250 MY LIFE [Chap. 



Within a mile of Pont-nedd-fychan is the Dinas rock, 

 a tongue of mountain limestone jutting out across the mill- 

 stone 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 gate- 

 way 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. 



