View from Eagles Nest, Chepstow. showing the Wye and in the distance, River Severn.
South Wales has perhaps the most unusual and mysterious water features anywhere in the world.
The Historia Regum Britanniae, translated as 'The History of the Kings of Britain' is a collection of histories dating back to around 828AD, surviving through numerous re-writes up to the 11th century. Written by Geoffrey of Monmouth, it was a work detailing the complete history of Britain that may itself have been based on an even older book. Historia itself begins with the Trojan Aeneas landing in Britain and twelve volumes later the history ends with the death of King Arthur and the resurgence of the Saxons.
In every known version of Historia, there is a list of twenty-six ‘Wonders’ of Britain (Mirabilibus Britanniae). The list of man-made and natural ‘wonders’ is not the same in every version of Historia between the ninth and twelfth century, but experts agree the ‘wonders’ can be fairly easily matched to each other, such as the pool surrounded by stone that’s warm when the weather is warm and cold when the weather is cold. This describes the famous spa which gave the Roman city of Bath its name.
Another of the wonders listed is harder for experts to agree on and is more obscure. The Severn Bore, or The Teared, as it was once called, is a phenomenon that is rare in the world, rarer still in Britain and unique in Wales. The Bore, or The Teared, is a wave created by water funneling into an increasingly narrow channel, which becomes narrower at high tide. This creates a tidal wave for approximately 130 days of the year (twice a day, so 260 bores in a year). Observers have noted the size is often higher and more impressive after a full moon and largest bores occur around the times of the equinoxes, with smaller examples seen throughout the year.
The Bore has been much studied in the last few decades and has benefited from becoming popular with surfers. Timetables for the height predictions are published annually, subject to fluctuation. According to Wikipedia, “a bore occurs between 7 a.m. and noon on bore days, and the other between 7 p.m. and midnight GMT, with the largest bores occurring between 9 and 11 in both the morning and evening.” The bores are associated with the phases of the moon. Maximum bores occur between one and three days after new and full moons, and smaller ones on the days that precede and follow the maxima” It goes on to say the bore height is increased by a strong southwest or west wind. No doubt marveled at by our ancient ancestors and duly noted by William of Monmouth as one of the most naturally impressive sights in all of Britain.
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As every well-studied school child should know, the River Severn is an impressive body of water on many levels. At 50 feet, it has the second highest disparity between high and low tide anywhere in the world. This may influence the water action of the Bore / Teared.
Surfers ride the Severn Bore near Sharpness, Gloucestershire.
História also describes a dramatic tidal wave and whirlpool at place called Lllin Lliwan, an area that has become 'lost’ in modern times. There are intriguing theories about its whereabouts, and these pose serious questions about the abilities of our ancestors to travel great distances across the Britain with relative ease.
Over the centuries have been many water reports of anomalies in the Wye the nearby Bristol Channel. Anecdotal evidence, however, has become noticeably less frequent and less dramatic in the last hundred years. Speculative conclusions have been made that the unique water features in this area were severely damaged in 1879 when engineers working on the Great Western Severn Tunnel railway line broke through into a water-filled passage 170 feet underground and 400 yards inland. Huge, uncontrollable deluges of water suddenly inundated the workers on the Wales side. Subsequent investigations found evidence the workers accidentally broke into an underground river bed, above the Limestone overhead, a natural spring of water for many on both Gloucester and Chepstow sides of the Wye. This theory lent credence by the amounts of fresh (i.e, not Severn) water that is pumped from the tunnel every day.
The main engineering triumph of the Severn Tunnel is not the initial feat of boring beneath 50 feet of water at high tide, or keeping the river water at bay; no, the engineering achievement which continues to this day is the network of tunnels that stem from the main railway tunnel. These tunnels house vast pipes that pump 23 million gallons of water from the above spring every day. If the pumps failed, it would take just 26 minutes for the tunnels to completely fill with water. After the tunnel flooded in 1879, it took two years to drain.
