CASTELL CERRIG CENNEN. 



167 



fig. 6.) The breadth of this valley between the escarpment of the carboniferous lime- 

 stone on the south, and the Silurian rocks on the north, does not exceed two miles. It 

 will hereafter be shown, that the full development of the Old Red Sandstone terminates 

 at Pont-ar-lleche, and as that system is not laid open by any transverse gorge in the 

 valley of the Cennen, no accurate opinion can be offered respecting all the strata of 

 which its chief and central masses may be composed. The uppermost beds, however, 

 present a thin zone of conglomerate and grit, and the lowest tilestones are distinctly 

 exhibited between Cerrig Cennen and the vale of the Towy. The intermediate strata, 

 upon some of which the limestone outlier of Cerrig Cennen is placed, are argillaceous 

 and sandy ; those on the north side of the castle being vertical and unconformable to 

 the bedding of the calcareous rock, while those at its southern base, though imperfectly 

 exhibited, are apparently twisted under the limestone. 



The calcareous beds of "Cerrig Cennen" are well exhibited, and consist of the black 

 and grey varieties of the adjacent carboniferous limestone, inclosing the same fossils * 

 viz., Productus hemisphfsricus , P. Martini, and large bunches of one of the corals pecu- 

 liar to this formation, (Lithodendron sexdecimale, PhilL, &c.) 



The precipitous face or escarpment of the rock on which the castle stands, is exposed 

 to the depth of about 60 feet, the summit of the hill being between 250 and 300 feet 

 above the Cennen. The remainder of the declivity from the base of the rock to the 

 river, is covered by a steep grassy slope, which conceals almost entirely the strata below 

 the limestone ; but at one point the Cennen has worn away a small portion of the talus, 

 and exposed a few feet of red sandstone, which appear to dip, as before said, unconform- 

 ably under the limestone of the castle hill. 



On the left bank of the Cennen the strata of Old Red Sandstone interposed between 

 the outlier and the main escarpment of the carboniferous limestone, are so concealed by 

 detritus, that traces of them are to be detected only in the bed of one small rivulet. 

 On ascending, however, to the site of the great fault already described, or towards the 

 break in the zone of limestone above alluded to, we find the upper member of the Old 

 Red Sandstone plunging to the south beneath the limestone on one side of the ravine, 

 at an angle of seventy degrees, and on the other of thirty or forty degrees to the south- 

 east (sources of the Lwchwr). Seeing that the sudden break in the continuity of the 

 main escarpment of the limestone is exactly opposite the outlier Cerrig Cennen, and that 

 this dislocated mass would, as nearly as could be estimated, fill the portion of the gap 

 which is between the spring head of the Lwchwr and the house of Cwrt-a-barddh, it 

 might at first be concluded, that the rock Cerrig Cennen had been torn from its parent 

 seat, and precipitated down the shelving escarpment upon the surface of the Old Red 

 Sandstone ; an inference which would not appear extravagant in this region of violent 

 disturbance. But though highly curious and different from that of Carreg-las already 

 described, or of any other detached mass along the frontier of the South Welsh coal- 

 field, the dislocation of Cerrig Cennen may be simply explained. It is evident from, 



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