Liesegang rings

The name for this unusual patterning in Moughton whetstone, found on our walk in Crummackdale today.  

I quote 
"Moughton Whetstone is a thin layer of siltstone with remarkable colour banding. The banding was formed by the rhythmic precipitation of hematite (iron oxide) as water percolated through the rock over millions of years. The iron diffused through the porous rock in pulses or waves creating distinct bands that scientists call ‘Liesegang rings’, but the process is still not fully understood."
The rock dates from the Silurian period. It was once used for sharpening razors, hence the name.