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ascent of the Beacons, a short account of them will be both interesting and instructive.
The northern face of the mountain is very rocky and precipitous, while on the southern and western sides easy slopes reach almost to the summit. The last few yards is, however, rather steep, and at the very top there is a thick layer of peat, which overhangs the rock a little. On surmounting this on the west side the visitor finds himself in a nearly flat triangular space, perhaps three or four acres in extent, bounded on the north by a very steep rocky slope, and on the other sides by steep but not difficult grass slopes. To the north-east he sees the chief summit about a quarter of a mile distant and nearly fifty feet higher, while connecting the two is a narrow ridge or saddle-back, which descends about a hundred feet in a regular curve, and then rises again, giving an easy access to the higher peak. The top of this ridge is only a foot or two wide and very steep on the northern slope, but the southern slope is less precipitous, and about a hundred yards down it there is a small spring where the visitor can get deliciously cold and pure water. The north-eastern summit is also triangular, a little larger than the other, and bounded by a very dangerous precipice on the side towards Brecon, where there is a nearly vertical slope of craggy rock for three or four hundred feet and a very steep rocky slope for a thousand, so that a fall is almost certainly fatal, and several such accidents have occurred, especially when parties of young men from Brecon make a holiday picnic to the summit.
What strikes the observant eye as especially interesting is the circumstance that these two triangular patches, forming the culminating points of South Wales, both slope to the south-west, and by stooping down on either of them, and looking towards the other, we find that their surfaces correspond so closely in direction and amount of slope, that they impress one at once as being really portions of one continuous mountain summit. This becomes more certain when we look at the whole mountain mass, of which they form a part, known as the "Fforest Fawr," or great forest of Brecknock. This extends about twenty miles from east to west and ten 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 Sandstone 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 débris due to which is carried away by the brooks and rivers. The geologist looks upon the rounded summits of these mountains 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 inspection two little triangular patches of the original peatcovered surface joined together by the narrow saddle, as
shown in the sketches opposite, showing a plan of the summits and a section through them to explain how accurately the two coincide in their slope with that of the original plateau. Every year the frost loosens the rock on the northern precipices, every heavy rain washes down earth from the ridge, while the gentler showers and mists penetrate the soil to the rock surface, which they slowly decompose. Thus, year by year, the flat portion of the summits becomes smaller, and a few thousand years will probably suffice to eat them away altogether, and leave rocky peaks more like that of Snowdon. The formation, as we now find it, is, in my experience, unique —that is, a mountain-top presenting two small patches of almost level ground, evidently being the last remnant of the great rolling plateau, out of which the whole range has been excavated. Double-headed mountains are by no means uncommon, but they are usually peaked or irregular, and carved out of inclined or twisted strata. The peculiarity of the Beacons consists in the strata being nearly horizontal and undisturbed, while the rock formation is not such as usually to break away into vertical precipices. The original surface must have had a very easy slope, while there were no meteorological conditions leading to great inequalities of weathering. The thick covering of peat has also aided in the result by preserving the original surface from being scored into gullies, and thus more rapidly denuded.
After we had completed most of our work at Trallong we had to go further up the valley to Devynock. This is an enormous parish of more than twenty thousand acres, divided into four townships or chapelries, the two eastern of which, Maescar and Senni, we had to survey. In these mountain districts, however, we only surveyed those small portions where the new roads or new enclosures had been made, the older maps being accepted as sufficiently accurate for the large unenclosed areas of mountain land. We first went to Senni Bridge, where both districts terminate in the Usk valley; but after a short time I went to stay in a little publichouse at Senni in the midst of my work, while my brother