Sources of salt in Worcestershire

– brine, saline springs and salty drinking water from Worcestershire’s rocks –

Section through the Droitwich Halite beds by Charles Hastings, 1835

The Droitwich area of Worcestershire is famous as a source of common salt (sodium chloride) and for its brine baths. This paper investigates the source of this salt and looks more broadly at the occurrence of common salt in the rocks of Worcestershire.

The salt works and brine baths have long been closed, but interest in the area as a source of salt remains. Brine Memories is a Heritage Lottery Funded living history project to collect and preserve memories of Droitwich Spa’s Brine Baths, while providing a broad understanding of “all things salty” in Worcestershire. This paper is inspired by a request for information from a geologist’s perspective about the Droitwich salt field and other salt sources in the county. Our attention was drawn in particular to some salty pools located close to Croome Court.  

The trust is pleased to be able to support this Heritage Lottery Funded project by providing this information for use in their project. For more information about the project, please visit the Brine Memories website.

This paper, prepared by Kay Hughes and John Payne is in three parts, covering:

• The extent of the Droitwich Salt Field;
• The occurrence of salt beyond the Droitwich area;
• The problem of salt in drinking water.

The Extent of the Droitwich Salt Field

Salt fields are found widely in the Mercia Mudstone (old name – Keuper Marl) strata of the UK, between the Bristol area and Northern Ireland, as shown in Fig 1 below.

Fig 1. Extract from L J WILLS. 1950. ‘The palaeogeography of the Midlands’. (Figure 22)). Solid black areas show where salt fields are found

The full extent of the Droitwich salt field is not well known.  There is little information available, other than the locations of brine springs and local place names, and the results from a few boreholes.  More recently the interpretation of seismic data could probably be of use but we know of no such information. 

The best attempt at mapping appears to be that of L J Wills, 1976: ‘The Trias of Worcestershire and Warwickshire’. Rep Inst Geol Sci, (76/2), 211pp, Figure 14), reproduced in Fig 2 below.  Here, the salt fields are shown by the cross-hatched areas, the lower one being the Droitwich field. Droitwich is marked by ‘D’, Tewkesbury by ‘T’ and boreholes by small circles.  Ordnance Survey grid lines are shown at 50km intervals.  From the map, the Droitwich salt field extends roughly from a few kilometres south of Tewkesbury (SO9098) to Stoke Prior (SO9766), with a width east-west of about ten kilometres.  Wills does not list the data set on which his map is based.  Spring positions are available from other sources and are discussed later.

Fig 2. Extract from L J WILLS, 1976. ‘The Trias of Worcestershire and Warwickshire’. Rep Inst Geol Sci, (76/2), 211pp, Figure 14

There is particular uncertainty about the extent of the salt field defined by Wills except in the vicinity of Droitwich and Stoke Prior. More recent reports by British Geological Survey (BGS) are in broad agreement with this picture.

The BGS, the national authority for naming rock formations, names the saltfield the Droitwich Halite Member, which occurs within the Sidmouth Mudstone Formation of the Mercia Mudstone Group, formerly the Keuper Marls of Wills’ sketch map. Barclay et al and Worssam et. al in the BGS memoirs of the Worcester and Tewkesbury districts respectively state that the existence of the Droitwich Halite member in these districts has not been proved. Barclay however postulates that the deposition of salt in Triassic times was fault controlled. He argues that the halite beds would have formed in the lowest part of the Worcester basin at that time, bounded to the west by the Smite-Pirton-Tewkesbury fault and in the east by the Inkberrow fault.   

Fig 3 below shows the positions of these faults relative to the proposed saltfield area of Wills, and displays a high degree of agreement between the two viewpoints. It is notable that the areas of discrepancy are not contradicted by any of the borehole evidence used by Wills. The blue diagonally hatching near the top of the figure indicates the area marked on the BGS map where the Droitwich Halite Member occurs at the surface.

Fig 3. Position of the fault-bounded basin proposed by Barclay relative to the salt field proposed by Wills. Faults are shown as red dashed lines: the Smite-Pirton-Tewkesbury fault (left) and the Inkberrow fault (right) define the western and eastern boundaries of the proposed basin respectively. Blue single hatching indicates the extent of the Droitwich Halite Member as shown on the BGS map.

