Vol. 67.] ASSOCIATED HOCKS OF NORTH-EAST MONTGOMERYSHIRE. 423 



At the head of the dingle, the beds are stained red, and the dip 

 repeatedly changes ; finally, they pass under the lied Conglomerate 

 of Cherry-tree Bank. 



On the western side of the dingle, the shales abut against the 

 conglomerate which forms the wall of the Welshpool Dyke. The 

 character of the junction indicates that the shales are cut off on the 

 west by a fault which throws down the Powis-Castle Conglomerate 

 and the Welshpool Dyke itself. Towards the east, the beds again 

 end abruptly against buff-coloured shales which yield Lower Ludlow 

 graptolites. The junction is best seen at the southern end of the 

 dingle behind the old Military Depot. Here, evidently, is a great 

 fault with a downthrow to the east of apparently over 1000 feet. 

 This fault may be called the Bron-y-Buckley Fault, from 

 the fact that the wood in which Trilobite Dingle is situated is 

 known by this name. On the south, shales occur under the 

 Red Conglomerate of Powis-Castle Park, which are probably the 

 southern extension of an inlier of Trilobite-Dingle Beds. 



On the north, similar beds occur in the ditches by Cefn-sych 

 Farm, in the dingle below Sale Farm and in Trelydan Dingle. 

 They strike north-north-eastwards, and are folded into a series of 

 small anticlines. The strata are jointed at right angles to the 

 strike, the joint-planes dipping 65° to 70° south-eastwards. Towards 

 the top of the section in Trelydan Dingle the shales dip in opposite 

 directions on opposite sides of the stream : on one side up stream, 

 on the other down stream. This is due to one of a series of faults 

 which cross the stream here. 



Under Trelydan Cottage a great fault throws down a small anti- 

 cline of Silurian beds, including a limestone-band which contains 

 fossils of Wenlock age. This fault is the northern continuation of 

 the Bron-y-Buckley Fault. Farther north, the beds are hidden 

 under deposits of drift and alluvium. In this area the shales do not 

 contain the abundance of Trinucleus concentricus which characterizes 

 those of Trilobite Dingle ; this is due to the fact that these strata 

 represent a slightly lower horizon than is exposed in Trilobite 

 Dingle. The same graptolitic fauna is obtained from both localities. 

 On the western side of the valley, the group is again found cropping 

 out in the fields south of Brookland Hall. The best exposure here 

 is seen in the dingle by Penbryn. 



The upper beds here become massive mudstones, with flaggy 

 bands towards the top. The beds are extremely fossiliferous in 

 places, and contain lamellibranchs and graptolites in the lower 

 and more shaly strata, 1 ' while Trinucleus characterizes the mudstone 

 group. The strata are again linked together by the occurrence of 

 similar graptolites, which become more rare in the upper part of 

 the group. 



On the north they are again well exposed in Gwern Heylin, 

 where the strike swings round to the west. A fault of unknown 

 throw occurs just below the lake. This locality yielded practically 

 no fossils. 



