I think its surface mineral workings as I think this may be one of a number of pics.... I will look as I think I have this set somewhere. π
Edit -Great Treveddoe mine perhaps or even the St.Erth alluvial workings.... my first guess????
π
Edit Edit - Kenton Marsh Wolfram deposits also just been suggested! π
Edit & So on!!! Goss Moor is another bet!
Aaaaaaaaaaaaaaaaaaagh Creeper!!!!!