part 4] THE UPPER ORDOVICTAN OF THE P.ERWYN HILLS. 487 



18. The Upper Ordoyician Rocks of the South-Western 

 Berwyn Hills. By William Bernard Robinson Kino, 

 O.B.E., M.A., F.G.S. (Read May 16th, 1923.) 



[Plate XXVI— Trilobites.] 



Contents. 



Page 



I. Introduction, and General Topography • 487 



II. Previous Literature 489 



III. The General Succession 490 



IV. Local Details 490 



V. Correlations with Neighbouring Areas 500 



VI. General Summary of the Sequence of Events in the South- 

 Western Berwyns 502 



VII. Palffiontological Notes 503 



I. Introduction, and General Topography. 



The area to be described is part of North-Western Montgomery- 

 shire. All place-names mentioned will be found on the 1-inch 

 Ordnance Survey Map, Sheet 136 (Bala), or on the accompanying- 

 sketch-maps (figs. 1, 2, & 3, pp. 488, 493, & 496). . 



Geologically, this area is the western part of the southern flank 

 of the great Berwyn dome of Ordovician strata ; but a glance at 

 the geological map shows that in this south-western portion the 

 beds are pinched up so as to form a wedge-shaped south-westerly 

 extension, one flank of which runs with remarkable straightness 

 for several miles. The reason for this straightness of outcrop lies 

 in the fact that in this belt of country the strata are vertical, or 

 have acquired a slight reversal of dip from the south-eastward 

 overfolcling. 



To this cause must also be attributed the parallel arrangement 

 of hill and valley, each band of hard rock standing out as a straight 

 ridge, and each soft bed forming a parallel depression. Thus the 

 sandstones of the Caradocian and the grits and massive mudstones 

 of the Salopian stand out as ridges, while the softer Ashgillian and 

 the shales of the uppermost Caradocian form the marked valley 

 which is followed by the old pack-road connecting the Upper 

 Yvrnwy valley with that of the Tanat. 



This strike-valley is drained by various streams. In its north- 

 eastern part to Pen-y-garnedd (Sheet 137) it is drained by a 

 tributary of the Tanat, then for a short distance the headwaters 

 of the Cain drain the water down past Blaen-y-cwm to Llanfyllin, 

 while towards the south-west the Marchnant brook carries off the 



Q. J. G. S. No. 316. 2 l 



