Hi Coggy.
A couple of years ago, when I was trying to find the old deep salt mine near Droitwich, I was told that there were a couple of fields nearby with blocks of gypsum around the edges. I've never found them, but I believe the old deep salt mine to be around SO933656.
Reports of the salt deposits in Droitwich als refer to layers of gypsum in the local rocks and it's not too far from Spernall. Where did you find the possible adit?
Where there's a mine or a hole in the ground.
That's where I'm heading for that's where I'm bound
So follow me down Cousin Jack
(Grateful thanks to Show of Hands)