What is Salt?
Salt is the name commonly used for SODIUM CHLORIDE (NaCl) a naturally occurring compound of Sodium and Chlorine.
Common salt occurs abundantly in nature either as a solid: ROCK SALT or in solution in sea water, salt lakes and natural springs as liquid: BRINE. Rock Salt is composed of the mineral HALITE (from the Greek ‘Hals’ for salt) and is one of the minerals that precipitates out as water evaporates. These salty minerals are called EVAPORITES.
Table 1 The sequence of precipitation of evaporite minerals
1st Calcite (CaCO3) and dolomite (CaMg(CO3)2)
2nd Gypsum (CaSO4.2H2O) and anhydrite (CaSO4)
3rd Halite (common salt, NaCl)
4th Potassium and magnesium salts
Evaporite minerals (including Halite) are precipitated out in the sequence shown in Table 1 above. Evaporite formation tends to occur under hot, dry conditions such as those that exist today in Israel (the Dead Sea), the Persian Gulf or southern California (Death Valley).
Geological history of Salt in Cheshire
Under Cheshire’s flat and gently rolling rural landscape dotted with meres and flashes and small market and industrial towns, lie vast resources of halite. The salt beds in Cheshire are of TRIASSIC age, 220 million years old. During this time Cheshire was part of the supercontinent PANGAEA (from the Greek: combined Earth) when all the continental crust was together as one. Cheshire was in the tropics (15oN) in the arid, desert region of Pangaea just north of the Equator.
Pangaea started to break up as tectonic plates moved away from each other. In what is now Cheshire the land sank as a fault controlled basin formed: The Triassic Cheshire Plain. The basin floor kept sinking as the crust was pulled apart. First the basin was filled with massive sand dunes shaped by strong, persistent winds from the east. At the same time, gravelly sand banks were formed in wide shallow riverbeds by rare flash floods coming down from the surrounding rugged mountains. From the Lion Salt Works look east towards Macclesfield to see what is left of those mountains today – The Pennines. These Triassic sands and gravels are now called the SHERWOOD SANDSTONE of Alderley Edge and the Sandstone Ridge from Frodsham to Whitchurch.
As the mountains were eroded by wind and water erosion, the sinking basin started to flood as the sea crept over the sand dunes and sandbanks. In this shallow arm of the sea, with PLAYAS or salt lakes and tidal flats, layers of red muds and silts settled out of the still water over the Sherwood Sandstone to harden over geological time forming the Triassic MERCIA MUDSTONE.
The shallow water coupled with the hot, arid climate meant the salt lakes and lagoons frequently evaporated leaving behind crystals of solid salt with some anhydrite, gypsum and thin coverings of mud, and silt and sand blown in by the winds. The shallow briny water was topped up time and time again, as the sea re-flooded the sinking basin. In rainier times the salt layers were covered by thicker layers of mud. A great pile of mudstone and salt beds built up – at its maximum about 1500 metres thick.
The centre of the basin was just north of Middlewich at Byley where the thickest accumulation of the salt beds occurs. The quantity of halite in Cheshire has been estimated to be about 1200 million m3.
In the Triassic Mercia Mudstone the salt is concentrated into two major rock salt members. The upper member – the WILKESLEY HALITE about 300m separated by 400m of mudstone from the lower NORTHWICH HALITE about 200m thick. Each halite member is made up of thick, massive beds of reddish to pink to white rock salt, up to 40m thick interbedded with red mudstone, siltstone and some sandstone.
Two very pure and thick rock salt beds in the Northwich Halite have been heavily used by mining and brine pumping – the 20m thick TOP BED and the 30m thick BOTTOM BED. After the Triassic, Cheshire’s sandstones, mudstones and halites were tilted and faulted, covered by shallow seas and younger rocks as plate tectonics drove Cheshire and Britain northwards. Faulting means that the rock salt beds occur both near the surface and deep underground.
What were the effects of the Ice Ages on the salt deposits?
During the Ice Ages (starting 2 million years ago) the younger rocks above the Triassic rocks were systematically scraped away by ice sheets. When the last ice melted ~16,000 years ago, it left behind glacial till and outwash sands and gravels over the mudstone and salt beds. As the glacial water drained away, deep river valleys, such as the River Weaver and River Dane, were cut. When the floods of glacial meltwater decreased, they left behind POROUS AND PERMEABLE (holding water and allowing water to pass through) loose sand in the river valley bottoms.
Cheshire’s landscape was shaped by this massive splurge of glacial freshwater. Melt water streams and temporary lakes soaked down to the solid salt just below the surface, the salt dissolving to brine. The distinctive meres of mid and eastern Cheshire formed as the ground surface gave away and sank forming massive linear hollows some flooding to form meres. You can easily trace the sunken ground from Marbury Mere through the dry deep hollow south of Great Budwoth to Pickmere, Tabley Mere to Lower Peover; and the subsidence hollows and meres stretching from Rostherne to Tatton Park through Knutsford to Booths Mere.
The ancient faulting and permeable sands above the halite are pathways for the natural brine springs found all over Cheshire’s salt district; the reason for the salt towns of Northwich, Winsford, Middlewich and Nantwich.
What are the clues for salt underlying the landscape?
