency by a downthrow northwards: for north of 

 the Llanelidan Fanlt the Carboniferous Limestone rests upon the 

 higher part of the Lower Ludlow, while, south of that fault, on 

 Cyrn-y- Brain it lies upon Ordovician. It must also be pointed out 

 that the Limestone crosses the Cyrn-y-Brain anticline by trans- 

 gressive overlap, much of the lower part being absent. The post- 

 Carboniferous movement along the Llanelidan Fault is, therefore, 

 less than is suggested by the displacement of the outcrops (from a 

 point immediately west of Minera to a mile and a half south-west 

 of Llandegla). 



Mynydd-Cricor Nodal Anticline. — In the area between 

 the Llanelidan and the Brj^neglwj^s Faults the dominant sti'uctural 

 feature is the periclinal dome of Mynjxld- Cricor, the northern 

 half of which is cut off by the Llanelidan Fault. 



The core of Ordovician slates and grits of Mynydd-Cricor is 



