438 



ME. A. WADE ON THE LIANDOVEKY AND [Aug. I9II* 



Pig. 6. — Map showing ilie Wenlock zones 

 exposed in Gullsfield Brook. 



1 mile.] 



are completely overlapped between Powis Castle and Y Prochas. 

 In the stream- section running down the west side of Y Prochas to 

 Ty-brith, strata of Wenlock age again occur. The basement beds 



are sandy flagstones, 

 but above they 

 change to blue flags 

 and shales with 

 massive mudstones, 

 which contain what 

 have been described 

 as 'large concretions 

 iTa Moei-y-g-arth Farm of limestone.' In 



this case, I doubt 

 the concretionary 

 nature of the frag- 

 ments, but think 

 rather that they are 

 boulders of an early 

 Wenlock or late 

 Llandovery lime- 

 stone, broken up 

 during a period of 

 denudation which 

 accompanied the 

 overlapping of the 

 upper beds already 

 noticed at Tyn-y- 

 llywn. This is sug- 

 gested by the fact 

 that some of the 

 blocks are brecciated 

 limestones or lime- 

 stone-conglomerates, 

 which could hardly 

 be concretionary. 



The circumstance 

 that the zone of 

 Cyrtograptus lund- 

 greni rests upon 

 these brecciated 

 beds, shows that 

 the overlap of the 

 Wenlock Shales fol- 

 lowed closery upon 

 a period during which the Llandovery Beds were being subjected 

 to denudation along the axis of the anticline. In Guilsfield Brook, 

 between Coed-y-garth and the Park Parm, the zones of Cyrtograptus 

 linnarssoni, C, rigidus, and C. lundgreni are fairly well seen (fig. 6) ; 

 it is possible, however, that other zones may be present. Fragments 



Coed-y-g-arth# 



