Symud i'r prif gynnwys

scientific importance for the excellent facilities it provides for the investi- gation of cave morphology, hydrology and ventilation. The total length of passages discovered to date is more than 4 J miles. There seems little doubt that most of the cave is phreatic in origin and that vadose modification has been on a limited scale. Ogof Ffynnon Ddu is developed in the Middle Dibunophyllum sub-zone (D2) in which the beds dip southward at an average inclination of 15 degrees. It has been recorded that in one mile of dry passageways practically all the joints are tight and have not provided means of access for water from above. Where a joint has been eroded this has almost invariably been from below rather than above. On the other hand, there is almost a complete agreement of roof slopes with bedding planes whilst in nearly all the passages there is a close conformity of these with the floors, the only exception being in stretches where the latter are obscured by deposit. There is consistent evidence from all parts of the system for a former complete filling with clay and silt, a fact which lends support to the evidence for its phreatic origin. In age it pre-dates the late Glacial diversion of the Byfre Fechan stream and is probably of pre-Glacial origin. Porth yr Ogof, or the White Horse Cave, is the best known of the caves in the tributary valleys of the river Neath. It contains one enormous low chamber with a roof extent of about 6,000 square feet. A feature of this area is that several of the lesser known and smaller cave systems are developed in the topmost beds of the Carboniferous Limestone (here the Middle Dibunophyllum or D2 sub-zone) so that the roofs may be very near or even partly consist of the overlying quartzitic sandstones or quartz conglomerates of the Basal Grit. The series of caves grouped under the heading of Cwm Pwll y Rhyd are those occurring near a limited area of the Little Neath river bed at and below the point where this stream begins its subterranean course. Caves here include the Deep Pit Cave, the Railway Tunnel, the White Lady Cave and the Town Drain, which can be followed throught a series of passages for a distance of some 500 yards. Located on the moorland to the west of the Little Neath valley is Pwll Pant Mawr, a 6 5 -foot pothole which is the nearest approach in South Wales to the well-known potholes of the West Riding of Yorkshire. This pothole opens out into a fine chamber from which a passage leads north and south in the latter direction the large opening can be followed for a few hundred yards before being blocked by falls of rock. Recent explorations have found a way beyond these and are proceeding in a southerly direction. East of the Hepste valley, cave systems discovered to date are on a more restricted scale. They include Ogof Fawr which receive the waters of Nant Cadlan in Pant Sychbant, two miles east of Penderyn Ogof y Ci and Ogof Rhyd Sych in the Nant Glais valley, Pontsarn; and a more