Geology and Geomorphology

Dr. A R Farrant (British Geological Survey)

Ogof Draenen is one of the longest caves in Britain with a surveyed length in excess of 70 km and is at the time of writing one of the top twenty longest caves in the world. If gypsum maze caves and hypogenic caves (sulphuric acid dissolution caves such as Lechuguilla and Jewel Cave) are excluded, Ogof Draenen jumps up into the top ten longest cave systems. Clearly, this discovery has provided an excellent opportunity to understand not only how cave development has occurred, but also how the landscape and cave evolved through time. To gain insights into the past history of the cave, a detailed geomorphological and geological survey was of the cave was begun. This was helped immeasurably by the availability of a comprehensive detailed survey produced by members of the Chelsea Spelaeological Society (A. Millet and J. Stevens) and numerous discussions with many of the cavers involved in the discovery of this amazing system.

This account is essentially a summary of how the cave developed, based on the current known extent of the system at the turn of the Millennium, with a brief introduction to the geology of the area.

Geology of the Pwll Ddu area
The northern crop of the South Wales coalfield is dominated by four major rock units. These are in order of age (oldest first): the Old Red Sandstone, the Carboniferous Limestone, the ‘Millstone Grit’ and the Coal Measures. Of these, the Carboniferous Limestone is the most important as this is what the cave is developed in and will be discussed further. For more information and local details, read the British Geological Survey memoir for the area (Barclay, 1989)

The sequence can be divided up into a number of units and subunits (Groups, Formations and Members) which are shown in Table 1. Each different rock unit influences style of cave development and imparts a major control on the appearance and character of a particular passage.

Table 1. Lithostratigraphical classification of the Carboniferous Limestone.

The Carboniferous Limestone succession in the Llangattock-Pwll Ddu area of South Wales was deposited on the north-eastern margins of a shallow sea which deepened gradually towards the south-west. This carbonate ‘ramp’ extended across much of South Wales, but this area lay at or close to the contemporary shoreline. Further south in south-west England deeper water (non-carbonate) rocks were deposited, while to the north and east, no limestone was deposited as the area was a landmass (St George’s Land) at the time. A consequence of this proximity to the shoreline is a marked thinning of the limestone sequence in this area compared to elsewhere in South Wales. The shallow nature of the sea coupled with sea-level fluctuations has caused parts of the limestone sequence to undergo sub-aerial erosion and karstification during Carboniferous times.

Furthermore, uplift and erosion prior to the deposition of the overlying river derived coarse sandstone and conglomerates of the Millstone Grit has removed much of the upper limestone sequence. This is probably due to tectonic movement along the ‘Usk Axis’ to the east; a prominent anticline running north-south through Usk which underwent periodic uplift during Carboniferous times. On the Blorenge, where most of the strata above the Llanelly Formation has been removed, the limestone is less than 100 m thick whereas in the Gower, over a kilometre of carbonate sediment was deposited.

The Lower Limestone Shale Group

The Lower Limestone Shale Group is comprised of two Formations; the Castell Coch Limestone and the Cwmyniscoy Mudstone. The former predominately comprises of sandy oolites (round grains of carbonate material deposited in a high energy environment) and limy mudstones, while the latter consists largely of dark grey silty mudstones. These are exposed on the eastern crop in many places and in the floor of the Afon Llwyd Valley. Underground these rocks are rarely seen; the Cwmyniscoy Mudstone may occur in the Snowball and War of the Worlds area where dark shaley limestones occur. These passages are characterised by much collapse and abundant gypsum. The Pontnewyndd Rising occur in this unit.

The Abercriban Oolite Group

Above this is the Abercriban Oolite Group. This group is marked by a series of massive oolitic limestones and interbedded thin dolomites. The oolites were deposited in high energy shallow water conditions and are often coarse grained cross-bedded rocks. Dolomites are made up of calcium-magnesium carbonate [CaMg(CO3)2] and tend to be less soluble. They are either primary in origin in that they were deposited as dolomites, or are secondary, formed by replacement of limestone with dolomite at a later date by magnesium rich fluids (usually derived from seawater) circulating through the sediment. Further north towards Llangattock and Llangynidir, the oolites become more prevalent; south towards Pontypool dolomites become increasingly important and south of the Blorenge the whole Group becomes dolomitised along the east crop. Here, the finer grained dolomites (the Sychnant Dolomite, Pantydarren Beds and the Coed Ffyddlwn Formation) were probably primary dolomites, the massive oolites were probably dolomitised at a later date. This can be seen in the cave; the northern parts of Draenen are relatively little dolomitised, but further south beyond the Players Tunnel and Agent Blorenge, dolomitisation is pervasive and widespread.

At the base of this group is the Sychnant Dolomite Formation, overlain by the Pwll-y-Cwm Oolite, Pantydarren Beds and the Blaen Onnen Oolite which is less than 10 m thick in this area. Dolomitisation of these units renders identification of these units very difficult, but the Blaen Onnen Oolite probably outcrops at the start of Gilwern Passage near Tea Junction. Much of White Arch Passage, Gilwern Passage and The Score is formed in these dolomitised units. The dolomite forms a very distinctive style of passage with lots of spleen-busting fins, upstanding pendants and knee crunching pocked rock. Most of the lower part of the Abercriban Oolite Group is exposed along the streamway, but the individual units are hard to distinguish owing to the mud, stalagmite and gypsum encrusted walls.

