518 DIVERGENT DIRECTIONS OF LOCAL DRIFTS — USK, BRECON, ETC. 



It must be observed, that these bowlders are not strewed in the direction of the ex- 

 isting water courses, but have been propelled to the east and north-east, thus meeting 

 the ancient or north-western drift 1 . It has also been shown, (p. 441 ,) that the centre of the 

 uplifted mass of Silurian rocks near Usk, is as free from gravel or other detritus as the 

 valley of elevation of Woolhope, all the broken materials having been shot off from the 

 centre to the flanks. But although the interior of the Silurian tract of Usk has been 

 shielded in its southern part, from the sterilizing effects of an overflow of carboniferous 

 detritus, the northern portion, including the hills of Lancayo, Trostrey, and Clytha, as 

 well as a large tract of Old Red Sandstone, extending by Penrose to Llantillio, exhibit 

 some large bowlders with much small gravel of the coal-fields. This is brought forward 

 as an addition to the other proofs which have been adduced, to show that the periods 

 of elevation of the Silurian and carboniferous rocks were independent and distinct ; 

 and it by no means affects the inference, that all such materials were accumulated under 

 the sea. In the tracts further removed from any coal-field, as at Ragland, are no large 

 bowlders, but small pebbles only of millstone grit, mixed up with vast quantities of 

 fragments of the Old Red Sandstone. In some parts of the district between Clytha 

 and Llantillio, lying to the south and south-east of the Holy Mountain, which is com- 

 posed of Old Red Sandstone, the bowlders consist exclusively of the hard beds of that 

 formation, and are of so great a size as to be split up for building purposes, in the 

 absence of regular quarries. In following these bowlders from Llantillio southward, 

 they become smaller as we recede from the flanks of the mountains, and at Clytha all 

 the gravel is round and finely comminuted, of great thickness, and it alternates 

 with layers of sand. Not a vestige of trap or of any rock foreign to the drainage of 

 the district, is to be found in these deposits. On descending the Usk from the town 

 to the river's mouth, the coarse detritus also gradually disappears, giving way to fine 

 sand and silt, through which the stream winds to the estuary of the Severn. 



Another striking example of a mass of coarse drift, thrown off by elevation, is dis- 

 played on the south-eastern talus of the Brecon anticlinal, and all the phenomena 

 agree with those previously described. A sharp and highly dislocated ridge of Upper 

 Silurian Rocks has been elevated on a line from north-east to south-west, and the 

 country on the north-west being higher than that to the south-east, the drift has all 

 been carried in the latter direction. The great transverse faults by which this ridge has 

 been fissured, have necessarily become the beds of the existing water courses of the two 



1 This ancient drift is very distinct from the modern detritus carried down from the gorge of Pontypool, and 

 described by Dr. Buckland (Geol. Trans., vol. v. Old Series, p. 531.) as forming a naked strand of pebbles 

 beneath the escarpment of mountain limestone. The dissimilarity between the river deposits and the ancient 

 drift will be hereafter pointed out in the case of the river Sowdde. The reader will bear in mind, that all the 

 transported matter described by Dr. Buckland as diluvium, and which he considered to have been swept over 

 pre-existing land, constitutes in my vjew submarine detritus of various dates, while his post diluvium is simply 

 a subaerial deposit. 



