View from Eagles Nest, Chepstow. showing the river Wye and  in the distance, River Severn.


South Wales has perhaps the most unusual and mysterious water features anywhere in the world.

In every surviving version of Historia Britannum there is a list presenting Wonders of the World. The list is not always the same in each version but most of the wonders can be fairly easily matched to eachother. Such as the pool surrounded by stone that’s warm when the weather is warm and cold when the weather is cold. Of course, this describes the famous spa which gave the Roman city of Bath it’s name. One of the wonders listed is the Severn Bore, or The Teared, as it was once known. This is a phenomena 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. The Severn has the the second highest disparity between high and low tide (50 feet!) anywhere in the world which naturally increases the water action of the Bore / Teared.

Surfers ride the Severn Bore near Sharpness, Gloucestershire.

Histioria also describes a dramatic tidal wave and whirlpool at Lllin Lliwan. By looking more closely at the name and its various different spellings (Llyn Lliuan. Llinn Lliuan etc.) ,we can safely infer this area is located somewhere across the Bristol Channel, possibly on the side of the channel furthest opposite Weston Super-Mare. Lllin Lliwan seems to be an area that has become lost in modern times, and the point at which the mouth of the Severn becomes The Bristol Channel is unclear. Many anomalies have been reported in the Wye, and in the Bristol Channel over the years, though these have become less frequent and less dramatic in the last hundred years. This has led to speculative conclusions that many of the unique water features in this area were severely damaged 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. The workers on the Wales side were suddenly inundated with huge, uncontrollable deluges of water. It’s believed the workers accidentally broke into an underground river bed, above the Limestone overhead. Many believe this was an underground spring, a source of water for many on both Gloucester and Chepstow sides of the Wye. This theory is bourne out by the amounts of fresh (i.e not Severn) water pumped from the tunnel everyday.

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

The water 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 from the freshwater spring. Typically, the Severn Tunnel pumps have to work harder three weeks following heavy rainfall, but again figures showing the amount 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. This can be partly explained by the soil and sand content beneath the riverbed but may also be explained by looking at other unusual water features of the area.

“The Great Spring”.  Freshwater pumped from the Severn Tunnel, near Sudbrook, Wales.

Another water oddity can be spotted 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 considered to be a stream and 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 beceause 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 safely build upon. 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 southern walls of Venta Silurum are aligned to follow 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. Using special dye, it has been proven that 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. It has also been noted 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 dry, is marked by a deep hollow. There is a tale circa 1910 of a farmer whose son had been 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 everyday, a dark ages lake that although now much receeded, 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 compelling evidence linking Caerwent and Caerleon as Camelot of Arthurian Legend. At the very least it seems probable a garrison of soldiers was once stationed there. In the 8th century, when Arthur is believed to have reigned, Venta Silurum would still have been in good condition. The area was chosen by the Romans, and likely the Silures tribe before them as straegically 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. The mouth of the Severn estuary makes the area easily defensible; anything approaching via the Bristol Channel would be seen miles and would have to slow significantly as it approached the conjoining rivers Wye and Severn. If myth is 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.). This lake was reputed to be so large that it was 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 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. Due to the difficulty of getting down there and the precise timing of the excursions, these natural wonders can only be accessed by 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 on those that have watched it empty itself. Eye witnesses have described unusual 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?

N Otter Hole Calcite Crystal Balls on thin straws Paul Fretwell
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 eminate? 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 in 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, completely wrecked by the digging of the Severn Tunnel. 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 fully be understood, which begs the question; How many Wonders listed in Historia Britannium have we unknowingly destroyed forever more?

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 this website for further reading on the subject of Caerwent’s claim to being Camelot and more of the mysterious Lake : http://members.madasafish.com/~cj_whitehound/Fanfic/A_true_original-Appendix.htm

Today’s post has been cribbed from my research for my fictional, unpublished novel the Scrying Stone.

Copyright Martin Gregory 2018




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