Pumps working hard in the Severn Tunnel, adjacent to the railway line tunnel
Water from this underground lake is pumped out at Sudbrook, near Caldicot. The “Great Spring”, as it has become known, contains a bit of a mystery because there is very little historical correlation between the amount of rainfall and water flowing from the freshwater spring. Typically, the Severn Tunnel pumps have to work harder three weeks following heavy rainfall, but again, figures showing the amount of water pumped out at Sudbrook versus rainfall in the preceding weeks rarely correlate. The most notable was an exceedingly high level of rainfall recorded in 1964, which didn’t affect the amount of water pumped out of the spring until 11 weeks later. The soil and sand content beneath the riverbed may provide explanation, but looking at other interesting water anomalies in the vicinity raises some interesting questions.
“The Great Spring”. Freshwater pumped from the Severn Tunnel, near Sudbrook, Wales.
Another water oddity lays just seven miles further in land from Sudbrook, in the small town of Caerwent, once the bustling Roman market town, Venta Silurum. Outside the impressive and well-preserved ramparts of the outer Roman wall, four brooks converge. One of these, Troggy Brook (Cas Troggy) is a stream that floods into a pool during wet weather which extends outwards. The pool comes and goes more than one would expect, and like the Great Spring, is largely unaffected by high rainfall.
Cas Troggy is interesting because instead of the ground rising smoothly from the current course of the brook, it sits in a depression around three hundred yards across, with defined boundaries which rise steeply to a ten meter contour and then slopes back more gently.
Archaeological evidence shows ancient dwellings clustering along the edge of the contour but avoiding the area inside it, suggesting this marks the boundary of a lake, or of an area of marshland around a lake, too soggy to build upon safely. There is an alluvial deposit (silt left by an old river) which follows the course of the Troggy but spreads out to cover all or most of the area bounded by the contour. Perhaps coincidentally, the alignment of the southern walls of Venta Silurum follows the edge of the silt deposits.
When water levels are particularly low, not only does the lake disappear, but so too does a two-mile stretch of the brook itself. Yet the water still flows above and below the dry stretch – showing that as well as the visible, overground stream-bed, the water is following a roughly parallel course underground. A dye test revealed some of the water from the Great Spring is from Cas Troggy.
The south wall of Venta Silurum, Caerwent.
Early archaeologists noted iron mooring-rings still surviving in the south wall of Venta Silurum. Some believe this pool of the Troggy had been a permanent feature in the Dark Ages, when the water table was higher and the Troggy emptier of silt. Experts believe the Troggy is connected to a subterranean reservoir (or reservoirs) via several narrow fissures acting as siphons. The fractures and hydraulics are so complex, it is impossible fully to predict how such a system would behave, but it seems likely that as the levels rose and water poured into the lake, it flowed through into this hidden reservoir. When the level fell, the fissure became a siphon, which caused the excess water in the reservoir to be emptied back out into the lake, until the two bodies of water were at the same level. This activity would almost certainly have caused localised currents beneath the surface. The lake is known to have had at least three whirlpools, known locally as Whirlyholes; water being sucked into, or ejected from, the underground system: one of them, now mostly dry, is noticable as a deep hollow. One of the historical anecdotes I mentioned earlier is a tale circa 1910 of a farmer whose son was lost down a Whirlyhole and never found.
Nedern, another of the four brooks converging around Caerwent, now flows into the Severn Estuary, two miles due south of Caerwent at Caldicot Pill (Pill, Old English. Pyll: outlet of a tidal stream) but originally its outlet was nearly a mile to the east, at Sudbrook where there was also once a Whirlyhole, now partially tucked underneath the railway line on the edge of the fields of what is now Southbrook Farm. This is where the system was wrecked in 1879.
Llyn Lliwan reborn? The Troggy in full flood (2007). The road is the M48.
So, we have an underground spring with an unknown source that dispels millions of gallons of water every day, a dark ages lake that although now much receded, still comes and goes regardless of the amount of rainfall. Those iron mooring rings in the walls of Venta Silurum are significant because they suggest an almost permanent body of water once existed there.
