164 MY LIFE [Chap. 



or twelve miles from north to south ; and in every part of it 

 the chief summits are from 2000 to 2500 feet high, while 

 near its western end, about twelve miles from the Beacons, is 

 the second highest summit, Van Voel, reaching 2632 feet. 

 Most of these mountains have rounded summits which are 

 smooth and covered with grassy or sedgy vegetation, but 

 many of them have some craggy slopes or precipices on their 

 northern faces. 



Almost the whole of this region is of the Old Red Sand- 

 stone formation, which here consists of nearly horizontal 

 strata with a moderate dip to the south ; and the whole of 

 the very numerous valleys with generally smooth and gradually 

 sloping sides which everywhere intersect it, must be all due 

 to sub-aerial denudation — that is, to rain, frost, and snow — the 

 dibris due to which is carried away by the brooks and rivers. 

 The geologist looks upon the rounded summits of these moun- 

 tains as indications of an extensive gently undulating plateau, 

 which had been slowly raised above the surface of the lakes 

 or inland seas in which they had been deposited, and subjected 

 to so little disturbance that the strata remain in a nearly 

 horizontal position. When from the summit of any of these 

 higher mountains we look over the wide parallel or radiating 

 valleys with the rounded grassy ridges, and consider that the 

 whole of the material that once filled all these valleys to the 

 level of the mountain-top has been washed away day by day 

 and year by year, by the very same agencies that after heavy 

 rain now render turbid every brooklet, stream, and river, 

 usually so clear and limpid, we obtain an excellent illustration 

 of how nature works in moulding the earth's surface by a 

 process so slow as to be to us almost imperceptible. 



This process of denudation is rendered especially clear to 

 us by the singular formation of the twin summits of the Brecon 

 Beacons. Here we are able, as it were, to catch nature at 

 work. Owing to the rare occurrence of a nearly equal rate 

 of denudation in four or five directions around this highest 

 part of the original plateau, we have remaining for our in- 

 spection two little triangular patches of the original peat- 

 covered surface joined together by the narrow saddle, as 



