500 THE UPPER ORDOYICIAN OF THE BERWYN HILLS, [vol. lxxix. 



Silurian. 



It is not proposed to describe the Silurian rocks in detail, but it 

 may be noted that in the area under consideration much of the 

 Upper Valentian appears to be missing by strike-faulting, although 

 a few graptolites of Middle Gala age have been found. The 

 Salopian is, however, well developed, and forms high ground. 

 There are numerous beds of coarse grit in the south-west of the 

 area, and in the Blaen-y-cwm valley three beds may still be 

 distinguished, one forming the bold crag of Craig Fach, and 

 another forming the crags immediately south of A 3 and Ordnance 

 bench-mark 918 - 6 (fig. 3, p. 496). This is the lowest grit, and 

 is the only one that can be traced north-eastwards. This grit 

 also thins out so rapidly that in the stream-section, just before the 

 Silurian is faulted out by the Bwlch-y-greolen Fault, it is repre- 

 sented by a thin sandstone. East of that point no grits appear at 

 this horizon. 



V. Correlations with Neighbouring Areas. 



(«) Welshpool. — The lower beds have much in common in 

 the two areas ; but, as the main land-mass appears to have lain on 

 the east, the beds at Welshpool are liable to be somewhat coarser. 

 Much of the Gaer-fawr Grit series is extremely like the Caradocian 

 sandstones of the Berwyns, and there can be little doubt that the 

 Gwern-y-brain Beds are of the same age as the Pen-y-garnedd 

 Shales. These have been shown to be much more closely related 

 to the Caradocian than to the Ashgillian. The fauna recorded 

 from the Gwern-y-brain Beds is peculiar ; but the absence of the 

 typical basal Ashgillian as developed in the Marchnant Valley 

 made correlation difficult. It would now appear, however, that 

 the Ashgillian is entirely missing in the Welshpool area, and, if 

 ever deposited, was eroded prior to the deposition of the PoAvis 

 Castle Conglomerate. 



(5) Glyn Ceiriog. — The connexion with the Ghyn Ceiriog 

 area is naturally close. Volcanic action Avas more rife in the Glyn 

 area in Caradocian times, the northern flank of the Berwyn Hills 

 coining under the influence of both the western or north-western 

 vents and the vents situated on the south-east near the Carboni- 

 ferous Escarpment ; Avhile the South- Western Berwyns only 

 received small showers of ash, presumably from a north-westerly 

 direction. 



The strike-fault at the base of the Dolhir Beds obscures the 

 sections at the base of the Ashgillian ; but it is clear that the 

 Blaen-y-cwm Beds are the equivalent of the Pen-y-garnedd Shales. 1 

 while perhaps the Ty'n-y-twmpath Beds are contemporaneous with 

 the dark Lower Ashgillian of the Marchnant Valley. 



1 See L. J. Wills & B. Smith, Q. J. G. S. vol. Ixxviii (1922) p. 186. 



