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Once upon time, there was a shallow inland sea, leftover meltwater from a previous ice age trapped because of nearby mountain ranges. The arid climate with almost no rain and weak flow to and from the sea ensured that the water became salty over time. Especially common was the formation of brine pools at the bottom of this sea.

Then the sea slowly began to recede as the ground was rising, leaving only a thin strip of brine pools and later salt plains after the last water had evaporated as well as unpleasant marshes and plenty of salty little lakes.

Or so I have been told about the history of that inland sea. Anyway, there is a problem with those salt plains and their salt: something makes it useless for trade or unwanted for everyday usage. I have confirmed that it must be something in its composition or contamination that cannot be removed with late iron-age or very early medieval techniques. Otherwise, people would be flocking in and harvesting this salt as the marshes are no more dangerous than marshes of our world—maybe even less without those pesky salt water alligators around.

It cannot be an overly toxic reason, because one can (barely) live on those marshes, and some tougher flora and fauna exist. However, one needs be very desperate to live in those marshes. Or, unlucky enough that their story takes them into that place.

So in short, what is in this salt of those salt plains which makes it useless?

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    $\begingroup$ You're describing the Great Salt Lake. $\endgroup$
    – RonJohn
    Jul 20, 2023 at 14:04
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    $\begingroup$ Not surprising. Great Salt Lake and Lake Bonneville were parts of research to see if this kind of inland sea would have been at least plausible. $\endgroup$ Jul 20, 2023 at 15:20

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It has a lot of sodium sulfate, specifically Glauber's salt.

sodium sulfate decahydrate (Mirabilite) is a strong laxative, so people might occasionally harvest it for medical uses, but no one is going to willing use it for food preservation. It is a normal salt found in high concentrations in some lake deposits, and its not really toxic just highly unpleasant, so people can live there, they just don't want to. When Mirabilite showed up in normal salt mines it was valuable in small amounts (again strong laxative) but if the whole mine/basin is loaded with it the salt is useless for the normal uses of salt.

The conditions the basin formed in you describe is completely consistent with this kind of deposit so you don't even have to change anything.

For full potency combine with smaller amounts of sodium sulfide and it becomes foul tasting and irritating so it won't even be valuable as a laxative. As a side effect the combination salt would be an effective treatment for cyanide ingestion, so plot hook.

Now in a few hundred years this will be highly valuable for all kinds of chemical production, but with an early medieval tech level it makes the location barely habitable and is almost useless, certainly not an economic draw.

"Why don't you use the salt?"

"It tastes disgusting and gives you a really bad case of the trots"

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    $\begingroup$ I like this answer and it would be easily rationalized by the presence of a low-pressure hot springs that became apparent as the inland sea evaporated to taint all the salt. The NACL plus the S (and atmospheric O) gets you the laxative NA2SO4 (and HCL Yay! Acid!) but it will also create some NA2S, which is definitely not a laxative. $\endgroup$
    – JBH
    Jul 19, 2023 at 23:40
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    $\begingroup$ @JBH there are several existing salt flats formed from evaporative lakes with a lot of Glauber's salt and other sulfur salts, today it is highly valuable for all kinds of industry but pre-chemistry it just makes the salt useless. You just need a lot of sulfur in the water to get it. $\endgroup$
    – John
    Jul 19, 2023 at 23:45
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    $\begingroup$ @JBH there are actually many ways, its fairly common in different concentrations in saline lakes or payas, it just about what your source rocks are but bacterial facilitated, volcanic mixing, sublimation heavy evaporation induced, or even just secondary redeposition of existing salt deposits are also known .link.springer.com/article/10.1023/A:1014565619506 you just need sodium and sulfur rich water evaporating a low temperatures or reworking of other sodium sulfur salts. there are deposits from Antarctica to Arizona. $\endgroup$
    – John
    Jul 20, 2023 at 0:43
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    $\begingroup$ The salt flats are home to many bizarre creatures, including the salt rat. Salt rats are rodents of burden, essential to agricultural success on the harsh marsh. Nations across the land covet the industrious nature of the salt rat, but unfortunately, they cannot survive anywhere beyond the salt flats. Without their native salt, they die of constipation. $\endgroup$
    – skeep
    Jul 20, 2023 at 13:09
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    $\begingroup$ I wonder how should I have forgotten our little humble sulfur, despise it being one of alchemical elements. Can I assume that it also gives those marshes "pleasant" scent? $\endgroup$ Jul 20, 2023 at 13:16
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It tastes bad

There are a number of naturally occurring sulfates that could render your salt tasting really bad if introduced in non-toxic quantities.
Some are so bad that they can leave a bad taste in your mouth for up to 24 hours.
While these salts will not necessarily kill you to eat a little bit of, they will taste way too bitter or metallic for anyone to want to season their food with them.

