Vol. 67.] ASSOCIATED ROCKS OF NORTH-EAST MONTGOMERYSHIRE. 427 



Tho great difference, besides the lithological distinction, between 

 these beds and those already dealt with, is the presence of 

 vast numbers of brachiopods. The lower beds yield very few 

 species of Orthis, and such as are found are insignificant forms- 

 but poorly preserved as a rule. The deeper-sea conditions of the 

 lower and more shaly beds evidently did not favour brachiopod life; 

 but in this shallower-water type they appear to have flourished 

 abundantly. Fossils can be obtained from numerous localities on 

 Moel-y-garth, the chief locality being an old quarry at the western 

 end of Moel-y-garth Wood. Good exposures also occur on the- 

 summit of the ridge itself and on its eastern flanks. 



The grits of Moel-y-garth are cut off against Guilsfield Brook,, 

 where they form a magnificent fault-scarp. Since the Moel- 

 y-garth Fault is a dip-fault, the lateral displacement is very- 

 considerable ; and one has to proceed nearly half a mile on tfoe- 

 northern side of the fault before the Gaerfawr Beds come in again 

 at Groes-lwyd, west of Guilsfield. Here the beds, broken off by 

 the fault, come in again, forming another well-marked escarpment 

 bounding the western side of the Guilsfield Valley, and having its 

 finest development in the Gaerfawr ridge. The series crops out 

 from under the Powis-Castle Conglomerate, striking now in the 

 normal north-easterly direction and dipping usually at an angle of 

 about 40° north-westwards. As the beds are traced to the north, 

 it becomes very evident that a higher sequence is exposed than is. 

 seen in Moel-y-garth, and the unconformity of the overlying con- 

 glomerate series becomes distinctly noticeable. 



An old quarry in the field above Trawscoed Bough gives the best- 

 exposure of the middle strata of the Gaerfawr Series. About 

 50 feet of alternating massive grits and bastard-limestone bands are 

 exposed. The 'limestones' are beds of sandy grit crowded with 

 fossils, chiefly Orthis, of which only hollow casts remain. The 

 hollows left by the removal of the calcium carbonate are filled as 

 a rule with soft powdery limonite, which imparts to the rock a 

 reddish-brown coloration. 



In Gwern-y-brain, a complete section exposing about 1000 feet 

 of grits and flags can be seen. Near the entrance to Gwern-y-brain 

 by Twll, some faulting is observed, bringing the grits against the 

 Pwll-y-glo Group. Farther up the valley, the beds roll slightly, 

 and strike-faults occur : the exact thickness cannot, therefore, be 

 measured. It seems very likely that it is greater than that men- 

 tioned above. A series of old quarries line the road up the valley,, 

 exposing hard massive grits. These contain few fossils, except in 

 the occasional softer bands. 



Two quarries, now being worked near the top of the valley, 

 expose a higher series of the grits than I have seen elsewhere. 

 The beds become, in fact, limestones, usually bluish in colour,, 

 and frequently impure and muddy. The lowest beds exposed rest 



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