The potentially large area of the Droitwich salt field does not imply that the salt beds have great thickness, or even exist, in all parts.  The salt accumulated by the evaporation of salty water from pools which occupied the lowest ground in land-locked basins, similar to the situation of the Dead Sea and the Great Salt Lake today. The pools would have been fed by rivers and probably occasionally flooded by sea water. Such evaporation events were followed by the deposition of wind-blown dust and possible later pool formation. The series of separated salt layers thus formed is just what is recorded in borehole logs. The conclusions of L. J. Wills regarding this are in the appendix.

The limits of the salt layers under Droitwich itself are likewise not accurately known.  Firm information is limited to the results from just a few boreholes.  The best data are probably those of Poole and Williams (1981) (‘The Keuper saliferous beds of the Droitwich area’. Report of the Institute of Geological Science, (81/2), 19pp.)  The map below, which is from their report, gives much more detail than the BGS map.  The salt beds are shown by the diagonally hatched area. Some subsequent seismic work in the area apparently did not indicate changes in the map.

The thick line shows the estimated route of the ‘wild brine run’ which flows under the town from Yew Tree Hill to Stoke Prior.  The course of this is determined mainly from the subsidence effects at the ground surface.  For instance, at SO900612 on Yew Tree Hill a depression of about one metre depth and ten metres width once ran across the field in a NNE to SSW direction.  This area is now occupied by housing and so the feature has probably disappeared.

Fig 4. Extract from Poole and Williams (1981) (‘The Keuper saliferous beds of the Droitwich area’. Report of the Institute of Geological Science, (81/2), 19pp.)

The occurrence of salt beyond the Droitwich area

In the nineteenth century mineral waters were a valuable source of income from wealthy people with the leisure to move around the country, while by the early twentieth century the provision of fresh drinking water to the entire population had become an important concern. Geologists were employed to sample and analyse ground and spring waters for their mineral content and were correlating their results with the geological formations from which they came.

Hastings paper ‘On the salt springs of Worcestershire’. Analyst, 2 (12), 252-279.(1835) names the locations of nine saline springs in the county, at Hampton near Evesham, Hasler, Defford, Bourn Bank, Churchill, Stone Bow Turnpike, Cauldwell, Abberton and Pinvin. Hastings also mentions spas at Tewkesbury and Cheltenham as having a high salt content. To these can be added a known spring at St Mary’s church, Croome Hill, which is very close to the salty pools south east of Croome Park, identified by the Brine Memories group.

The salt content of some of these springs is given by Hastings, measured in grains per gallon. They are translated to modern units in the following table. Also listed for comparison is the salt content of a few familiar fluids. Note that the salt content of spas and spring waters mentioned by Hastings is similar to that of popular food products of today. The strongest Droitwich brine well, by contrast, was said to produce fully saturated brine at an eye-watering 260 grams per litre.

Brine from the rock salt of the Droitwich Halite beds contains predominantly sodium chloride salt, but the other springs listed contain variable quantities of other mineral salts including magnesium, calcium, carbonates, sulphates and “sulphuretted hydrogen” with its notorious bad eggs smell, while a few contain some iron (termed ‘chalybeate springs’).

Most of these springs can be identified on historic or modern maps and are plotted as blue dots on the geological map below. Major towns are shown as well as the two proposed areas of maximum extent of the Droitwich Halite Member.

Fig 5. Location of saline springs and spas identified by Hastings plotted on the BGS geological map

Most but not all of these springs lie within the areas where Droitwich Halite beds may occur in the underlying rocks, but those near Evesham and at Cheltenham to the south are beyond the maximum extent of these beds. We cannot therefore assume that the salt of the Droitwich Halite beds is the source of the salt in these waters.

Two of the springs come directly from the Droitwich Halite beds and there is no doubt of the origin of the salt there. The Droitwich Halite occurs within the Mercia Mudstone Group, coloured salmon pink on the geological map and occupying most of the Western part of area shown.