Above the Blaen Onnen Oolite is the Coed Ffyddlwn Formation. The lower parts of this unit are fine grained brown dolomites which often form very square shaped blocky passages with much breakdown, including White Arch Passage. The top of this is marked by a very distinctive horizon; the Daren Ddu Beds (the ‘DDB’s’). It is a very shallow water deposit which often contains shaly material with ripple marks. This unit is well exposed in the cascades in Pitch Bypass between the base of the Scaffolded shaft and the rope climb and forms a very thinly bedded passage cross section with many shelves. It can also be seen in the roof at the base of Lamb and Fox Chamber where some nice ripples an be seen..

The Gilwern Oolite is the most easily recognised unit in this group. It forms a pale grey, almost white rock which is often very well sculpted by the water. It can be seen in the at the base of the scaffolded shaft in the entrance series, in the Indiana Highway-Megadrive area, Waterfall series, above Big Bang pitch and around Circus Maximus in the Dollimore series. It contains many fossils near its base including shark fins spines which look like bits of fossil wood standing proud of the passage wall. These gave Gyracanthus Loop near the entrance its name. In places it has been dolomitised, often along joints to form a sandy honey-coloured brown rock, notably at the end of Raiders Passage and at the northern end of Knees Up Mother Brown (Yes Passage) in the Waterfall series. The decrease in solubility often causes a marked decrease in passage size. The top of the Gilwern oolite has undergone sub-aerial erosion, palaeokarstic solution hollows infilled with Llanelly Formation material is present in some places around the Clydach Gorge.

Llanelly Formation and above

The strata above the Abercriban Oolite Group comprises the Llanelly Formation (after Llanelly Quarry in the Clydach Gorge), the Carn Gaws Sandstone and the Dowlais Limestone. The Llanelly Formation is made up of fine grained intertidal limestones, mottled clays (fossil soil horizons) and oolites. It occurs around Gilwern Hill but is removed by intra-Carboniferous erosion prior to the deposition of the Millstone Grit on the Blorenge and the eastern side of the Afon Llwyd. It occurs in the entrance series of Draenen between the entrance and the scaffolded shaft and elsewhere as odd blocks in boulder chokes. It is also well seen in the quarry below Siambre Ddu; here, the lower part of the face is massive Gilwern Oolite, the upper third is thinly bedded Llanelly Formation. Above lie the sandstones of the Millstone Grit in which Siambre Ddu is formed as a collapse feature. The upper two Formations are completely cut out to the east of the Clydach Gorge and do not occur in the area underlain by Ogof Draenen. The Millstone Grit rests directly on the Llanelly Formation at Pwll Ddu, on the Coed Ffyddlwn Formation near the Foxhunter car park on the Blorenge and on the Cwmyniscoy Mudstone on the Blorenge itself.

Much of the cave is overlain by Millstone Grit and in some places by the Coal Measures. This impermeable cap explains the lack of drip waters in much of the cave and hence the absence of stalagmites. Occasional blocks of Millstone Grit can be seen in some of the larger boulder chokes in Bolder Passage, Upstream series and War of the Worlds indicating significant collapse.

Geological Structure

The geological structure of the area is relatively simple. The rocks dip to the south-west at between 10-28° on the east side of the Afon Llwyd and swing round to a south-south-west direction dipping at 10-12° around Pwll Ddu. Thus many of the north-west/south-east passages in Ogof Draenen are oriented along the strike. The rock is heavily jointed, at least three joint sets are apparent which have a strong influence on cave development. The major set runs at 150°-330° and as it is also co-incident with the strike, it controls the orientation of many of the major passages including Gilwern Passage, the Main Streamway, Megadrive, Bolder Land, Into the Black, War of the Worlds and the Canyons. A second set runs at 300°-120° and controls parts of Raiders Passage, Oo Crawl and Agent Blorenge. The third set runs approximately north-south and can be seen in Big Country and in the Agent Blorenge II-Squirrel Rifts-Wyvern Series area, and in Luck of the Draw. This last set is coincident with the fault orientations in the area. At least one fault has been identified in the cave in the aptly named Fault Chambers. At the southern extremity of the system, the Pontnewydd Rising are situated on the Trevethin fault which has a maximum displacement of over 100 m. This downthrows the Carboniferous Limestone against the Coal Measures in the valley floor and effectively terminates the southerly development of the cave system.

Mineralisation

A notable feature in many parts of the cave is a preponderance of gypsum. This is calcium sulphate (CaSO4) and is formed by the oxidation of iron pyrite (FeS) which occurs as finely disseminated crystals in many of the darker mudstones within the limestone sequence, notably in the Cwmyniscoy mudstone. Oxidation by ground-water of the iron pyrite causes the sulphur to be liberated which then reacts with the carbonate in the host rock to form calcium sulphate. As the gypsum crystal grows from its base (i.e. the pyrite grain) rather like a blade of grass, it forms a elongate crystal ‘flower’ or more rarely a ‘hair’. Gypsum deposits can be seen in abundance in the Snowball area and gypsum hairs occur in the Cantankerous Surveyors Series. The snowball itself may have originated from a very pyrite rich block.

Other minerals occur in Ogof Draenen. Apart from calcite and dolomite, the commonest is barytes which is a relatively common and heavy mineral formed of barium sulphate. It can be easily recognised by its distinctive heavy feel and yellowish brown colour. Veins of it occur around the Fault Chambers, in Big Country, Gilwern Passage and in the Snowball-War of the Worlds area. Many of these veins have been emplaced along small faults or major joints, most commonly along the 150°-330° joint set. In places in War of the Worlds and Big Country, these veins also contain traces of copper ore, notably malachite and azurite. In the Reactor, this copper deposit has caused the blue-green tinge to the south-east wall which is formed along one of these veins.