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.
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.
