Blimey, hope it wasn't rat poison. Might be worth checking with the farmer or someone.
(Sorry to be alarmist, but I have experience of this being done without warning notices being put up).
That actually is quite logical, chalk powder for pentagon making perhaps? I have found food up at WKLB on a top ledge, but I doubt poison would be put down where the public goes, but leaving food does encourage rats, etc... they have got a particular disease rats which we can catch, but can't recall the name..
we washed it off with scrumpy afterwards! i dont think it was rat poison- it was like a chalky white paint. i think it was more to do with some magick ritual as i found a few different man made objects in there too. or it may have been some kind of naturally occuring residue