A look at layers in the walls at the Kansas Underground Salt Museum (recently renamed Strataca) in Hutchinson, Kansas:
The salt deposit that is being mined in Kansas forms a bed about 400 feet thick, 150 miles wide, and 200 miles long, and contains approximately 30 trillion tons of salt. This was all laid down in the Permian period, some 275 million years ago.
At the time, Kansas was in the northern tropics and was partially covered by a shallow sea. Some branches of this sea were apparently cut off from the general circulation — at least from time to time. So when this branch saw little inflow from streams, it could evaporate, leaving a deposit of relatively clean and clear salt behind. When sediment was washed into it from streams, a thin dark coating of mud settled over the last salt layer.
A variety of processes can be used to clean rock salt for use in food for people, but this deposit is mostly used as-is for road salt and in livestock feed.