FORMATION AND EXTENT OF THE CHESHIRE DEPOSITS
The theories propounded and the conclusions arrived at on the
subject of the formation of the Cheshire salt beds do not differ in
any important particular from those which have been put forward,
investigated, and accepted with regard to rock-salt deposits in all
parts of the world, but, because of the enormous geologic and climatic
changes that have occurred in the English county since a salt basin
was in course of formation there, scientists were slower in accepting
those conclusions in respect of our home deposits than in the case of
the salt areas which are found in the Runn of Kutch, at Lake Elton, or
Black Gulph on the eastern side of the Caspian Sea.
The facts that the chief accompaniment of every known deposit of
rock-salt is clay, and that clay is deposited in water, formed the
basis of the erroneous theory that because salt is a deposit out of
water, and sea-water contains salt, all salt beds must have been
deposited in the sea. But salt does not mix mechanically with water and
has not been deposited like sedimentary rocks; it forms a solution, and
not until the solution becomes super-saturated does it crystallize out.
Now sea-water rarely contains more than 3½ per cent. of salt, and since
the solution must contain at least 26 per cent. of salt before the salt
will crystallize out, and, provided it is left from contact with the
air, a solution of this strength may be left for an indefinite length
of time without a single particle of salt depositing, the old theory
that all salt beds were deposited in the sea had to be abandoned.
The theory of the sea-water deposition of salt beds having been
disposed of, it was long a popular idea that the beds of rock-salt
owed their formation to volcanic action. Professor C. Thompson was
of opinion that some enormous electrical force had been at work in
its crystallization; Professor Silvestri found quantities of chloride
of sodium varying from 50 to 90 per cent. in different sublimations
in the lava which was erupted from Etna in 1863; Bunsen discovered a
considerable but less important sublimation of chloride of sodium in
the lava erupted from Hekla in 1854; and G. F. Rodwell and H. M. Elder
also recognized small traces of sodic chloride as one of the products
of volcanic action. In a paper contributed to the Manchester Geological
Society, in 1842, on “An Inquiry into the Origin of the Salt Field of
Cheshire,” so respected an authority as Ormerod stated his conclusions
as follows--
“(_a_) That from the lithological character of the accompanying
beds and partings, and from the regularity in the thickness of the
respective beds, as far as the same were now known, these salt beds
were, in his opinion, deposited from an aqueous menstruum, and had not
been injected.
“(_b_) That from the absence of marine remains, from the salt deposits
containing matter not found in the ocean, and from similar beds of salt
not being in any place known to have been formed from the ocean, he
considered that there were not satisfactory reasons for ascribing the
origin of the salt found in the new red sandstone of England to marine
deposits.
“(_c_) That from the minerals found associated with the salt, and
adjoining red sandstone rocks, being similar to those found together
with it in volcanic districts in other parts of the world; that from
former or present volcanic action being apparent at localities in
various parts of the globe, at which beds of salt of similar character
are found, and the origin of which can be evidently traced to that
cause, and from the salt beds in England being always found accompanied
by neighbouring traces of volcanic action, he considered that there
were satisfactory reasons for ascribing the origin of the salt fields
of England to volcanic agency.”
Ormerod was not only convinced that the Cheshire deposits were
the result of volcanic action which had impregnated neighbouring
lagoons and formed the aqueous menstruum from which those beds were
precipitated, but that these lakes lay in depressions of the upper
New Red Sandstone, and that the alternation of the strata of rock and
salt had arisen from subsidences, followed or accompanied by fresh
discharges of the same impregnating matter.
This theory is untenable, for beyond the fact that salt has been
ejected in volcanic eruptions there is practically nothing to support
it. Volcanic action is always accompanied by intense heat, and the fact
that the pure rock crystal is one of slow growth in a cool liquid, and
is not of rapid formation in a hot fluid, conclusively disposes of the
volcanic theory. Particles of chloride of sodium in volcanic ejections
were no explanation of the formation of huge deposits of rock-salt, and
since it was realized that salt in large quantity can only be obtained
from salt water, and that it cannot be got naturally from the sea, it
became evident that what man does in isolating tracts of sea-water to
produce salt by solar evaporation, must have been practised by nature
on an extensive scale in all ages. And as an isolated tract of salt
water is a salt lake, we are directed to the obvious conclusion that
all rock-salt formations have been deposited in salt lakes.
