In the following descriptions the grid references have all been checked by site investigation. In many instances the descriptions themselves are based on personal exploration. However, in those cases where it has not been possible to make a personal visit, the descriptions have been taken from the literature. The length and vertical range of all caves is quoted where it is known.
To help in the assessment of the speleological significance of the sites they have been classified as:
Unconformity Caves (U)
The site reference numbers at the head of each description relate to the numbering used on the general area map of the Mellte Valley.
The literature reference numbers at the bottom of descriptions relate to the bibliography which will form Part 6 of this study.
The inclusion of any site in this listing does not imply that a right of access to it exists. Indeed, in some instances it almost certainly does not and such access, if sought, would almost certainly be denied.
27) MORGAN'S HOLE (S,O) Alt. Name Mellte Top Sink, Ogof Mellte Uchaf (Upper Mellte Cave)
N.G.R. SN 9340 1337
A small hole in the eastern bank of the Mellte just above the stream. The entrance is covered by stone slabs and leads to a short silted stream passage blocked by boulders.
The main stream can be seen to sink both into its bed and into the bank immediately below the entrance. It reappears briefly inside the cave before disappearing into a constricted rift close to the entrance. Water also sinks into the river bed at other locations immediately upstream of this cave. The water has been traced to Porth-yr-Ogof.
References: 3, 12, 27 (inc. survey), 35
28) CHURCH SINK (S) Alt. Name Mellte Main Sink
N.G.R. SN 9314 1331
This is the main sink from the Afon Mellte and engulfs its entire flow in dry weather. It lies just to the north of the Penderyn-Ystradfellte Road in the east bank of the stream where the water sinks at various points around a bluff of exposed rock which protrudes into the river bed. The fissures into which the water sinks are usually blocked by boulders and organic debris. However Croydon CC have dug here on three occasions during dry weather and entered short lengths of tight bedding cave and phreatic-type tubes. Progress stopped where the water dropped into boulder filled vertical rifts below the river bed.
The water from Church Sink reappears in the upstream sumps at Porth-yr-Ogof about ½km away.
In wet weather the stream backs up at Church Sink very rapidly, demonstrating the limited capacity of the entrance passage at the sink. The river may then flow in the surface river bed as far as Porth-yr-Ogof or it may sink in a series of ill defined points in the stream bed between Church Sink and Porth-yr-Ogof depending on the volume of water in it.
References: 2, 4, 5, 12, 13, 14
29) Dig (O)
N.G.R. SN 9294 1268
A boulder ruckle at the front of the old river terrace cliff upstream of Porth-yr-ogof shows signs of acting as a sink/resurgence in flood conditions. Scalloping can be seen on some of the rocks indicating water flow in the past. Digging here might 9ive access to passages upstream of the Top Entrance sumps in Porth-yr-Ogof.
The site is possibly a disused river sink. At least two other similar sites can be found along the length of the old river cliff (although these others look less promising).
30) PORTH-YR-OGOF (S,R,O) Alt. Name White Horse Cave, Cwm Porth Cavern
L > 2220m VR = 6m (Main Entrance to Resurgence)
N.G.R. of the eight principle entrance areas.
a) Main Entrance SN 9281 1241
b) Resurgence SN 9272 1221
c) Top Entrance SN 9288 1253
d) Cwmbran Entrance & Dig SN 9285 1250
e) Tradesmans Entrance SN 9284 1245
f) Maze Entrances SN 9278 1242
g) Right Hand Series Entrances SN 9278 1237
h) Downstream Entrances SN 9274 1227
This is the principal cave system in the Mellte Valley being p art of the main underground conduit of the Afon Mellte itself. Details relating to the formation and hydrology of the cave were discussed in Part 2 of this study in Pelobates 52.
No fewer than fifteen entrances exist to the system which were coded A-N by UBSS in the most comprehensive study of this cave çRef.13). To simplify the matter a little however the individual entrances can conveniently be grouped into eight locations as follows:-
a) Main Entrance (UBSS E)
This is a huge natural arch 17 1/2 metres wide and 5 metres high at the base of a cliff into which the main river bed leads. It is only in wet weather, however, that the main river flow sinks here; the only water entering the system in normal conditions being that which emanates from Porth-yr-Ogof Rising just upstream of the entrance. This entrance gives direct access to an impressive through system about 270m long. This starts as a broad, boulder floored cavern which leads to the main canal, a deep water filled, passage 80m long. This leads in turn to the main bedding cave which has a boulder floor and a low roof which spans as much as 30m. This part of the cave becomes intermittently strewn with tree trunks and other debris which has been washed into the cave during flood conditions. A major inlet joins the main stream just upstream of the main bedding cave.
b) Resurgence (UBSS K, L, M, N)
After the bedding cave the main stream flows into a deep pool just inside the downstream exit from the cave. The stream issues over a small waterfall before resuming its normal surface course. In normal conditions a small air space exists above the water allowing a "swim through" to the outside. The exit pool is about 7m deep and contains undercurrents which have contributed to several accidents at this location. Above the resurgence proper a low bedding plane can be entered giving access to a rift which runs parallel to the main stream. This can be followed back to rejoin the main bedding cave above the exit pool.