Running roughly north to south through the map is the escarpment that marks the transition from soft, upper Triassic mudstones to some harder rocks, which lie on top and in which all of the remaining spas and springs are found. Hastings calls these rocks the “Lias Formation”, which includes four formations in today’s stratigraphy (four coloured areas in the geological map). They were formed at the end of the Triassic and very beginning of the Jurassic period. Starting with the oldest, lowest formation (left on this map) we have:

1. Wilmcote Limestone, (pale dull green) consists of limestones and mudstone interbedded;
2. Saltford Shale (darker dull green) is mostly mudstone;
3. Rugby Limestone (pale dull green) and like the Wilmcote limestone in composition;
4. Charmouth Mudstone (bright pink) is mostly mudstone, similar to the Saltford Shale.

Many of the springs on these ‘Lias’ rocks lie on or very close to the interestingly named Saltford Shale (darker dull green on the map). Perhaps the salt in these springs originates directly from the Lias rocks, rather than from the halite within the underlying Mercia Mudstone.

The evidence is unclear on this point, but a number of factors may have a bearing on the question.

Many metres of relatively impervious mudstones lie between the halite beds, where present, and the Lias formations that lie above them, although weaknesses and mineral veins in the Mercia Mudstones could allow water to filter through in places.

Wells sunk into the Mercia Mudstones were, in the early nineteenth century, typically able to supply adequate drinking water to a small number of households and the water was effectively free of salt. This was true even in households in the Droitwich area.

The Lias formations were formed in a relatively quiescent, shallow marine environment, rich in mineral salts.

Wells in Worcestershire, sunk in these Lias formations and in use in the early twentieth century, generally yielded water with some salt content, and often the levels of salt made the water unfit to drink. (see next section).

Saline waters also occur within the same Lias formations in Somerset, where there is no suggestion of any halite beds in the underlying rocks.

The town of Saltford is located on the Saltford Shale formation, but neither the BGS memoir for the district nor an initial internet search has yielded any suggestion that the groundwater there is salty.

The famous nineteenth century geologist, Roderick Murchison, certainly believed that the source of the salt in the Cheltenham Spa waters was a “great sub-terranean store of rock salt”, and that other minerals were added as this water passed through the Lias formations.

Geologists no longer believe that rock salt is present in the mudstones beneath Cheltenham. Waters circulating deep underground commonly contain salt, so there is no need to postulate a concentrated source in the underlying formations. For instance, results from a deep borehole at Kempsey in the middle of the Worcester basin show that water in the sandstone strata beneath the Mercia Mudstone has salt concentration nearly up to that of sea water. As a second example, Tenbury Wells, at the far north west of the county (see fig 6) lies on entirely different and much older rocks and has a heritage of a spa with salty spring water from deep in the earth’s crust.

Problems of salt in drinking water

Further information on the mineral content of groundwater has come from investigations in the early twentieth century. The provision of healthy drinking water throughout the county had clearly become a political issue. Samples of water from domestic and community wells as well as streams were being assessed for their mineral content and degrees of contamination. A Geological Survey report by Richardson in 1930 on the wells and springs of Worcestershire assessed the state of drinking water across the entire county. For each parish, there is a summary of the state of water supplies at the time. These descriptions are supported by the results of more than 600 analyses, selected from more than 8000 made by the County Analyst between 1899 and 1924. As far as possible the results selected by Richardson were those rated as good or satisfactory, so we must assume that results of poor quality water have been included only if there is no alternative supply in the vicinity. The results do however provide an empirical overview of the levels of salt found in the groundwaters of different locations and rock formations within the county.

Of the 624 samples selected by Richardson, 70 contained levels of common salt higher than permitted by modern UK standards for drinking water. These are plotted in Fig 6 below.

Fig 6 The occurrence of common salt in Worcestershire water sources in 1930 relative to bedrock geology and proposed maximum extent of halite in underlying Mercia Mudstone

In seventeen of the sites plotted, the presence of salt would have been scarcely discernible, and in only a few wells was the salty taste moderate to strong, with householders having to travel some distance to obtain usable drinking water. Many of the water sources were condemned for reasons other than salt content, including contamination or the presence of sulphur and other minerals. These are not considered here.

Water sources with excess salt are clearly clustered within the area of the Lias beds, which  Richardson names the “Lower Lias” (the formations coloured dull green and bright pink in the east of the county). He says “The Lower Lias is a notoriously bad formation in which to seek water. Wells cut in the limestone beds tap limited supplies of very hard water, while those sunk in the succeeding clays sometimes fail to tap any at all, and where water is found, the bulk of it is more often than not surface or subsoil-water. Moreover, the water is hard, as a rule saline, and not infrequently impregnated with sulphuretted hydrogen”.