In support of this theory we have the evidence of the salt-forming
process that is now in operation in Southern Russia, America, and
India. It is evident that at one time the low-lying country to the
west and north of the Caspian Sea was part of that inland sea, and
that, when its surface was contracted by shrinkage, the retreating
water left behind it numerous swamps, which now form salt lakes, and
tracts of intervening land which, in the dry season, are covered with a
saline afflorescence. The large quantities of salt which, in ordinary
seasons are deposited in these salt lakes, are collected by the Russian
Government. In India there are many salt lakes, such as Lake Sambhur,
in Rajpootana, which in the rainy season has a length of from fifteen
to twenty miles, but in the dry season is only three or four miles
long, the remainder of its course consisting of a succession of small
salt pools alternating with stretches of salt-encrusted ground. In the
great desert of Mongolia many square miles of country are spread with
salt incrustations; and in America similar tracts are found which once
formed the beds of considerable lakes. In Nevada, at the sink of the
Carson River, is an area of five square miles which was once the bed of
a salt lake. The famous Great Salt Lake, between the Wahsatch Mountains
and the Nevadas in America, is the remains of a large inland sea which
once covered the district, and should the climate become drier than it
is now, the shrinkage, which went on for ages, will be resumed, and a
huge salt deposit will be formed.
The salt lakes in rainless districts soon dry up, and the salt, being
quickly deposited, is almost pure, but such instances are not usual,
and, in dealing with existing salt-depositing lakes, we find continual
references to the salt and clay mixtures, or alternations of the
deposits. Herr Cech tells us that the yearly layers of salt in Lake
Elton are separated from one another by a layer of black mud; beneath
the fourth layer is found black clay, and beneath this are further
layers of salt of a more solid quality. Schleiden, in speaking of
Lake Elton, says: “On this old salt is deposited a blackish mud layer
(salt clay) which separates the salt from the next succeeding layer.
In 1805 Göbel bored, in the very shallow lake, about 1½ miles from the
shore. He found forty-two distinctly separated layers of rock salt,
the uppermost from 1 to 4 inches thick, the lowest 9 inches thick. The
deeper he bored the more solid the salt was, and the more pure. At the
hundredth layer the salt was so hard that the iron tool broke.”
From the foregoing, which are among a great collection of accepted
data, it will be seen that, in whatever quarter of the world salt
lakes occur, the same characteristics are encountered, viz., salt
depositing on mud and covered by mud. Every shower of rain creates a
certain amount of mud or sand, and every brook and stream running into
the salt lakes during the rainy season brings in a certain quantity of
the same material. The mud represents the wet season of the year, and
the salt the dry season. The geological conditions must have been the
same when salt was deposited in Cheshire, and with the instances of
modern salt-forming regions before us, and the strata of the Cheshire
salt country to guide us, it must be concluded that the genesis of
rock-salt, modified by local circumstances, must have been the same
in every case. Indeed, in the face of the evidence, it seems certain
that the Cheshire beds of rock-salt have been crystallized out of the
saturated waters of salt lakes, and that their admixture of marl has
been caused by streams running into the lakes during the wet seasons,
and that the peculiar amorphous mixture of marl and salt known as
rock-salt is the result of the continual growth of pure salt crystals,
and their partial destruction by mud-bearing fresh waters.
This conclusion on the subject, which is now generally accepted, is
based on the theory that the Cheshire salt lake was situated in a
desert, or more probably a salty _steppe_, such as are found in the
region of the Caspian Sea, and that the climate was divided into wet
and dry seasons. The presence of rock-salt supports these ideas,
because the marls could only be formed in periods of heavy rainfall,
and the salt could only crystallize out in dry, water-evaporating
periods. It is further evident that the lake, though extensive in area,
was shallow, and that the dry seasons produced extensive shrinkages
and caused salt to form in the saturated water that remained in the
deeper parts, while the occurrence of the deep deposits in a shallow
lake is explained by the constantly varying elevation and depression
of the earth’s surface. The difficulty of explaining how the salt in
this lake could be renewed to enable the waters to go on depositing for
a geologic age is recognized, but it is no greater than that which is
presented by scores of existing salt lakes out of which thousands of
tons of salt are taken annually without causing any apparent diminution
in the salt which forms year by year. And when it is considered that,
in a lake having a probable area of from 500 to 1,000 square miles, the
known salt deposits do not occupy 50 square miles, and in many portions
contain 50 per cent. of marl, the difficulty does not seem to be
insuperable. It is, moreover, safe to conclude that, when the bar rose
that eventually cut off the Cheshire lake from the sea, it would be
many years before the high tides ceased to wash over it and replenish
the lake, and Dr. Ball’s theory as to the enormity of the tides that
occurred in past ages--owing to the moon being nearer to the earth
than at present--reveals a means by which the lake might continue to
receive fresh accessions of sea-water for many generations.