The existence of shallow rock salt is supported by evidence of brine springs, a landscape of ground surface movement, sink holes and subsidence.
Man’s use of salt
Since Celtic and Roman times Cheshire salt has been exploited from brine springs, hand dug brine pits and wells. Salt production grew quickly in the late 1700s onwards when wind and steam powered wild brine pumping started, followed by salt mining from the 19th century. In 20th century controlled brine pumping was developed to reduce subsidence. Now in the 21st century gigantic salt cavities are engineered to store natural gas underground.
What is the difference between Dry rock head and Wet rock head?
Where the area of rock salt occurs just below the ground level and is easily reached by water from the surface it is known as the WET ROCK HEAD as under Northwich, Winsford, Middlewich, and Sandbach. The wet rock head area is where natural brine springs and subsidence are present, and where wild brine pumping and salt mining has a long history and is still taking place.
Where rock salt is deeper below a protective layer of mudstone stopping water from reaching the salt the area is known as DRY ROCK HEAD. One such area stretches from Lostock and Plumley, south to Byley and Middlewich and another is under Warmingham. The dry rock head area is well suited for controlled brine pumping and gas storage in salt cavities.
What causes subsidence?
Salt is soluble in water so where water from rivers, streams and rain soaks into the ground finding pathways through loose sands, it dissolves any rock salt it taking away support for the overlying ground which collapses. The ground keeps collapsing as cavities migrate upwards and sinks holes and subsidence hollows form at the surface.
What is left behind is a COLLAPSE BRECCIA (from Italian – gravel) a leaky capping to the rock salt made up of broken mudstone, sands, clays and soils. Underground on top of the solid rock salt this collapse breccia is awash with brine flowing through the broken rocks and soils as an underground stream that can stretch for miles – BRINE RUN. Brine runs can sometimes be traced on the surface by lines of sink holes and flooded hollows such as the Moston Long Flash between Middlewich and Sandbach.
Where surface water collects in hollows it can leak through the collapse breccia to the underlying solid salt feeding in more fresh water so more salt dissolves and more sink holes appear. Solution will keep on taking place as long as there is an input of fresh water and a removal of the brine by springs. If there is no brine discharge at the surface then the ground water at the salt interface will become saturated with brine and salt solution will cease, the ground surface will stop sinking. However, if HYDRAULIC CONDITIONS (amount and movement of water above and below ground) change then salt solution, brines runs and ground surface collapse could be initiated or re-activated.
‘The possibility of salt solution and therefore subsidence exists throughout the salt subcrop or wet rockhead area.’ further subsidence could occur if ‘there is a fundamental change in the hydraulic regime in the ground’.
Earp J R and Taylor B J 1986 Geology of the Country around Chester and Winsford BGS
For salt mines, collapse breccia makes a weak, leaky roof to a mine. The salt miners dread fresh water – if this meets the solid rock salt, the salt dissolves taking away any support holding up the ground above the mine.
‘I have frequently explained that fresh water is the greatest destroyer of rock salt, and the worst enemy the rock salt miner has to encounter’
Thomas Ward 1898 Rock Salt Deposits of Cheshire and their explanation: Transactions of the Manchester Geological Society.
A case study of subsidence
THE GREAT COLLAPSE of Northwich and Wincham salt mines in 1880 led to the formation of Neumanns’s and Ashton’s Flashes. The ground sank dramatically as the collapsed breccia roof above dry rock salt mines disintegrated after fresh water from Wade Brook leaked down to the solid rock salt dissolving it.
‘A little before 6 o’clock in the morning of the 6th December 1880, before the men went down, the inundation came. The land slipped from under the Peover (now Wade) Brook and water poured in…… the Weaver flowed backwards and filled the mine and the adjoining old workings at a rapid rate’
Joseph Dickinson HM Inspector of Mines
A Report on the Flooding of the Platt’s Hill Salt Mine, near Northwich.
Not all the flooded 19th Century mines collapsed. Mines such as the Witton Hall Mine and Baron’s Quay Mine under Northwich were full of saturated brine with a few pillars of rock salt remaining solid and intact. For 100 years after mining ceased, the pillars gave little support to the ground above which suffered intermittent subsidence. Even in the 1980s and 1990s the ground was only strong enough to support light buildings or none at all. It took until the 21st Century to fill the mines and stabilize the area in a massive engineering project that pumped a grout mix of pulverised fly ash and salt into the old flooded salt mines beneath Northwich.
But what of the future?
Climate change will lead to changes in the hydraulic conditions, changes to more intense, more prolonged rainfall, increased river flow, higher water table and sea level. Sink holes are still active and as Cheshire’s weather turns wetter could new ones form and old ones be reactivated?
The changing landscape of past, present and future ground condition in the Cheshire salt fields poses geotechnical and engineering challenges when building houses, development and transport routes. Cheshire has developed a distinctive style of architecture – the black and white timber framed ‘jackable’ buildings, road, railway and canal bridges on ‘adjustable’ footings. Even the planned M6 Motorway route was moved further east to avoid the tricky ground of the Cheshire salt district.
The Lion Salt Works Museum gratefully acknowledge the assistance of The Curry
Fund of the Geologists’ Association