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Your salt plains happen to be on a moderately-sized radium deposit. Not only do you have tasty sea salt in the plains, but also spicy salts like radium chloride and radium bromide. There are pockets where the concentration is low enough that folks can build a village. But if you venture too far out into the flats, you develop an acute case of radiation sickness.

Alternatively, every few years the Interstellar Racing League drops by for their salt flats racing circuit. They don't mind the locals, but the alien racing tech has a way of incinerating most living creatures that don't keep their distance. Every culture has its local folklore. Instead of sasquatch, folks on the salt plains are always telling tales about strange creatures that emerge from the marshes to ride their thunderslugs.

enter image description here

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    $\begingroup$ +1 for "thunderslug"! BrontoLimax? Any sufficiently abundant heavy metal would do to dissuade individuals from ingesting the salt. IF they are able to make the connection to the affects of the salt to sicknesses. $\endgroup$
    – Gillgamesh
    Jul 19, 2023 at 20:15
  • $\begingroup$ @Gillgamesh I don't know what BrontoLimax means, but it sounds like great name for a thunderslug. I came up with the word thunderslug because (a) slugs are very aerodynamic creatures and (b) thunder is the sound a slug makes at Mach 1. $\endgroup$
    – skeep
    Jul 20, 2023 at 2:21
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    $\begingroup$ Barosaurus = Thunder Lizard. Limax is latin for slug. Cheers! $\endgroup$
    – Gillgamesh
    Jul 20, 2023 at 12:29
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    $\begingroup$ Brontolimax salsus. I like the sound of it! $\endgroup$
    – skeep
    Jul 20, 2023 at 13:15
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Too Much Magnesium

It's still usable and perfectly functional as salt. It just tastes bad. Turns out people would rather pay extra for salt that makes their food delicious over salt that turns everything bitter, so it has minimal value and just stays put.

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    $\begingroup$ You’re about to become a magnesium-based lifeform. The age of the primitive carbon-man is done. No longer must mankind rely on slow-working background radiation to take us further into our genetic destiny. This is the era of guided evolution, and magnesium is the key. $\endgroup$
    – SPavel
    Jul 21, 2023 at 18:14
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Salty sea? No, I mean Salton Sea!

Your salt and marshes are similar to the real life Salton sea. It's not that the salt isn't useful. It's that the surrounding marshes have a "unique" and "pervasive" smell, along with massive algal blooms, that dry into dust, get vaporized and then inhaled by anyone unlucky enough to be there. Birds regularly drop dead. So does the occasional person. Local legends spring up of the dried land with dust that kills.

Sure, some enterprising businessman might show up and have his people drag blocks of salt out, but it's all contaminated with toxic algae. Dragging stuff through the dried, toxic algae filled mud is also terrible, frequently stirring up dust clouds that cause massive respiratory distress.

It makes salt pan work feel like a trip to the seaside

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That dalt is laced with lead, due to the region being rich in lead-zync ore deposits (stuff like galenas and calcite).

The local wildlife has evolved to cope with it. But humans? Wikipedia has some not so nice things to say about lead:

Lead poisoning, also known as plumbism and saturnism, is a type of metal poisoning caused by lead in the body. The brain is the most sensitive. Symptoms may include abdominal pain, constipation, headaches, irritability, memory problems, infertility, and tingling in the hands and feet. It causes almost 10% of intellectual disability of otherwise unknown cause and can result in behavioral problems. Some of the effects are permanent. In severe cases, anemia, seizures, coma, or death may occur.

In 2013, lead is believed to have resulted in 853,000 deaths worldwide.

People in your world are aware of this and want nothing to do with that salt. Also it might contain enough lead to poison people but not enough that you could smelt actual usable lead from it.

