WOOTTON BASSETT'S MUDSPRINGS
In the small copse at Templar's Firs, Wootton Bassett, near the recently re-opened stretch of the Wilts & Berks canal is a phenomenon that has perplexed geologists for the last few years. Grey mud oozes out of 2 or 3 vents, and flows away like a cold version of a lava flow. It can't be water leaking from the canal because the springs have been there a long time. There appears to be no similar thing anywhere in the world so far reported. All this came to a head when plans for a bypass showed the road going through the mudsprings' area. Now there are hopes that the nation-wide publicity will help get the site protected.
The site is a group of grass-covered domes which contain liquid mud of a grey creamy texture. If you walk on them the ground has a strange springy or wobbly feel in places. It seems a chamber of mud lies below, retained only by the knit of vegetation and topsoil. The pressure of a person's weight even as far away as 7 metres is enough to cause oscillation and renewed oozing of mud out of a vent. It then flows away at about 5 cm per second in a river up to half a metre wide, swallowing up the grass like a lava flow. A note of caution: The springs are dangerous to the unwary especially children, so WCC (who own the site at the moment) have fenced it off with barbed wire plus warning signs. Farmers say cattle have disappeared into the mud and 100 tons of rubble was also devoured without trace! It would be great if the W&BCAG obtained this area of land to form part of the canal's amenity.
The questions are, why is it mud, not just clear water, what is replenishing the mud chamber, are there several domes fed by one parent mud chamber or is it individual chambers? How deep down does the mud come from and has it flowed laterally for some distance? These questions need the attention of a geologist to solve. I have suggested that vibration and pressure from high speed trains on the GWR main line could cause underground discharges strong enough to stir clay and water into mud. The The water most likely comes under pressure from the bed of limestone which forms the backbone of Wootton Bassett old town. The Coral Rag limestone, which holds water like a sponge, forms a layer sloping down southwards under the waterproof Ampthill clay, so the latter forms a retaining cap. Above that comes a rich seam of fossils marking the base of the Kimmeridge Clay. A geological fault might cause a leak through the clay to the surface.
I have found lots of fossils in the seam such as ammonites, oysters, and even a vertebra from an animal's backbone. Small brass-coloured ammonites may be from the mudsprings because I found them in the stream only downstream from the springs, where they may have been winnowed from the mud poured into the stream. Some of these fossils have already given some fame to the area because they proved useful in the British Geological Survey's ongoing update of the country's strata. Also, the B.G.S. drilled a borehole near the PMH, Swindon, specifically to unravel the local geology. The seam of fossils includes the Inconstans Bed named after the odd-shaped brachipod Torquirhynchia Inconstans which occurs only in this thin seam of strata.
The fossils probably have no bearing on the cause of the mudsprings, yet they add some extra interest to the area. A common misapprehension concerns lots of shell found in the mud dredged from the canal. These are not fossils but are the leftovers from canal boatmens' meals! They seemed to live on clams possibly kept alive in salty water, brought from fishmarkets.
Mud Springs in Britain.
As the Authority's geologist I went to investigate this phenomenon. In Templar's Firs, on the right bank of the stream, I found three mounds each about 10 metres long by 5 metres high. They were 'mud blisters', consisting of a more or less liquid mud core contained within a living skin created by the roots of rushes, sedges and other swampy vegetation, including shrubs and small trees. The workmen had cut into the end of one blister, allowing it partly to deflate. The liquid core was (and is) at least 2 metres deep, as I ascertained by probing with a stake that had been sharpened and left there by other investigators. Grey liquid mud oozes from splits in the skin. The ground surrounding the blisters is level; thus they must be nourished from below them.
The landowner told me that the 'bogs' in the copse have always been there and that cattle have been lost in them. They are underlain by 10-20 metres of Kimmeridge Clay resting on the Coral Rag sub-division of the Corallian. I am inclined to rule out mud diapirism arising from a Wootton Bassett accretionary complex as the source of these mud springs and plumb instead for artesian leakage up through a weakness in the Kimmeridge Clay from the Coral Rag, which is locally a minor aquifer. But why the springs are of mud, and why the mud remains liquid instead of settling out of suspension, need to be explained.
Housing and industry are spreading out from Wootton Bassett and the mud springs of Templar's Firs are not far beyond some new estates. I know of no similar springs and have suggested to the Nature Conservancy Council that they merit SSSI protection, without response so far. Do readers of Geology Today know of other mud springs in the UK?
Mud Springs at Wootton Bassett.
There are two vents, each about 15 cm wide, from which liquid has obviously flowed recently. The one that seems to have been the more active lies about 10 metres north of the stream and has extruded mud flows on a broad front to the stream. The mud is of a similar consistency to intertidal harbour mud, such as that in Chichester Harbour. No grass appears to have started growing on or through it. By jumping on the mat of vegetation that still projects above and near to the vent, I could induce liquid mud to ooze out and flow away across the older flows.
The other vent is actually in the stream bed, partially arresting the water flow. The mud previously extruded is lining the stream bed for several metres downstream. Here the water has winnowed the clay into soft lumps about 3 cm across.
Numerous fossils have been found in this stream, in the clay exposures forming its banks or in various building works to the north-east. I had some fossils that I collected identified by B.M.Cox of the BGS. The most interesting was Rhactorhynchia inconstans, of which about a dozen specimens have been found between SU 082817 and 081823. As the iconstans bed is taken to be only a few feet above the base of the Kimmeridge Clay, it encourages the theory of artesian leakage from a permeable layer layer of Corallian limestone below. A borehole at 070818,shown on the 6" geological map, passed from Kimmeridge Clay into Upper Corallian at 78 metres above OD, and Templar's Firs is only a little higher at about 92 metres, estimated from the 2.5 " map. Neither the borehole nor the map mention the Red Down clay surveyed by W.J.Arkell as lying between the Coral Rag and the Kimmeridge Clay. If this clay is present, the water assumed to be responsible for the mud springs would have a greater thickness of clay to cross. The aquifer could, alternatively, be a bed of Red Down Ironsand lying just below Kimmeridge Clay.
Finally, a possible explanation of how the water and mud became so well mixed is that the swaying of trees growing near the underground channel was transmitted through their roots, acting as a stirrer.