This section is based upon the survey of Crescent Cave, the British Geological Survey map of the area, Coal Board borehole data and W. Gascoine's geological diagram of the Trefil area in the book 'Limestones And Caves Of Wales' © Cambridge University Press 1989.
This text has been reviewed by Andrew Farrant.
The Trefil Moors under which Crescent Cave is situated consist mainly of a millstone grit cap - comprised of coarse sandstones and conglomerate - over limestone. The maximum thickness of the limestone is about 140 m although it is likely that the caves under the Trefil Las Moor (the one containing Crescent Cave) will never reach the maximum thickness due to the existence of the Llanelly Formation (shales) at about 70 m below the top of the Dowlais (upper) Limestone. Below the Llanelly Formation lies the Abercriban Oolite. The faults that show up in the area have a general directional trend of 150°. The general dip of the rock is between 5° South near Crescent Cave and 3° South Southeast near the resurgence.
The first 12 m depth of Crescent cave are in millstone grit and the remaining 69 m are in limestone. Therefore the cave has probably reached its limit of depth in the local area, although this seems certain to change should a major horizontal system be encountered. See paragraph on possibility of passing the shale band below.
The altitude of the resurgence of Crescent cave is 355 m (Ffynnon Sion Sheffrey - dye trace 48 hrs). This is about 185 m below the entrance to Crescent Cave giving a maximum depth for the cave of 185 m plus whatever may be below the water table.
The faulting shows up very clearly in the directional nature of Crescent Cave with many passages being stacked above each other. The fault does not, however, show up on the surface. It is likely that many other feeders to the main system exist along the line of Crescent Cave (about 150° from the entrance) including the nearby Chartists Cave. Chartists Cave is of a similar length to Crescent Cave although it has never displayed the vertical characteristics of Crescent. It does however give an idea of the complexities that could be expected in the main system.
From the appearance of the end of the streamway in Crescent Cave, it seems that any extension will remain small until the major system is encountered. Any major system encountered is likely to be gently dipping and will probably be running at the bottom of the Dowlais Limestone. Inlets are likely to be of a similar nature to Crescent Cave, being steeply dipping and due to extensive faulting, very much linear. It is also possible that many fossil passages exist from older, perhaps glacial, systems.
Relatively few systems have managed to pass through the shale band. Those that I can remember are: Pwll Coedog - dye trace to Ogof Capel, Pwll Y Pasg - dye trace to Agen Allwedd and Llangatwg Swallet - dye trace to Daren Cilau. There is a small possibility of extra depth being gained in the close vicinity of the currently known Crescent Cave, that being the existence of the strong faulting allowing the cave to pass through the shale band. As the fault is likely to exist in the beds beneath the Dowlais Limestone, it is possible that a route has been opened through the Llanelly Formation into the Abercriban Oolite below. Should this have happened, it would be easy for the water to wash away the shale to open up the route. This would mean that any greater depth gained in the local area would be very unstable initially but may reach systems similar to the nearby Agen Allwedd. This is unfortunately unlikely as unlike the systems mentioned earlier where the resurgence is in the lower Abercriban Oolite, the resurgence for Crescent Cave is in the millstone grit above the Dowlais Limestone. If the Llanelly Formation had been passed, it would require passing again to return to the upper beds and also would give less open streamway so a longer dye trace time.
There is also a major fault line running down the Trefil Valley, parallel to the fault within Crescent Cave. This, it would seem, collects water from the moors on either side and resurges it once the limestone begins to dip significantly under the millstone grit. There is little evidence of cross faulting so quite where the water leaves the fault in which it is currently running within Crescent Cave, and passes to the fault under the Trefil Valley, remains to be seen. It is most probable that the passages connecting the faults are phreatic and running in bedding planes rather than rifts or faults. This allows for about 2-3 km of stream passage before the final sump, assuming that the system continues above the Llanelly Formation. It is most probable that the system will not spend too much time underwater as the dye trace was only 48 hours, showing a vadose streamway for the most part. This is also supported by the other dye traces done in the area: Ogof Ap Robert and sinks on both sides of the Trefil Valley all taking 48 hours.
All of this section is based on speculation from the locations of passages and sinks with respect to each other, and not from dye tracing (except the trace to the rising).
The stream that enters the cave at the entrance sinks into the floor of the first - Crescent - chamber. This re-enters the known cave at the first pitch and the choke above the Z-Bend. This sinks into the floor in various places between the first and second pitches, to re-enter at the aven in The Lower Streamways Part 1. From here it flows into The Lower Streamways Part 2.
The sink to the South of the entrance to Crescent Cave lies between the passage up the aven before the Second Pitch and the choke at the furthest reach of the Highest Level. The water sinks down to both of these and flows almost immediately out again to re-enter in the inlet rift in the Highest Level. This flows out through the blind pitch in the floor to enter The Main Level - Right at the inlet. This then flows through the first part of Motorway Passage to pass to The Lower Streamways Part 2 at the aven by the 4 m climb.
Both streams now combine to flow out of the end of The Lower Streamways Part 2 and flow to Ffynnon Sion Sheffrey.
It would appear that the history of flows within the cave would be that in its earlier stages several drainage routes existed where short drops connected different levels in which the water flowed from end to end of the cave. Some of the level sections were phreatic, e.g. The Main Level - Right. Towards the present day, newer routes were opened up allowing the water to drop more quickly without having to flow from end to end first. Once the water reached the Llanelly Formation, the cave became horizontal which is how it remains now.