250 MY LIFE 



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 accom- 

 panying 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 convul- 

 sion of nature — a favourite expression of the old geologists. 



I have already (in chap, xi.) described one of the curious 

 " standing stones " near the source of the Llia river, but there 

 is a still more interesting example about a mile and a half 

 north-west of Ystrad-fellte, where the old Roman road — the 

 Saru Helen — crosses over the ridge between the Nedd and 

 the Llia valleys. This is a tall, narrow stone, roughly quadri- 

 lateral, on one of the faces of which there is a rudely inscribed 



