348 



UPPER SILURIAN ROCKS — CAERMARTHENSH1RE. 



On both sides of the ravine, these tilestones inclined 60° and 65° to the south-east pass 

 downwards into grey-coloured rocks. The junction is well observed to the west of Horeb chapel, 

 near which the Upper Ludlow Rock, rising from beneath the tilestones, ascends into the peaks 

 called Cerrig-bw-bach. The ends of the underlying strata are visible on the north side of the road, 

 but are more clearly exposed on the opposite or south-western bank in small parallel ridges se- 

 parated by combs, and the decomposing ends of the beds which protrude through the grass contain 

 a few fossils. As the angle of inclination increases, the strata pass down into others, charged with 

 casts of large orthocerata, corals, Sec, and these are succeeded by dark grey flagstones and coarse 

 slaty beds exposed in the longitudinal depression of Cwm-meirah and in the opposite bank of the 

 stream near the half-way house. Among the fossils at this spot are Asaphus caudatus, Productus 

 depressus (var.), Atrypa affinis, and many corals, Sec. 



These lower strata are still more highly inclined, than those of the Upper Ludlow Rock and Old 

 Red Sandstone, for they become absolutely vertical, and in some parts are even so bent over as to 

 dip to the north-west, apparently passing under the Lower Silurian Rocks which flank them in that 

 direction. There can, however, be no doubt that the beds are only partially inverted, since there 

 is the clearest evidence, that the strata and their fossils which lie between this spot and Llandovery 

 belong to the Lower Silurian Rocks. These beds, where vertical or most bent, are broken through 

 by numerous, parallel, vertical, small joints, transverse to the strike, the interstices being often 

 filled with quartz crystals 1 . The fossils on the whole agree with those of the Lower Ludlow Rock, 

 although there are one or two species not found in Shropshire, an addition to be expected in the 

 same formation at distant points. These strata are succeeded by beds of black shivery shale, which, 

 though generally perishable, have occasionally (as on the south-eastern face of the adjoining 

 parallel ridge of Cefn-erthan) so fissile a structure, as to have been quarried for rough slates. 

 They occupy precisely the same position as the coarse slates of Hennalt near Builth, and complete 

 the parallel between the Upper Silurian Rocks of the Cwm-dwr and those on the Wye. (See 

 previous chapter.) These rocks range to the south-west along the escarpment of Mynidd-myddfai, 

 and are well seen in the transverse section of the gorge of Pont-ar-llechau (banks of the Sowdde 

 river, PI. 34. f. 5.). The junction beds beneath the Old Red Sandstone are grey, fine-grained, very 

 slightly micaceous, hard sandstone, of conchoidal fracture (the corner or building stone of the 

 district), which passes down into thinner flaggy beds with casts of fossils, succeeded by strata of 

 compact sandstone, again alternating with other flag-like fossil beds. These beds in the gorge of 

 Pont-ar-llechau dip at angles of 70° and 80° beneath the Old Red Sandstone, and are in parts much 

 affected by a slaty cleavage, occasionally so complete as to pass through the laminee of deposit and 

 cut completely through the organic remains. Ludlow rocks possessing all these mineral characters 

 are underlaid on the sides of the road between the Cwm-dwr and Llandovery, and in the tract 

 between that road and the river Sowdde, by arid, sandy shale, usually devoid of fossils. It 

 generally folds over in undulating and sometimes in large sub-concretionary masses, in which lines 

 of true stratification are exceedingly obscure. At Pwll-calch, near Myddfai, the upper part of this 

 shale contains a stratum sufficiently calcareous to have been burnt for lime. The mass lies in ver- 

 tical strata, which strike 30° south of west and 30° north of east, and is from 30 to 40 feet wide 3 . 



1 This is one out of a multitude of examples clearly exhibiting a jointed structure, the direction of the joints 

 changing with every change of the strike. 



2 My attention was first called to this small patch of limestone by Mr.W. Rees of Llandovery, to whom I am 

 obliged for several good hints respecting the topography and structure of this neighbourhood. 



