OUTLIER OF PEN CERRIG CALCH. 



163 



"Pen Cerrig Calch" {Angl. the limestone Crag), I ascended to ascertain why such a name 

 had been used in a district appearing to consist exclusively of the Old Red Sandstone. 

 (PL 31. fig. 1.) I also perceived that the lower sides of these hills were precisely similar 

 in structure to those of the Old Red Sandstone in the opposite escarpment of the coal-field. 

 The quartzose conglomerate or top of the Old Red Sandstone (which will be described in 

 the next chapter) stands out in the great ' 1 ecroulement " of rocks named Daren, and 

 also in the Beacon Hill, both acting as "contreforts" or supports to the higher mountain 

 summit ; and lastly I found in the culminating point of the arid and lofty mountain of 

 Pen Cerrig Calch, 2200 feet above the sea, that the limestone, not less than fifty feet thick, 

 occupied an isolated yet distinct escarpment covered by the true millstone grit, the latter 

 having a thickness of about one hundred and fifty or two hundred feet, and thus all the 

 relations were completely established. In this outlier, therefore, we find a much smaller 

 development of the limestone than in the main escarpment of the coal-field, where the 

 formation occasionally swells to a thickness of several hundred feet. The chief mass is 

 thick-bedded, compact, and subcrystalline, of a cream colour and without fossils; but in 

 some of the thinner beds on the southern face of the mountain, where they disappear 

 beneath the cap of millstone grit, they become oolitic and contain a few organic remains, 

 — Productus, Terebratula, Corals, and Encrinites, but all those which I collected were 

 in too imperfect a condition to be capable of specific description 1 . 



The grooved and sinuous surfaces upon the weathered bosses of this rock will re- 

 mind the traveller of those limestones, which have been long exposed to the atmosphere 

 in very lofty situations ; for they strongly recalled to my memory similar appear- 

 ances in the calcareous* peaks of the Eastern Alps. The beds of Pen Cerrig Calch are 

 inclined at the very gentle dip of six or seven degrees to the south-east, and if the planes 

 of stratification be prolonged towards the Blorenge mountain near Abergavenny, to 

 which they point, we find that this limestone would be carried much below its natural 

 position in the escarpment of the coal-field ; in other words, the limestone would be 

 brought far beneath the surface of the Old Red Sandstone, the rock which in undis- 

 turbed positions forms its natural support. Again, the inclination of these hills is to 

 the south-east, whilst the same beds in that portion of the escarpment of the coal-field 

 which is nearest to Pen Cerrig Calch, dip to the south. (See Map.) We have thus 

 clear proofs of violent dislocations, along the external margin of the coal-field, by 

 which certain members of the Old Red Sandstone have been so thrown up as to circle 

 round the basin, whilst the chief masses of the system from which they have been de- 

 tached, still occupy the broad mountainous range of the Black Forest, and of which 

 Pen Cerrig Calch forms a part. This fact is a strong additional testimony, that although 

 the adjoining carboniferous deposits may have been accumulated in a depression, yet 

 their present basin shape has been produced by great dislocations. The removal of 



1 This limestone was formerly burnt for lime, but is now abandoned on account of the expense, caused by the 

 distance from coal, the height of the mountain, and the want of roads. 



