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Comments on ‘The lithostratigraphy, biostratigraphy and hydrogeological significance of the mud springs at Templars Firs, Wootton Bassett, Wiltshire’ by C. R. Bristow, I. N. Gale, E. Fellman and B. M. Cox with I. P. Wilkinson and J. B. Riding
CORRESPONDENCE
Comments on 'The lithostratigraphy, biostratigraphy and hydrogeological significance of the mud springs at Templars Firs, Wootton Bassett, Wiltshire' by C. R. Bristow, I. N. Gale, E. Fellman and B. M. Cox with I. P. Wilkinson and J. B. Riding W I. S tallion writes: The research described in this paper marks a significant step forward in under stand ing these curious phenomena and establishing details of the local geology. There are , however, two matters that need some clarification. Th e first concerns the density and viscosity of the liquid mud , which is important to the hydrogeological interpretation. The rising mud car ries macrofossils and othe r stony objects to the ground surfa ce, partly because it is significantly denser than water. But, assuming (for the sake of discussion) that its density is 1.5, it would not be able to lift sto ny objects of density 2.4, for example, unless it was viscous enough to prevent the objects falling through it. Suppose that the liquid mud , density 1.5, fills a sub-vertical shaft or chamber 20 m deep in the Ampthill Clay above the Cora llian (Bristow et al., 2000, fig. 4). If the mud column is in cont inuity with groundwate r in the Corallian aquifer, the balancing column of groundwater, density 1.0, would be 30 m high. Thu s, unless the hydro static head driving the water/mud circulation greatly exceeded 10 m there would be no movement of the viscous liquid mud (indeed, for heads much less than 10 m the circulation could, theoretically, go into reverse). It is not at all clear that suitably high hydro stat ic heads could exist in the Corallian, especially at the Greenhill Common mud springs which are only 400 m from, and several metres higher than, the point where the Corallian aquifer out crops in the Brinkworth Brook (Bristow et al., 2000, fig. I). Th e artesian head (water) of 8 m recorded in the BGS Templars Firs borehole could be a purely local balancing head generated by the adjacent column of liquid mud. It would be interesting to know at what stage(s) in the borehole constructi on it was measured. Density considerations are relevant also to the schemat ic cross-section through the Templar s Firs mud springs (Bristow et al., 2000, fig. 4). If the liquid mud filling the large chamber and the extensive lateral extensions to the vertical column is significantly less dense than the Ampt hill Clay, which presumably is not very rigid, the latte r would tend to sett le into them, expelling the liquid mud and causing ground subsidence. The Corallian in this region is not a major aquifer. Due to its variable calcareous and argillaceous facies, groundwater flow routes through the confined zone Proceedings of the Geologists' Association, 112, 9 1-92.
beneath the Ampthill and Kimmeridge clays are likely to be indirect and unpredictable. Even so, it is by no means certain that hydrostatic heads high enough to drive the water/mud circulation at either spring group could be generated within the Corallian . To take this discussion further , a representative series of density measurements of the liquid mud from different sites and depths is desirable. Measuring the viscosity would be less straightforward and, possibly, not relevant, given the evident thixotropic nature of the fluid beneath active mud mound s. This probabl y rises natu rally as a gooey paste (Stanton, 1995) rather th an the runny liquid that currently spills out of Templar s Firs vents I and 2 which have suffered so much human disturbance. The second matter tha t would benefit from further discussion is defined in the Introduction : the aim of the investigation 'was also to assess the mud springs for their uniqueness, with the possibility of their being designated an SSSI' . Certainly the authors' valuable discovery of a second group of mud springs at Greenhill Common mean s that the Templars Fir s group is not unique, but the two gro ups are only 1500 m apart. The question remains: ifmud springs are a normal consequence of artesian pressure in an aquifer beneath a confining clay layer, why are they not mo re common? What is the special factor that create s IS mud springs, uniquely in this country so far as is known, in a small region near Wootton Bassett, while everywhere else the springs that break up thr ough confining clay strat a are of water? 1. N. Gale replies: The Corallian in the area is a minor aquifer characterized by variable facies and low permeability. Gr oundwater movement is slow and hydraul ic gradient s are likely to be steep. This is testified to by the age of the mud (ofa few millennia) as indicated by the isotopic analyses at the Templ ars Fir s site. However, the hydrauli c pressure head could be transmitted through the aquifer relatively quickly. There is some anecdotal evidence of the flow in the mud springs increasing about one month after periods of heavy rainfall, presumably in response to recharge at outcrop beneath Wootton Bassett. The hydraulic head in the BGS borehole was measured in the Coral Rag at a depth of about 20 m, on completion of drilling. Despite the high artesian head of about 8 m abo ve ground level, the flow was 0016-7878/01 $ 15.00
© 2001 Geologists' Associati on