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Perhaps it was an alkaline soda lake (or became one at some point). This is where a lake contains a large amount of dissolved carbon dioxide and carbonates and there is no magnesium or calcium in the surrounding rocks to precipitate out as calcium and magnesium carbonates. This leaves sodium carbonate levels to build up along with other brines.

https://en.wikipedia.org/wiki/Soda_lake

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Kind of summarizing a bunch of other answers here, but:

There are enough traces of water-soluable salts other than NaCl that the salt is not pleasant to consume for some reason.

Essentially, if the contaminants are at least moderately water soluble, they won’t be separable without using chemical reactions.

Magnesium chloride and sodium sulfate have both been brought up by other answers in detail. The first is nastily bitter, and the second is a rather annoyingly effective laxative.

But there are all kinds of other salts that would work for this. Just off the top of my head:

  • Sodium carbonate: Non-toxic, but moderately alkaline in solution. Occurs naturally in some salt beds (the specific minerals to look up are natron and thermonatron). This actually has culinary uses for a number of reasons, and in fact would help the salt preserve the food a bit better, but on it’s own it tastes kind of nasty (think baking soda, but stronger), and the flavor/texture of things preserved with it is objectionable to some people (it’s similar to lutefisk).
  • Ammonium chloride: Non-toxic, also occurs naturally in some salt beds (sal ammoniac is the usual mineral name). This one is also used in some cases in cooking (most recognizably in the salty liccorice that is popular in the Nordic countries), but it’s also got a distinctly different flavor from regular sodium chloride that is objectionable to some people.
  • Sodium nitrate and sodium nitrite: Non-toxic, can occur naturally in salt flats. These are actually used as food preservatives in small amounts, especially sodium nitrite (which reacts with the myoglobin in meat to give it the rather nice pink color often associated with hot dogs and bologna sausage). They have very little flavor though, so ‘salt’ with high concentrations of either doesn’t really taste all that ‘salty’, making it less than ideal for other culinary uses.
  • Lithium chloride: Non-toxic (probably, literature is inconclusive, but most cases of supposed toxicity reported appear to actually be cases of hyponatremia instead), but it tastes somewhat strange (exact description varies by individual, but one of the common aspects is a sharp metallic aftertaste) and it has interesting (but largely unexplained) effects on the human central nervous system (some lithium salts, especially lithium carbonate, are actually used to treat mood disorders). It’s also strongly hygroscopic, which would result in high enough concentrations causing the harvested salt to clump together more readily than pure sodium chloride would.
  • Iron (II) chloride and iron (III) chloride: These aren’t technically toxic, but you really do not want to eat them. In very small trace concentrations they would impart a ‘rusty’ or metallic flavor due to the iron ions. In larger concentrations they cause nasty chemical burns (to the point that touching the powder while damp could cause such a reaction).
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There are many different salts, and we mainly use Na+Cl-, sodium cloride. This could be another salt, or it could mainly be sodium cloride, but too much of other salts mixed in to be healthy. It all boils down to the geology nearby. What ions will be extracted from the soil and rocks there. Copper sulphate is poisonous for instance, and sodium fluoroacetate is bad enough to be used as rat poison, and occurs naturally.

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Just get the toxicity in the right range. Makethe toxicity just large enough that generously seasoning your food with it can make you moderately ill, but not just low enough that you can live their without becoming seriously ill. Preferably make the winds either quite mild or have them prefer a specific direction, areas upwind will probably be considerably more habitable due to lower contamination.

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  • Due to a freak of topography, your marshlands have too much salt. The salt itself is fine, but living there even for a short harvesting trip is hell.
  • Nearby are wind-swept potash deposits, which lead to respiratory illnesses among the harvesters.
  • I was wondering if there was too much sylvite in the halite, but I guess iron age workers could separate them sufficiently by removing the brine at the right point.
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    $\begingroup$ the OP specifically says they already considered and rejected this answer. $\endgroup$
    – John
    Jul 19, 2023 at 23:56
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    $\begingroup$ @John, it was rejected to have them overly toxic. What I describe would be livable, just barely, and not a nice place. $\endgroup$
    – o.m.
    Jul 20, 2023 at 4:13

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