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Upper Flood Geology

Following the breakthrough in Upper Flood Swallet on 10/09/06, Andy Farrant made this prediction on 18/09/06:

I gather on the grapevine that Upper Flood has gone! Congratulations...!!

Before I get to see the place and the survey, I thought I'd offer you my predictions on what is likely to be found, and see whether they will be bourne out by exploration - it'll be an interesting exercise either way. See what you think....

Certainly the Charterhouse area has huge potential, probably more so than any other sink on Mendip, mainly because of the low dip of the limestone and the amount of water sinking in the area, plus the length of time speleogenesis has been taking place in the area.

geology - click to enlargeBeyond the old limit, (Golden Chamber) the cave should be a fairly straightforward vadose canyon, trending probably north-south or NNW-SSE along major joints, but with some dog-legs along strike oriented passage segments, (in a similar manner to parts of the GB Mud passage route). Locally it may even trend ESE along strike near the entrance. I would expect a considerable number of tributary inlets, both active and relict, including tribuarties from Waterwheel and Grebe. Whether Upper Flood is the main drain, or is a tributary into a main drain, I wouldn't like to say, but it is certainly in the right position to have been the original sink, but I suspect the stream has sunk at a number of places over the years, and you are more than likely to get a series of active and relict inlet passages, all initially at relatively shallow depth and all fairly gently descending (compared to GB or St Cuthberts) converging on a larger streamway. GB for example has had numerous inlet points during its history. There may well be some sections where you loose the stream into small oxbows (like Barnes loop and the bit before sump 1 in Swildons)

The cave should descend quite gently at first, certainly the phreatic tube at roof level will probably follow the dip, but you may find that you get pretty deep vadose canyon, with few if any true pitches and a series of minor cascades. If like the old part of Upper Flood, the passage meanders round along strike for part of its route, then the local graident will be very shallow. The overall gradient should be less than Swildons or GB, perhaps more like Salubrious in OFD. However, further south, the dip increases and you may start to get some steeper, more typically Mendip passages. Like Swildons 1, the only pitches you may get will probably due to stal or bolder blockages/collapses of the passages, or where inlets enter the cave from a higher level. I wouldn't envisage any deep pitches like Thrupe or Rhino. Having said that there is a possibility that the streamway may hit a major N-S fault/mineral vein cavity and descend to the water table rapidly, but I suspect (hope) not.

I would not be surprised if some of the passage followed some of the 'neptunean dykes' or sediment filled fissures known to exist in the area. These generally trend N_S or NNW_SSE and are infilled with Triassic or Lower Jurassic sediment. Some were identified in Grebe Swallet, and are also associated with mineralisation. I suspect that by the time you get under the Ubley warren mineral rakes, you may well be at a reasonable depth (60-70 m?) and thus avoid much of the near surface mining, which worked the near surface residual ore bodies. As you know from Willie's work, the lead veins become very thin and patchy with depth, but these veins may well still control passage orentation, giving riser to some straighter sections with much calcite mineralisation and possibly collapse. However, you may have problems with inwashed lead tailings, and it is entirely possible that some of the neptunean dykes may have been washed out and provide routes back up towards the surface.

Once you get to an elevation of c. 138-140 OD ( 100 m below surface), you may well find a relict series of high-level fossil passages trending off, like Ladder Dig in GB. Similar relict high level passages may also occur at 120 m OD (120 m depth) and possibly 100 m OD. These again may well be strike and or joint controlled, but may well be partially/totally choked. In GB there is also a former water-table level (associated with a relict series of phreatic passages) at 238 m OD, but I think Upper Flood is too young (and low) for this to be developed. These relic passages are essentially independant fossil phreatic conduits leading towards Gough's, but with associated drawdown passages and links back to the streamway.

I wouldn't expect a true water-table sump until you get to a depth of c. 80 m OD (160 m depth) or thereabouts, which could make Upper Flood one of the deepest caves on Mendip. This coupled with the low dip around Charterhouse, makes a long vadose section of cave a possibility (long compared to say GB, rather than Draenen!!!!). If we assume a gradient of around 10 degrees, then we are likely to hit the water table at about a kilometre south or south-south-east of the entrance. It will be at this point that the streamway is likely to turn phreatic with many N_S orientated segments along phreatic fault guided rifts like Reservoir Hole or deep dip aligned phreatic loops such as in Gough's sump 2 and 3.