[Illustration: STREET-RAISING IN PROGRESS--HIGH STREET, NORTHWICH]
Irrespective of all theories, the outstanding fact remains that
enormous beds of salt were deposited in the Cheshire salt lake, and an
examination of the strata in the appended Northwich section will enable
the salt to tell its own history.
_Depth._ _Thickness._
Ft. in. Ft. in.
1 6 1 6 Soil.
9 0 7 6 Drift composed of brown sand mixed with clay
varying from 1 to 100 ft. in thickness.
27 0 18 0 Brown clay with greenstone, etc., boulders.
132 0 105 0 Marl in thin bands, brown and blue with thin
beds and streaks of gypsum to the rock head.
216 0 84 0 Rock-salt, top bed.
222 0 6 0 Upper blue marlstone mixed with brown, which
falls on exposure.
229 0 7 0 Brown marl and marlstone, with vein of red
rock-salt.
234 0 5 0 Lower blue marlstone, very compact, hard,
and does not fall on exposure.
(This forms the foundation for the
wedging-curb of the shaft cylinders.)
246 0 12 0 Marl and rock-salt mixed in about equal parts.
330 0 84 0 Rock-salt, bottom bed.
334 0 4 0 Brown and blue marlstone, with rock-salt.
417 0 83 0 Ditto with thin veins of rock-salt, ramifying
in various directions.
320 0 3 0 Rock-salt, almost pure.
501 0 81 0 Brown and blue marlstone, with thin veins of
rock-salt.
507 0 6 0 Rock-salt, almost transparent.
525 0 18 0 Hard blue marlstone, not sunk through.
The formation has only been bored through to a depth of 525 ft., where
we find an unpierced stratum, 18 ft. thick, of hard marl. Above it
are 6 ft. of pure rock-salt, then 81 ft., of marl with thick veins of
rock-salt, then 3 ft. of nearly pure salt, then 83 ft. of marl with
thin veins of salt, and above it 4 ft. of marl and salt. So far it is
evident that the wet seasons predominated, and that marl was deposited
far more extensively than salt. For a time, a cycle of dry seasons
prevailed; a great change occurred, and a bed of rock-salt, 84 ft. in
thickness, was deposited. In other parts, the bed of rock-salt varies
from 80 ft. to over 100 ft. in thickness, none of which is perfectly
pure, and not more than 20 ft. of it is sufficiently pure to be of
commercial use. The greatly changed seasons are indicated by these
formations. A portion near the bottom, containing less clay, shows
a less copious or less protracted rainfall, and these periods were
followed by wet seasons and the presence of much clay. After a time,
so much rain fell that for a period sufficiently long for about 30 ft.
of marl to deposit, practically no salt formed. Here and there in this
deposit are veins of salt, and as these are perpendicular and run as if
deposited in rifts or cracks of the marl, the salt doubtless belongs to
the next period, when another change occurred and another bed of salt,
varying from 50 to 80 ft. in thickness, was deposited. The whole of
this bed is fairly full of marl, and, for an untold period, marls were
deposited, covering up the rock-salt.
The cycles of greater or less rainfalls are traceable in the varying
preponderance of marls, in the crystallization of salt, and in the form
in which the rock-salt is found. Each minute cube starts as a crystal
from some independent point of rock salt, and these increase in numbers
until they form a mass of crystallization possessing no distinct lines
or features. Had the dry season continued for a long period a thick
mass of rock-salt would have been formed. The floor of the lake would
have been covered with salt crystals, like the crystal floor of a
mine, and the moment the rainy season commenced, and the brooks began
to bring in fresh water and mud, these crystals, being attacked by
non-saturated water, immediately lost their sharp angles and became
covered with a fine layer of mud. As soon as the crystals became
completely covered they ceased to dissolve, but the angles and cubes
disappeared, and a shapeless mass of mixed salt and mud was formed.
With the next dry season, crystallization again set in and another
crystal floor was produced, to be again destroyed by the succeeding wet
season. This constant growth and destruction of crystals went on for
ages, until the salt beds were formed and the water ceased to become
super-saturated.
