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and backed by mountains from fifteen hundred to eighteen hundred feet high. The view up this valley is delightful, its sides being varied with a few houses peeping out from the woods, abundance of lateral valleys and ravines, with here and there the glint of falling water, while its generally straight direction affords fine perspective effects, sometimes fading in the distance into a warm yellow haze, at others affording a view of the distant mountain ranges beyond.
At twelve miles from the town we come to the little village of Pont-nedd-fychan (the bridge of the little Neath river), where we enter upon a quite distinct type of scenery, dependent on our passing out of the South Wales coal basin, crossing the hard rock-belt of the millstone grit, succeeded by the picturesque crags of the mountain limestone, and then entering on the extensive formation of the Old Red Sandstone. The river here divides first into two, and a little further on into four branches, each in a deep ravine with wooded slopes or precipices, above which is an undulating hilly and rocky country backed by the range of the great forest of Brecon, with its series of isolated summits or vans, more than two thousand feet high, and culminating in the remarkable twin summits of the Brecknock Beacons, which reach over twenty-nine hundred feet. Within a four-mile walk of Pont-nedd-fychan there are six or eight picturesque waterfalls or cascades, one of the most interesting, named Ysgwd Gladys, being a miniature of Niagara, inasmuch as it falls over an overhanging rock, so that it is easy to walk across behind it. A photograph of this fall is given here. Another, Ysgwd Einon Gam, is much higher, while five miles to the west, near Capel Coelbren, is one of the finest waterfalls in Wales, being surpassed only, so far as I know, by the celebrated falls above Llanrhaiadr in the Berwyn Mountains. From the open moor it drops suddenly about ninety feet into a deep ravine, with vertical precipices wooded at the top all round. In summer the stream is small, but after heavy rains it must be a very fine sight, as it falls unbroken into a deep pool below, and then flows away down a thickly wooded glen to the river Tawe.
Within a mile of Pont-nedd-fychan is the Dinas rock, a tongue of mountain limestone jutting out across the millstone grit, and forming fine precipices, one of which was called the Bwa-maen or bow rock, from its being apparently bent double. Lower down there are also some curious waving lines of apparent stratification, but on a recent examination I am inclined to think that these are really glacial groovings caused by the ice coming down from Hirwain, right against these ravines and precipices, and being thus heaped up and obliged to flow away at right angles to its former course.
But the most remarkable and interesting of the natural phenomena of the upper valley is Porth-yr-Ogof (the gateway of the cavern), where the river Mellte runs for a quarter of a mile underground. The entrance is under a fine arch of limestone rock overhung with trees, as shown in the accompanying photograph. The outlet is more irregular and less lofty, and is also less easily accessible ; but the valley just below has wooded banks, open glades, and fantastic rocks near the cave, forming one of the most charmingly picturesque spots imaginable. It is also very interesting to walk over the underground river along a hollow strewn with masses of rock, and with here and there irregular funnels, where the water can be heard and in one place seen. The whole place is very instructive, as showing us how many of the narrow limestone gorges, bounded by irregular perpendicular rocks with no sign of water-wear, have been formed. Caves abound in all limestone regions, owing to the dissolving power of rain-water penetrating the fissures of the rock, and finding outlets often at a distance of many miles and then gushing forth in a copious spring. Where a range of such caverns lies along an ancient valley, and are not very far below the surface, they in time fall in, and, partially blocking up the drainage, cause the caverns to be filled up and still further enlarged. In time the fallen portion is dissolved and worn away, other portions fall in, and in course of ages an open valley is formed, bounded by precipices with fractured surfaces, and giving the idea of their being rent open by some tremendous convulsion of nature-a favourite expression of the old geologists.