Its further south and at depth where things may get interesting. I would expect the (phreatic) streamway to continue on a general north-south or slightly SE trend (with some E-W strike oriented segments) to a point about 1.5-2 km south of the entrance, ie the towards the core of the Cheddar Syncline, ie around King Down Farm/Yoxter. It is here the streamway may start trending west along strike towards Cheddar. Any phreatic conduits aligned along strike (E_W) are likely to be relatively shallow. From here the streamway may well pick up the major conduit from Tor Hole/Wigmore and Lodmore, flowing along strike around the northern side of the North Hill pericline. The infilled Triassic wadi which stretches from Longwood to East Harptree may cause complications but I suspect you may be too deep to see the Trias by this stage. I also suspect the conduit will be phreatic at this point and only accesible by diving. From here the conduit will trend along strike to the west or southwest, along strike along the northern limb of the Cheddar syncline, interspered by N_S rifts along the major joints/faults.

Attached is a word document giving a very rough indication of the likely form of a drainage network leading to Goughs. Only the main conduits are shown, and the diag does not predict the actual location of the conduits, but more their general dispoition - view it like a London underground map, rather than an accurate survey!!

I'd love to come and see the place, but I'll wait until exploration, taping and surveying (as well as squeeze enlarging and boulder stabilisation) is complete, before doing the geomorph of the place.

Plenty of potential whatever!

[See also Mendip geology on the British Geological Survey website]

Upper Flood Swallet - Geology

Mark Tringham, Grampian Speleolgical Society

Preliminary Geological Observations in Upper Flood


A single trip was made into upper Flood on the 8th December 2007 in support of the surveying and digging efforts by MCG. Due to the length and of the cave and duration of the trip, the emphasis was put onto seeing as much of the cave as possible and scoping out what could usefully be done in the future on a more systematic basis and with greater accuracy. It is anticipated that two more long trips should be sufficient to make a more scientifically robust geological analysis of the cave as known so far, fit for inclusion on future publications on the cave exploration, morphology and its origin. The cave is especially interesting due to its intimate relation to mineral veins and nearby old mine workings. A geological analysis of the cave when considered alongside published surface geology should assist in predicting where future digging efforts might best be concentrated. The geology would be better described using photographs, cross-sections and maps, however the narrative below will have to suffice until further work is done.

Preliminary Observations:

Entrance Passageways as far as Midnight Chamber.

These are mostly aligned along mineral veins with calcite and grey clay infill evident. The veins are commonly obscured by stal deposits. Black and rusty brown markings on some stal was thought by the explorers to be due to rust or oil from a dumped car overlaying the cave, however my view is that these are more likely a natural product from iron and manganese mineralization.

Bypass Passage as far as Lavatory Trap.

This passage is aligned along a fault . The fault plane surface is visible cutting across the roof of the passage with slickenside markings. The fault plane is more gently inclined than the passage itself and hades (dips) approximately south at about 40 degrees.

Red Room and Passageway immediately below.

This is formed along a major fault with abundant iron mineralization. The limestones on the south side of the fault below Red Room show significant folding with 'reverse drag'. The thin-bedded shaley limestones steepen up from about 10 degree notherly dip to about 75 degrees southerly dip within a few meters.

Departure Lounge and 550m Way.

This passage generally shows the streamway to be heading down-dip with clean thicker-bedded limestones initially until after a fault is crossed (Threadneedle St ?) , where more shaley limestones are encountered again.

Passage Below Netherwood Inlet.

This follows a major fault zone with a high rift passage inclined at about 75 degrees along the main fault plane. East Passage follows a series of fault aligned rifts with much calcite veining, commonly with en-echelon arrays.

En-echelon arrays are formed by shear failure of the host rock, the failure angle generally being at about 30 degrees to the maximum principal stress. They commonly form conjugate sets with opposing sense of shear. The veins themselves form due to secondary extension within the shear zone. Other well-known en-echelon vein sets occur in OFD at Marble Showers and also in the Carboniferous Limestones on Clifton Down in Bristol, where the childrens slide has polished the bedding surface very well, enabling the veins to be seen clearly.

Royal Icing Grotto.

What looks superficially like a sedimentary conglomerate is in fact cobble sized limestone clasts within a mineral vein. The wall of the grotto is partly aligned parallel to the vein. The clasts have a rind of weathering alteration and are enclosed in greyish-white calcite or other white carbonate mineral.

West Passage.

This passage follows a series of veins and joints . The choke at the far end is formed at the intersection of a N-S oriented vein with the others that are approximately E-W. The N-S vein has abundant iron and manganese mineralization.

Mark Tringham 07/02/08

Upper Flood Swallet - Dowsing

Dr John D. Wilcock has carried out dowsing exercises in 1987 and 2008 in the Blackmoor valley. To view his results, click here

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Page last updated 21 June 2008

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Mendip Caving Group. UK Charity Number 270088. The object of the Group is, for the benefit of the public, the furtherance of all aspects of the exploration, scientific study and conservation of caves and related features. Membership shall be open to anyone over the age of 18 years with an interest in the objects of the Group.