St Cuthbert's Swallet - young or old?

As you may be aware from the article I wrote in the BB last year, myself and Pete Smart have been working on a project, one of whose aims was to estimate how old the Mendip landscape is.  Studying cave deposits provides one of the best methods of obtaining a handle on the age of the surface topography, as caves are protected from the effects of erosion and weathering, can be related to surface topography and water-tables, and contain datable material in the form of stalagmite.  Dating stalagmites from caves in the Wookey Hole catchment allowed us to estimate the age of these caves, and to determine whether caves such as Swildons Hole and St Cuthbert's originally drained to Cheddar or Wookey. This project hoped to resolve a long running debate which Derek Ford initiated back in the 1960's after his ground-breaking studies in the major swallet caves.  He originally suggested that Swildons Hole drained to Cheddar. However, dye tracing of Swildon's to Wookey by the MKHP, seemed to rule this hypothesis out.  However, as I explained in the previous B.B. article, some of the high level passages in Swildons Hole may have originally drained to Cheddar, with the stream subsequently being captured to Wookey. In St Cuthbert's, the lack of distinct groupings of high level abandoned passages at certain elevations suggested that there was no former drainage to Cheddar, and that the cave has always drained to Wookey.  The only way to potentially resolve this debate was to actually try estimate the age of both Swildons Hole & St Cuthbert's by dating the stalagmites within it, as they provide a minimum age for the cave.

In order to do this, a careful sampling program was undertaken with the help of Estelle Sandford and Chris Castle.  Most of the samples were obtained in Pillar Chamber and the Rabbit Warren area, from suitable un-anaesthetic flowstone.  These were dated using Uranium series dating methods at Bristol.  The sample locations and ages obtained are shown in Fig. 1.

Implications

As is shown in Fig.1, most of the ages are relatively young (geologically speaking!) nearly all are less than 120,000 years old, which is the time of the last interglacial.  By comparison, the oldest stalagmites in G.B. are over 780,000 years old.  The bulk of the dates occupy (within 2 sigma error) the Middle Devensian interstadial, a warmer interlude during the last glaciation, between 25 and 60 ka (ka = 1000 years).  No particularly old stalagmites were found, even though a concerted effort was made to only sample those stalagmites which appeared from their location or relation to gravels deposits to be the oldest.  This suggests that St Cuthbert's is relatively young when compared to other swallet caves such as G.B. Cave.

Also of note is the evidence of an almost total sediment fill in St. Cuthbert's.  A broken stalagmite within the gravel fill gave a date of 41 ka, thus implying the gravel is younger.  It appears that much of the cave was infilled with gravel during the last (Late Devensian) glaciation.  Thus, much of the early phreatic network was modified following gravel influx, under paragenetic conditions.  Paragenetic development occurs when sediment deposition within a phreatic passage smothers the passage floor, preventing dissolution by the stream. Instead, dissolution only occurs on the passage roof, thus the passage enlarges upwards, forming a phreatic rift or half-tube.  Alternatively, if other routes are available for the water, perhaps along suitable bedding planes, an interconnected maze of passages with phreatic half­tubes in the roof will result.  This is exactly the case in the Rabbit Warren, where there is still evidence of a total sediment fill, such as in the Railway Tunnel.

From this, I would suggest that St Cuthbert's is a relatively recent cave system, which has always drained to Wookey Hole.  Its large size and big chambers result from several factors.  The first is the large catchment, which may have been substantially larger in the past.  Secondly, the cave is developed on a plunging anticline which has open joints suitable for cave development along its axis.  Thirdly, this anticline has been fractured by at least three thrust faults (one can be clearly seen in the Rocky Boulder Series - ask Chris Castle or Dave Irwin!) as well as the major Gour Hall Fault, allowing rapid penetration of water and cave development.  In addition, the stream has sunk in several places around the depression, thus forming a complex of vadose shafts (which can develop very rapidly, c.f. Thrupe Lane) and phreatic tubes at depth.  These routes were then subsequently modified, enlarged and choked during the Late Devensian under paragenetic conditions.  Finally, enlargement of this passage network has caused extensive collapse, aided by the numerous fractures and bedding planes.

All these factors helped create a large cave system in a relatively short space of time.  Swildons on the other hand appears to have some old stalagmites (>350 ka), although a much larger sample collection is needed before we can reliably demonstrate that Swildons is indeed older.  The lack of suitable datable material in the upper levels such as S.E. Inlets doesn't help.

It is at Wookey Hole where the most interesting results were obtained.  Here, a comprehensive suite of samples was collected from all the dry parts of the cave, plus some from Wookey 21, thanks to Keith Savory. Uranium series dating has revealed that the upper parts of the high level passages above Chamber Nine are older than 350 ka.  By calculating the average rate of base-level lowering at the resurgence (12 cm per 1000 years) from U-series dates of stalagmites, the age of each passage can be crudely estimated.  The earliest passages (above Ch. 9) had developed at 'around 350-400 ka, but these passages are relatively small and cannot have drained a large area.  It is possible this was the resurgence for the early St Cuthbert's system.  It wasn't until much later (c. 175-200 ka) that the main show cave passages developed, possibly following the capture of the Swildons Hole water.  The modern river passages have developed subsequently, and are in the process of being captured to Glencot Spring today.

The data suggests that St Cuthbert's is probably too young to have drained to Cheddar, and the water has always resurged at Wookey.  The upper levels in Swildons may yet prove older than Wookey, but the lack of datable material in this key region prevents us being able to draw any definite conclusions. I hope I have shed a little light on the age and origins of the St. Cuthbert's and Wookey systems.  Some of this may be a little contentious, but hopefully it'll stimulate a bit of debate!  Although this dating project suggests some ideas, it by no means proves that Swildons Hole originally drained to Cheddar.  To prove it right or wrong, we've got to find the passage - get digging!

Acknowledgements

Thanks to Chris Castle and Estelle Sandford for showing me around St. Cuthbert's on often slow and tedious sampling trips; the BEC committee, Wookey Hole Show Caves and English Nature for permission to sample, and to Keith Savory for obtaining the Wookey 21 samples.

FIG.1 Uranium series ages for selected samples from the Priddy and Wookey caves

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