92
CORRESPONDENCE
only about 5 I min - I, again indicating the low permeability of the aquifer. The density of Ampthill Clay is 2.0 Mg m -3 (Forster, pers. comm.) and the mud has a calculated density of about 1.3 Mg m - 3. It is reasonable to assume the same hydraulic head in the Coral Rag beneath the mud springs at Templars Firs as in the BGS borehole (c. 100 m distant). This would support a column of mud of over 21 m. As the mud springs are at an elevation about 3 m below the site of the BGS borehole there is ample head in the Coral Rag to drive the flow of mud. The accuracy of these estimates could be improved by measuring the density of samples of the mud and the Ampthill Clay from the sites, as well as accurately measuring elevations. As a borehole has not been drilled at the Lower Greenhill Farm site it is not possible to confirm the depth to the Coral Rag or the hydraulic head in the aquifer beneath the mud springs. Estimates of groundwater elevation have been made where small windows in the Ampthill Clay permit the Corallian to crop out (Bristow et al., 2000, fig. I). Both of the outcrops to the
southeast of the mud springs are presumed to be in hydraulic contact with the upper reaches of the Brinkworth Brook and are at elevations of c. 97 and c. 93 m AOD. It is postulated that the hydraulic gradient is sufficient to make the aquifer artesian at the site of the mud springs. A borehole is needed to confirm or refute this hypothesis. The authors agree that further discussion is warranted as to why the mud springs appear to be unique to this area. Current evidence indicates that a combination of factors is needed, namely: a low permeability aquifer containing groundwater at a high artesian head that reacts with the confining layer so that clay particles remain in colloidal suspension. A route through the confining layer is then needed. This is probably controlled by the relationship between the artesian head and the thickness of the confining layer, as well as local geological structures. It would be interesting to compare these factors at the sites of the other springs mentioned in the last paragraph of Dr Stanton's comments.
REFERENCES BRISTOW, C. R., GALE, 1. N., FELLMAN, E., COX, B. M., WILKINSON, 1. P. & RIDING, J. B. 2000. The lithostratigraphy, biostratigraphy and hydrogeological significance of the mud springs at Templar Firs, Wootton
Bassett, Wiltshire. Proceedings of the Geologists' Association, 111, 231-245. STANTON, W. 1. 1995. Wootton Bassett: Fame at last for mud springs. Geology Today,ll, 172.
W. 1. STANTON, Kites Croft, Westbury-sub-Mendip, Wells, Somerset BA5 IHU, UK 1. N. GALE, British Geological Survey, Maclean Building, Crowmarsh Gifforq, Wallingford,
Oxfordshire OXIO 8BB, UK
Comments on 'The lithostratigraphy, biostratigraphy and hydrogeological significance of the mud springs at Templars Firs, Wootton Bassett, Wiltshire' by C. R. Bristow, I. N. Gale, E. Fellman and B. M. Cox with I. P. Wilkinson and J. B. Riding W I. S tallion writes: The research described in this paper marks a significant step forward in under stand ing these curious phenomena and establishing details of the local geology. There are , however, two matters that need some clarification. Th e first concerns the density and viscosity of the liquid mud , which is important to the hydrogeological interpretation. The rising mud car ries macrofossils and othe r stony objects to the ground surfa ce, partly because it is significantly denser than water. But, assuming (for the sake of discussion) that its density is 1.5, it would not be able to lift sto ny objects of density 2.4, for example, unless it was viscous enough to prevent the objects falling through it. Suppose that the liquid mud , density 1.5, fills a sub-vertical shaft or chamber 20 m deep in the Ampthill Clay above the Cora llian (Bristow et al., 2000, fig. 4). If the mud column is in cont inuity with groundwate r in the Corallian aquifer, the balancing column of groundwater, density 1.0, would be 30 m high. Thu s, unless the hydro static head driving the water/mud circulation greatly exceeded 10 m there would be no movement of the viscous liquid mud (indeed, for heads much less than 10 m the circulation could, theoretically, go into reverse). It is not at all clear that suitably high hydro stat ic heads could exist in the Corallian, especially at the Greenhill Common mud springs which are only 400 m from, and several metres higher than, the point where the Corallian aquifer out crops in the Brinkworth Brook (Bristow et al., 2000, fig. I). Th e artesian head (water) of 8 m recorded in the BGS Templars Firs borehole could be a purely local balancing head generated by the adjacent column of liquid mud. It would be interesting to know at what stage(s) in the borehole constructi on it was measured. Density considerations are relevant also to the schemat ic cross-section through the Templar s Firs mud springs (Bristow et al., 2000, fig. 4). If the liquid mud filling the large chamber and the extensive lateral extensions to the vertical column is significantly less dense than the Ampt hill Clay, which presumably is not very rigid, the latte r would tend to sett le into them, expelling the liquid mud and causing ground subsidence. The Corallian in this region is not a major aquifer. Due to its variable calcareous and argillaceous facies, groundwater flow routes through the confined zone Proceedings of the Geologists' Association, 112, 9 1-92.
