There was a reportedly huge chalk mine under the Plumstead area that caused serious subsidence problems in the 1950's, I think even killing one unfortunate council worker who slipped into a sinkhole so its surprising they allowed building over this place.
I've been a bit confused about the housing development too; I thought building on a chalk mine was dangerous - let alone when you're using that chalk to soak away excess storm water..! I think Dartford and Plumstead were part of a trio, along with Crayford. If my map skillz aren't too weak, then this would be the Crayford one:
I'm sure I read that some of the design decisions at the Dartford brickworks were inspired by issues encountered at Plumstead.
Kent County Concil is shocking for records on these (despite being previous owners ) , but I did see another potential one mentioned when looking at planning applications in the area. This one was alleged to have been in East Hill, but all I can find is references to old Gravel Pits there:
Then obviously you've got a whole plethora of possibilities from Stone to Gravesend, likely including a lot left over from the old cement industry.
Edit: didn't mean to hijack OPs thread; I've just got a bit of a bizarre soft-spot for geeking out on old maps!