saltstone
Salt that has accumulated and hardened into a rocklike consistency.
countable, uncountable
The amount of saltstone to be produced at Savannah River is very large, about 3 million tons over 28 years; […]
Where it comes from
From Middle English saltstone, salteston, salt stan, from Old English sealtstān (“rock salt, niter”), from Proto-West Germanic *saltastain, equivalent to salt + stone.
Adapted from the Wiktionary entry saltstone (CC BY-SA 4.0).