There is obscure evidence linking Caerwent and Caerleon as Camelot of Arthurian Legend. At the very least, it seems as if a garrison of soldiers has always maintained a presence there. Arthur, according to Historia reigned during the ninth century. Venta Silurum would still have been in good condition. The area was no doubt chosen by the Romans, and the Silures tribe before them because it was strategically important. The sudden rising land around the town is clustered with dense woodland and levels into high ridges which provide excellent, clear views across the Severn and the mouth of the Severn estuary, makes the area easily defensible; anything approaching via the Bristol Channel would be seen for miles, especially as vessels would have to slow significantly approaching the conjoining rivers Wye and Severn.
If myth is to be believed and Caerwent really is Camelot, then Cas Troggy could well be the famous lake of legend (Lady of the Lake etc.). If the inferences supplied by experts, science and anecdotal evidence are to be believed, the lake would have been so large that it would be possible to sail from Caerleon, past Venta Silrum, into the Severn, the Bristol Channel and down toward Cornwall, another county strong linked to Arthurian legend. In fact, not allowing for wind factors, it would have been possible to do a return trip via this route in under thirty-six hours!
Otter Hole is another fascinating peculiarity, unique to South Wales. Just ten miles from Sudbrook, Troggy Brook and Caerwent, located beneath Chepstow Racecourse and the Wye Valley path. Otter Hole is a spectacular network of chambers and caverns “found” (or possibly re-discovered) in 1974. Because of the difficulty of getting down there and the precise timing of the excursions, these natural wonders can only be accessed by enjoyed by highly experienced cavers. Sensitive to local rainfall, the height of the River Wye, and the level of the tide, it’s a challenge, even for experienced cavers, and it is this tidal sump that goes towards making Otter Hole so unique and dangerous. One of the chambers features another large ‘disappearing lake’. The sight of the lake as it drains itself of water is said to have a sobering effect on those that have watched it empty itself. Eye-witnesses describe unusual, unearthly sounds when the water drains away. As trapped air escapes, they describe a sound like a tube train rushing down a tunnel, or a “rhythmic gurgling”, and at some points something very much like a “very loud drumming sound”. Is there anything to link Otter Hole with the mysterious disappearing lake of Cas Troggy / Nedern Brook?
Stalactites and under ground lake of Otter Hole. Chepstow, Wales.
There is still much research to be done on these unusual water features that appear to be utterly unique. Where does the water from the Great Spring emanate? Why does the amount of water pumped from the Severn Tunnel fluctuate regardless of tide or rainfall? Why does Cas Troggy continue to flood, and where does the water go whenever it recedes? Why were there mooring rings on the walls of Venta Silurum? And why are the whirlypools no longer in evidence, even after significant rainfall?
The only inference that can be made is there was an unusual and unique system of water features, utilised, if not completely understood, artfully by our ancestors. A complex system completely wrecked by the digging of the Severn Tunnel and the knock-on effects of the flooding disaster in 1879. Is it possible industrial work wrecked one of the historical Wonders of Britain without knowing?
There are still many things to learn about this area, but it seems clear the workings of this complicated network of whirlpools, sumps, brooks and springs will never be understood fully, which begs the question; How many Wonders listed in Historia have we unknowingly destroyed forever more?
by Martin Gregory
Photo Credits: Surfing the Bore – Tim Cooper / Roman Wall (South) Caerwent – TripAdvisor / Otter Hole – Paul Fretwell / Severn Tunnel pumps – BristolPost / Sudbrook Spring – Mike Timms / / Troggy Flood – Eric Woods Eagles Nest (lead image) – Paula J James
I recommend these websites for further reading:
On the subject of Caerwent claim to being Camelot and more of the mysterious Lake: http://members.madasafish.com/~cj_whitehound/Fanfic/A_true_original-Appendix.htm
On Linn Liuan/Llynn Llyw : https://www.jstor.org/stable/40646470 / http://www.wondersofbritain.org/wonder6/
On Otter Hole I recommend this documentary. It’s a bit old but captures the drama well: www.youtube.com/watch?v=xR1vTkfQEWg
*This was originally written for noscriptforlife.wordpress.com in 2019. It has been updates and re-fact checked for anything changes / developments in.the interim.
Martin Gregory writes thought pieces drawing on lived experience and the human condition. I also write about my ‘non-line’ life, foregoing social media, apps and algorithms.
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