c) Top Entrance (UBSS A)
A small cave in the east river bank leads quickly to a sump which has been dived to a series of upstream sumps, bedding developments and submerged potholes. Progress upstream towards Church Sink is not yet exhausted but at present this is divers territory only.
d) Cwmbran Entrance & Dig (UBSS B)
An inconspicuous wet entrance at the base of the cliff in the east bank of the river leads to a sump which connects upstream to Top Entrance and downstream to Tradesman's Entrance. It operates as a sink when the river level is high. On the bank above this entrance was another small, unstable cave which collapsed on 4/7/80 causing a serious accident.
e) Tradesman's Entrance (UBSS C)
This name has been applied to several entrances to Porth-yr-Ogof in the past but seems to have stuck most firmly to this one which is also known as Entrance 1. It is located at the foot of an impressive cliff to the east of the Main Entrance where a moderately lar9e entrance gives access to the underground stream as it emerges from the upstream sumps. This feature has apparently been formed by the partial unroofing of the stream passage by the progressive collapse of the sides of the main river gorge. This alcove in the cliff may also at one time have been the main sink.
Downstream the tunnel can be followed in fast flowing water to a sump just before its junction with the main cave a short distance inside the main entrance. A series of bedding developments and sand chambers interconnect between this streamway and the main cave.
f) Maze Entrances (UBSS Dl & D2)
In the steep cliff at the west side of the main entrance, a number of enlarged joints give access to an extensive three dimensional maze. The lowest level of the maze consists of a complex of sandy passages with percolation water entering from the west side of the valley.
g) Right Hand Series Entrances (UBSS F, G & H)
A series of avens have breached the bottom of the dry river bed above the main cave. They can be descended to enter the lower reaches of the maze and a large dry oxbow which is a bypass to the main stream canal. One entrance is free clirnable, the other two each require 15m of ladder.
h) Downstream Entrances (UBSS I & J)
These are also sometimes known as Tradesman's Entrance (see (e) above). They are two large collapse features formed along the line of a fault in the dry river bed above the cave giving direct access to the main bedding cave.
References: 2, 8, 11, 12, 13 (inc. survey), 14, 26 (inc. survey), 27, 30 (inc. part survey)
31) Cave (O)
N.G.R. SN 9287 1250
A small bedding cave in the cliff on the east bank of the Mellte midway between the Top and Cwmbran entrances to Porth-yr-ogof. There is a timbered dig at the back of the cave amongst dangerous looking boulders
32) PORTH-YR-OGOF RISING (R)
N.G.R. SN 9278 1247
A strong perenial spring at the foot of the river cliff, a short distance upstream of Porth-yr-Ogof main entrance, on the west bank. Water emerges from clay and boulders in about three well defined places depending on the season.
There does not appear to be an y obvious geological reason for its occurrence at this point. Stratford (Ref.l4) suggests that some of the water originates at Hole by the Wall although this has not been confirmed by subsequent tests.
References: 12, 13, 14, 35
33) Rising (R)
N.G.R. SN 9266 1219
A strong spring emerges from boulder clay below tree roots high on the river bank immediately downstream of Porth-yr-Ogof. The source of water is not known but the bank below the spring is encrusted with tuffa indicating that the water originates in the limestone.
About three other similar, but less persistent springs can be found in the same bank of the river further downstream towards Ogof Glan Mellte.
34) Rift (O)
N.G.R. SN 9271 1215
A large rift in the limestone pavement immediately downstream of the Porth-yr-ogof resurgence. It is about 2½m deep but choked with rocks and rubbish. It is an example of joint enlargement similar to those seen in the upper parts of Porth-yr-ogof and probably of little importance.
35) OGOF GLAN MELLTE (R)
N.G.R. SN 9266 1168
L = 50m VR = negligible
A small poorly developed resurgence cave in the east bank of the Mellte just downstream of the start of the gorge below Porth-yr-Ogof. The entrance is about ½m above stream level amongst fallen boulders in the side of the gorge. It leads to a single contorted passage which varies in cross-section from a narrow rift 3m high to a small phreatic tube with a vadose trench in the floor which carries a small stream. It ends in a mud and calcite choke where water enters from the floor and also from the bedding on both sides of the passage. The presence of false flooring at the end of the cave and small stalactites in the passage indicate at least one period of rejuvenation has occurred.
The water source is almost entirely percolation water; the cave having formed as a phreatic tube before the At on Mellte cut down below its level thus allowing vadose development.
References: 6, 9, 12, 14, 18 (inc survey), 20
36) Spring (R)
N.G.R. SN 9261 1158
A spring emerges from several points in the west bank of the Mellte just upstream of the point where Nant y Carad joins it. This site lies on a fault line.