Scientific exploration work and a great number of borings have enabled
us to form a fairly accurate estimate of the area of the Cheshire
salt-beds, except in the region to the north of the deposits, where
systematic examination has still to be undertaken. Without quoting the
exact locations of bore-holes and distances between them--particulars
which would convey little or nothing to the general reader--it may be
broadly stated that the proved salt area in the Northwich district is
about four square miles, while the increasing quantity of marl that
is mixed with the salt to the northward favours the probability that
the beds soon die out in that direction. The Winsford salt district
comprises an area of six square miles, while it is calculated, with
less preciseness, that the Middlewich, Nantwich, and Lawton districts
all contain large quantities of rock-salt. At the bore-hole at Marston,
which appears to be on the highest proved portion of the salt-bed, the
salt is found at 47 ft. below ordnance datum, and from this central
point the surface of the salt falls away gently in every direction.
Mr. James Thompson, a recognized local authority upon salt and
salt-mining, writing on the subject nearly fifty years ago, gave the
thickness of the upper bed of rock-salt at about twenty-five yards, but
that thickness was only maintained within a circle of about three miles
in circumference, beyond which he found that it thinned off rapidly on
the upper surface. The extent of the second or bottom bed, from which
all the rock-salt produced in Cheshire since 1780 has been extracted,
is less clearly defined, but it is known to underlie not only the whole
of the upper bed, but a further considerable area in all directions.
[Illustration: THIS ROAD WAS RAISED TWENTY FEET IN TWENTY YEARS. NONE
OF THESE BUILDINGS IS NOW STANDING]
Professor Thompson, in calculating the period of time that was required
to lay the salt contents comprised in these deposits, fixed upon an
inch in ten years as a fair estimate of the rate of progress at which
it was accumulated, and found that it must have taken 21,000 years to
lay 60 yds. of rock-salt. With this figure before us, it is interesting
to study the following calculation of the salt contents of the Cheshire
deposits and of the quantity of mineral that is extracted from the
interior of the earth in the form of brine to produce the salt that is
made in the Cheshire districts.
Calculating the Northwich salt area at 3 square miles or 1,920 acres or
9,292,800 square yards, and
Taking the upper bed of rock-salt at an average of 25 yds.
thick, we have 232,320,000 cubic yds. of rock-salt.
Taking the specific gravity of rock-salt at 2·125, a cubic yard
of rock-salt weighs 32 cwts., therefore weight of rock-salt in
upper bed
232,020,000 × 32 tons / 20 = 371,702,000 tons.
Taking the bottom bed as extending over the same area, but having a
thickness of 35 yds., we find in it--
9,292,800 × 35 × 32 / 20 = 520,396,800 tons,
or, in both beds together, 892,108,800 tons.
The Winsford district, taking the beds of rock-salt at an average
thickness of 65 yds., which is 5 yds. less than the figure given by
Dickinson, we have 1,932,902,400 tons.
As the whole of the white salt has been manufactured from brine derived
from the rock-salt, it represents so many tons of rock-salt pumped
up. Now, as the specific gravity of rock-salt is 2·125, a cubic yard
contains 32 cwts. This being the case, we find the cubic yards of
rock-salt pumped up annually in each district to be, viz.--
In WINSFORD DISTRICT--
687,000 × 20 / 32 = 429,375 cubic yds.
In NORTHWICH DISTRICT--
587,000 × 20 / 32 = 366,875 cubic yds.
In MIDDLEWICH DISTRICT--
14,000 × 20 / 32 = 8,750 cubic yds.
In SANDBACH DISTRICT--
78,000 × 20 / 32 = 48,750 cubic yds.
Making a total of 853,750 cubic yds. This represents 176·5 acres of 1
yd. thick.
This is entirely independent of the rock-salt, which, at a low
estimate, equals 120,000 tons per annum, or, say, 75,000 cubic yds., or
15·5 acres of 1 yd. thick.
In these calculations no allowance has been made for wastage, and this
is very large. During the year every pan requires picking from six to
twelve times, the stoved oftener than the common. This necessitates the
pan being swept out and an enormous quantity of brine wasted. Besides
this, the pan scale contains a large percentage of salt. Again, in
_drawing_ the salt out of the pans a large quantity of brine is wasted.
Add to this also the leakage in pipes, overflow of cisterns, leakage
through defective pans, etc., and the total of wastage will be very
large. It is scarcely possible to estimate this, but if we calculate 10
per cent. we shall be under the mark. Thus, for waste, we may set down
136,600 tons. This would represent 85,075 cubic yds., or 17·65 acres 1
yd. thick.
We thus see that 209·65 acres of rock-salt 1 yd. thick is every year
consumed in the Cheshire salt district.