DENUDATIONS AT DIFFERENT EPOCHS (VALLEY OF USK). 517 



valley of the Usk, the narrow transverse valleys in which the minor streams flow 

 from the crest of Old Red Sandstone, as well as the south-eastern face of the moun- 

 tains, are loaded with bowlders of that rock alone. In the broad and deep valley of 

 the Usk itself, from the west of Crickhowell to Abergavenny, is much coarse gravel, 

 lodged sometimes several hundred feet above the present bed of the river. A part of 

 this gravel has also been transported from the mountains of Old Red Sandstone (on 

 the north-west), but the greater portion has, doubtlessly, been derived from the margin 

 of the contiguous coal basin of South "Wales, which from Crickhowell to Newport, con- 

 stitutes the opposite side of this deep depression. 



The enormous accumulations in the vale of the Usk, between Crickhowell and Aber- 

 gavenny, have been previously cited, as the necessary results of the powerful dislo- 

 cations and denudations to which this valley owes its origin ; and the amount of the 

 wreck has been satisfactorily explained by showing, that the insulated and lofty limestone 

 peak of Pen Cerrig-calch, now distant five miles from the edge of the coal-field, has 

 been separated from the main mass of the formation by the scooping out of the inter- 

 vening valley to the depth of 1200 or 1500 feet. (See p. 163.) 



The Usk, from its source to its mouth, winds round the escarpment of this coal-field, 

 and hence, in the valley watered by it, we find an intermixture of detritus produced 

 by currents, which have flowed from different centres of elevation. A great number of 

 the fragments of the Old Red Sandstone have been drifted from north-west to south- 

 east, owing to the ancient elevation of the region • but the broken materials of the coal- 

 field have been carried from the south, south-east, and south-west, towards the north, 

 inconsequence of the east and west movement before described (p. 406), each drift pro- 

 ceeding from independent points of disturbance. These drifts have been arrested upon 

 certain promontories only ; thus, for example, although they are in great force around 

 the town of Abergavenny, obscuring the lower sides of the surrounding mountains, yet 

 the lower limits of the Little Skyridd, reaching to Llangattock, are free from such 

 coarse detritus, presenting slopes of deep red loam, which terminate in the sandy and 

 gravelly bed of the Usk. In descending this valley to the town of Usk, the river flows 

 through a large, elliptically shaped mass of Silurian rocks, already described as rising 

 in the form of an irregular valley of elevation, (p. 441.) The western face of this in- 

 sulated mass of rocks, which approaches within a few miles of the external lip of the 

 coal basin, has been covered by a prodigious quantity of bowlders and coarse gravel of 

 the coal-field, the hard white grits predominating ; the red matrix and chief mass having 

 been derived from the surrounding Old Red Sandstone. The bowlders of the coal 

 measures can be traced up to the mouths of large transverse rents in the edge of the 

 adjoining coal basin (Pontypool, &c.) ; and the chasms prove what vast masses of 

 solid rock must have been abstracted from them. These materials were no doubt 

 poured down into their present situations during those periods of elevation when the 

 adjoining carboniferous strata were thrown up and fractured. 



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