In contrast, he says of the Mercia Mudstone, or Keuper Marl as it was then called (salmon pink areas to the west) “Dug wells, which are anything up to 200 ft in depth, appear to yield as a rule a sufficiency of water for limited requirements. The water derived from the marls comes through fissures in the rock: it is on the hard side, sometimes too hard to be satisfactory for ordinary domestic purposes, but outside the Droitwich-Stoke Prior area no occurrence of brine has been noticed.”

These observations are consistent with the concentration of unsatisfactorily salty water sources in the Lower Lias areas. Different approaches to obtaining satisfactory water had been deployed in these areas. Where there is a sufficient depth of superficial deposits, such as river terrace sand and gravel, a properly constructed well could yield water of acceptable quality. Indeed, Richardson observes: “Areas of [superficial] sand and gravel – particularly those on the surface of the Lower Lias – have determined the original site of many a village. The deposit contained water … readily reached by shallow wells.” Habitation around Bredon Hill, was able to exploit the Birdlip Limestone Formation at its summit (bright yellow on the map) into which rainwater collected, emerging as springs at the junction with the underlying mudstones which carried the fresh water in streams downhill. Hence in these areas, salty water supplies did not seem to cause a problem.

Conclusions

This paper has touched on a number of topics relating to the presence of salt in the rocks of Worcestershire. The key points are summarised as follows.

• Geologists postulate that rock salt such as is found in the Droitwich area may occur underground in patches within a large area, extending to the south of Tewkesbury. At the time the salt was formed (about 200 million years ago), this was the lowest area in the Worcester basin. Playa lakes in the area would have evaporated in the hot dry climate, leaving salt deposits. It is unclear how extensive such deposits were.
• Brine from the Droitwich Halite beds contains very high concentrations of common salt, several times stronger than seawater.
• Groundwater elsewhere in Worcestershire may be saline or brackish, containing much lower concentrations of salt.
• Most sources of salty water occur in the Lias beds, formed in relatively still shallow seas, where mud slowly accumulated and limestones were formed intermittently by the marine lifeforms of the time.
• It is unclear whether, or to what extent, the Droitwich Halite beds contribute to the salt content in the higher formations of the Lias beds.

Kay Hughes and John Payne, May 2021

References

LJ Wills, ‘The palaeogeography of the Midlands’, 1950.

LJ Wills, ‘The Trias of Worcestershire and Warwickshire’. Rep Inst Geol Sci, (76/2), 211pp, 1976

C Hastings, ‘On the salt springs of Worcestershire’. Analyst, 2 (12), 252-279.(1835)

L Richardson, “Wells and Springs of Worcestershire”, Geological Survey of England and Wales, 1930.

GH Mitchell, RW Pocock, JH Taylor, “Geology of the country around Droitwich, Abberley and Kidderminster”, Geological Survey of Great Britain, Explanation of Geological Sheet 182, HMSO 1962

WJ Barclay, K Ambrose, RA Chadwick, TC Pharoah, “Geology of the country around Worcester”. Memoir for Geological Sheet 199, British Geological Survey, 1997

BC Worssam, RA Ellison, BSP Moorlock, “Geology of the country around Tewkesbury”. Memoir for Geological Sheet 216, British Geological Survey, 1989

AJM Barron, TH Sheppard, RW Gallois, RRN Hobbs, NJP Smith, “The geology of the Bath district: a brief explanation of the geological map Sheet 265 Bath.” BGS 2011”

PRN Hobbs, DC Entwisle, KJ Northmore, MG Sumbler, LD Jones, S. Kemp, S Self,  M Barron, JL Meakin, “Engineering Geology of British Rocks and Soils – Lias Group”, BGS 2012

W Page, ‘Geology’, in A History of the County of Somerset: Volume 1, London, 1906, pp. 1-33. British History Online http://www.british-history.ac.uk/vch/som/vol1/pp1-33 [accessed 25 April 2021].

“Rediscovered/Restored: The mineral well of Tenbury Wells, Worcestershire”  https://insearchofholywellsandhealingsprings.com/2018/08/19/the-mineral-springs-of-tenbury-wells-worcestershire/ [accessed 01 May 2021]