Evidence from four recent boreholes in the Yorkshire potash field suggests that carnallite was an important original constituent of the Middle Evaporite Bed, and that much of the sylvine of the potash zone is of secondary origin.

The carnallitie rocks have been affected by complex replacements, an earlier series involving sylvine, halite, and anhydrite, and a later series involving rinneite, halite, sylvine, and carnallite.