Thanks both – Alexander, I know exactly what you mean.. i do want to use the surface for other desert surfaces too, so can’t do that (otherwise you’d be spot on).
Here’s the cave system ‘finished;’ (i.e. I’m bored now!) I think the black edges of the board help a lot.
For my next trick – a ravine, with rope bridge type situation. Indiana jones style! Just need some more ( a lot more) 18mm cork.