Lichenometry investigation

I came across lichenometry as a way of ageing exposed rock on mountains. 

The outer wall of Wigton Cemetery seemed like an ideal place to start. The cemetery was built in the 1850s so the wall will be about 170 years old. 

I looked for the largest ones I could find. The largest were 300mm across but up to 250mm was quit well represented too. It suggests that the growth rate of this lichen is between 1.5 and 2.0mm per year. Now to find more sandstone to test out the theory.