516 EXAMPLES OF PURELY LOCAL DETRITUS (HEREFORDSHIRE.) 



In the central parts of Herefordshire, the effects of powerful denudation are also ap- 

 parent, though few or no remains of the abraded strata are exhibited. Such action is 

 clearly displayed in the form of the two Pyons. These conical hills, seen at great 

 distances throughout the county, consist in their lower part of the argillaceous marls 

 of the Old Red Sandstone. The soft materials of which these strata are composed, 

 and by the former extension of which, these two conical hills were once doubtlessly 

 connected, have been entirely swept away, and the present form of the hills has alone 

 been preserved by caps of semi-conglomerate cornstone. Thus to the perishable nature 

 of the strata must be attributed the usual absence of gravel in the centre of Hereford- 

 shire. Even upon reaching the chain of the Abberley and Malvern Hills, where a lofty 

 ridge of syenite and greenstone has risen through the Silurian rocks and Old Red Sand- 

 stone, we find little or no coarse detritus upon the western side of the ridge, except 

 in two hollows immediately beneath the hills, in which are heaps of angular and disin- 

 tegrated syenite. One of these hollows occurs near Colwell Green, and another near 

 the Eastnor Obelisk. No gravel, however, properly so called, occurs on the western 

 flank of the Malvern Hills, except a small patch near Clencher's Mill, which has been 

 accumulated in a trough between the Silurian ridges of Malvern and Ledbury, from 

 both of which the materials have been derived. On the east side of the Malverns, on 

 the contrary, accumulations of the wreck of the syenite and Silurian rocks flank the 

 gravel of the valley of the Severn or the northern drift, the consideration of which oc- 

 cupies the next chapter. 



The absence of all fine or coarse detritus in the valley of elevation of Woolhope has 

 before been pointed out, the only remains of the broken materials which have been 

 swept out from the interior of that remarkable valley, being partially heaped up at the 

 mouth of the principal gorge by which it is drained. In such a case we should expect 

 to find the debris where it is, namely, lodged only at those points of its margin, which, 

 having been broken through, permitted the disintegrated materials to escape by trans- 

 verse apertures from the centre of elevation. Such examples offer, indeed, the most 

 convincing proof of purely local drift. 



From Herefordshire or the great water-shed of South Salop and Radnor, let us pass 

 into the drainage of the Usk, and see if it offers analogous evidence. The valley 

 of the Wye is separated from that of the Usk by the lofty chain of Old Red Sandstone, 

 called the Black Mountain, the highest points of which are about 2300 feet above the 

 sea. Whether we cross this dividing ridge in the neighbourhood of the Hay where it 

 is high, by the lateral valleys in which the minor streams flow from north-west to south- 

 east, or by the Monmouth Cap, where it is comparatively low, we meet with finely 

 levigated red silt only, or gravel both coarse and fine, derived from the cornstone or 

 other members of the surrounding Old Red Sandstone 1 . When we descend into the 



i The embayed flats south of Whitfield and north of Monmouth Cap, are good examples of the fertile soil 

 produced by the breaking up and disintegration of these materials, 



