UPPER SILURIAN ROCKS OF RADNOR FOREST AND W. OF RADNORSHIRE. 315 



or Liandeilo flags, they are so interlaced with trap rocks that they can only be described 

 together in the special account of these rocks which follows in the ensuing chapter. 



Lateral or Western Expansion of the Upper Silurian Rocks in Radnorshire. 



It has been shown that in the west of Salop and the contiguous parts of Mont- 

 gomeryshire, the whole Silurian system has been repeatedly folded over in undulating 

 masses by the protrusion of trap and the elevation of the Lower Silurian rocks on 

 parallel lines, throwing off in succession the younger deposits. In Radnorshire are 

 only two such lines of disturbance, one extending from Old Radnor to Presteign, along 

 which, as already shown, the Caradoc sandstone is brought to day. The other is that 

 of the Llandegley and Llandrindod district, where trap rocks and strata of similar age 

 reoccur. With these exceptions nearly the whole of the country to the east of the river 

 Ithon is occupied by the Upper Silurian Rocks, which, in the absence of subdividing 

 limestones to limit the Ludlow and Wenlock formations, is simply one great mass of 

 mudstone formed of innumerable strata of flag and shale. 



In many of the hills, particularly near the summits of those which are not highly inclined, the 

 Upper Ludlow Rock is well marked by its harder character and usual shells, as in the Maen Hill, 

 south-west of the Vale of Radnor, and near the top of Radnor Forest, which is capped by a gritty 

 sandstone (the firestone of that tract) representing the bottom beds of the Old Red Sandstone. 

 The Upper Ludlow Rock, perfectly characterized by its organic remains, is also exposed in va- 

 riously inclined strata in the gorges of the Teme above Knighton, affording at Felindre a most 

 illustrative passage into the Old Red Sandstone of Clun Forest. (See p. 191, and Plate 33. f. 1.) 

 In nearly all such cases the Upper Ludlow Rock is underlaid by shale, sometimes so hard as to 

 constitute flagstones, and these are succeeded by lower beds of shale of vast thickness, which 

 must therefore occupy the place of the Wenlock shale, for in the Llandrindod country the shelly 

 sandstone and grit appear at once from beneath them. The zone of hard flagstones is precisely 

 of the same age as those which occur in the Lower Ludlow Rock in Radnor Forest, the Black Hill, 

 and many places in Salop. They are very largely quarried, and in slabs of enormous size, at 

 Yechad and at Trevod on the south-eastern slopes of Kerry Hill, and, although not of durable 

 quality when exposed, they are very useful for in-door purposes. They contain Cardiola inter- 

 rupta and C. Jibrosa, together with GraptolHes, &c. Similar flagstones of the Lower Ludlow 

 Rock are extracted from the sides of the deep ravines in Radnor Forest with the same and other 

 characteristic fossils, including many Orthoceratites, and in slightly inclined positions underlying 

 the Upper Ludlow Rock. In like manner all the undulating hills south-west of the Vale of Radnor, 

 or between it and the Wye, belong to the Ludlow formation, the lower strata being well exposed in 

 many parts. In the gorges of the Wye, between Erwood and Builth, rocks of this age constitute 

 picturesque grey cliffs, the slightly inclined strata presenting their edges to the eye of the traveller, 

 the rock being in some instances exceedingly hard and compact. In one spot only I observed thin 

 impure calcareous bands, or bastard limestone, viz. near Aber Eddw, about four miles east of 

 Builth, in the hills overhanging the river. As these beds dip under unequivocal Upper Ludlow 



