THE BRECON ANTICLINAL OF SILURIAN ROCKS. 



337 



to the north-east beneath the same red sandstone. At Erwood, not a mile from the 

 eastern boundary, the bed of the Wye exhibits great confusion, a narrow wedge of red 

 sandstone being there dovetailed between masses of Ludlow rock. That this patch of 

 sandstone does not belong to the Silurian System is clearly explained by ascending the 

 hills to the south-west, where a promontory of Ludlow Rocks is found, constituting a 

 distinct prolongation of the Silurian frontier. The Silurian strata, in fact, dip both to 

 the north-west and south-east, constituting a marked anticlinal ridge, which throws off 

 the Old Red Sandstone on each flank ; and hence the narrow tongue of that rock at 

 Erwood is simply one end of the wide area of Old Red Sandstone which occupies the 

 wilds of Mynidd Epynt. (See Map.) 



Brecon anticlinal. — The Ludlow rocks form the above promontory amid the Old Red 

 Sandstone, extend in highly disrupted masses from Erwood, on the Wye, to Corn- 

 y-fan, six miles north-west of Brecon, where they terminate in a narrow point after 

 a course of about ten miles. This remarkable ridge offers, perhaps, the best type of 

 the Ludlow formation to the south of the Wye. It does not consist of one continuous 

 range, as I was at first disposed to think, but of large masses which are often dis- 

 jointed, trending in slightly divergent directions, and sometimes completely separated 

 by great transverse intervals occupied by Old Red Sandstone. (See map.) These 

 elevated rocks constitute the mountainous hills or wild moorlands of the Graig, Llan 

 eglwys, Ysgwidd-hwch, and Castel-madoc, including Gaer-fawr and Altt-fawr, Castel 

 rhiwan-nest, Mynidd-bach and Corn-y-fan, and rising to heights varying from 1200 

 to 1500 feet above the sea, are prominently distinguished by their grey surfaces from 

 the red lands through which they emerge. The mean strike of the strata is from 30° 

 north of east to 30° south of west, though there are considerable deviations from 

 this direction in the different masses. Thus, in the hills near Crickadarn and the Wye, 

 the strike is nearly north-east and south-west. In Mynidd-bach and Corn-y-fan, the 

 south-western extremity, it is E.N.E. and W.S.W. ; whilst in several of the smaller 

 intervening masses it is east and west. These differences in direction are the evidences 

 of powerful transverse dislocations along this anticlinal line, and afford the most con- 

 vincing proofs that the strata so affected have been forced up with intense violence. 



The picturesque and rocky point of Corn-y-fan, where the Ludlow rocks finally 

 subside, is a striking example of the truth of this remark. That rock presents on 

 one face an inclined plane dipping to the north-east at 33°, but in a few yards, or 

 its extreme point, the inclination changes to the south-west, in both cases the strata 

 of the Ludlow formation passing conformably beneath the Old Red Sandstone ; whilst 

 on the south-eastern face is a broken escarpment, probably due to the narrow form 

 and sharp inclination of the elevated mass, for in the bed of the adjacent brook, which 

 is parallel to it, and near the farm of Neuadd, the Old Red Sandstone is thrown off to 

 the S.S.E. at the very high angle of 80°. (See PI. 33. f. 9.) As they approach the 

 western extremity, the elevated masses of Upper Silurian rocks are most affected 