beneath the Ampthill and Kimmeridge clays are likely to be indirect and unpredictable. Even so, it is by no means certain that hydrostatic heads high enough to drive the water/mud circulation at either spring group could be generated within the Corallian . To take this discussion further , a representative series of density measurements of the liquid mud from different sites and depths is desirable. Measuring the viscosity would be less straightforward and, possibly, not relevant, given the evident thixotropic nature of the fluid beneath active mud mound s. This probabl y rises natu rally as a gooey paste (Stanton, 1995) rather th an the runny liquid that currently spills out of Templar s Firs vents I and 2 which have suffered so much human disturbance. The second matter tha t would benefit from further discussion is defined in the Introduction : the aim of the investigation 'was also to assess the mud springs for their uniqueness, with the possibility of their being designated an SSSI' . Certainly the authors' valuable discovery of a second group of mud springs at Greenhill Common mean s that the Templars Fir s group is not unique, but the two gro ups are only 1500 m apart. The question remains: ifmud springs are a normal consequence of artesian pressure in an aquifer beneath a confining clay layer, why are they not mo re common? What is the special factor that create s IS mud springs, uniquely in this country so far as is known, in a small region near Wootton Bassett, while everywhere else the springs that break up thr ough confining clay strat a are of water? 1. N. Gale replies: The Corallian in the area is a minor aquifer characterized by variable facies and low permeability. Gr oundwater movement is slow and hydraul ic gradient s are likely to be steep. This is testified to by the age of the mud (ofa few millennia) as indicated by the isotopic analyses at the Templ ars Fir s site. However, the hydrauli c pressure head could be transmitted through the aquifer relatively quickly. There is some anecdotal evidence of the flow in the mud springs increasing about one month after periods of heavy rainfall, presumably in response to recharge at outcrop beneath Wootton Bassett. The hydraulic head in the BGS borehole was measured in the Coral Rag at a depth of about 20 m, on completion of drilling. Despite the high artesian head of about 8 m abo ve ground level, the flow was 0016-7878/01 $ 15.00
© 2001 Geologists' Associati on
92
CORRESPONDENCE
only about 5 I min - I, again indicating the low permeability of the aquifer. The density of Ampthill Clay is 2.0 Mg m -3 (Forster, pers. comm.) and the mud has a calculated density of about 1.3 Mg m - 3. It is reasonable to assume the same hydraulic head in the Coral Rag beneath the mud springs at Templars Firs as in the BGS borehole (c. 100 m distant). This would support a column of mud of over 21 m. As the mud springs are at an elevation about 3 m below the site of the BGS borehole there is ample head in the Coral Rag to drive the flow of mud. The accuracy of these estimates could be improved by measuring the density of samples of the mud and the Ampthill Clay from the sites, as well as accurately measuring elevations. As a borehole has not been drilled at the Lower Greenhill Farm site it is not possible to confirm the depth to the Coral Rag or the hydraulic head in the aquifer beneath the mud springs. Estimates of groundwater elevation have been made where small windows in the Ampthill Clay permit the Corallian to crop out (Bristow et al., 2000, fig. I). Both of the outcrops to the
southeast of the mud springs are presumed to be in hydraulic contact with the upper reaches of the Brinkworth Brook and are at elevations of c. 97 and c. 93 m AOD. It is postulated that the hydraulic gradient is sufficient to make the aquifer artesian at the site of the mud springs. A borehole is needed to confirm or refute this hypothesis. The authors agree that further discussion is warranted as to why the mud springs appear to be unique to this area. Current evidence indicates that a combination of factors is needed, namely: a low permeability aquifer containing groundwater at a high artesian head that reacts with the confining layer so that clay particles remain in colloidal suspension. A route through the confining layer is then needed. This is probably controlled by the relationship between the artesian head and the thickness of the confining layer, as well as local geological structures. It would be interesting to compare these factors at the sites of the other springs mentioned in the last paragraph of Dr Stanton's comments.
REFERENCES BRISTOW, C. R., GALE, 1. N., FELLMAN, E., COX, B. M., WILKINSON, 1. P. & RIDING, J. B. 2000. The lithostratigraphy, biostratigraphy and hydrogeological significance of the mud springs at Templar Firs, Wootton
Bassett, Wiltshire. Proceedings of the Geologists' Association, 111, 231-245. STANTON, W. 1. 1995. Wootton Bassett: Fame at last for mud springs. Geology Today,ll, 172.
W. 1. STANTON, Kites Croft, Westbury-sub-Mendip, Wells, Somerset BA5 IHU, UK 1. N. GALE, British Geological Survey, Maclean Building, Crowmarsh Gifforq, Wallingford,
Oxfordshire OXIO 8BB, UK