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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
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.