checked out the tunnels trail. Just a random question; what’s on the walls? My friend that it was mould, but I assume it’s all checked out by the council. Couldn’t find any info about it on the tunnels trails website.
Great walk / ride though! 
Calcite deposits built up over 150 years! Looks slimy but while wet it’s actually hard.
1 Like
Wonder where the calcium carbonate comes from?
Usually when it’s dark like that it’s got lots of iron in it, but maybe it’s the dark bricks behind it. I’m used to seeing it on pale limestone walls
Thanks Gerard. Good to know. 