Geograph - photograph every grid square

An igneous intrusion, or dyke, penetrating the plum-coloured slates on the Holywell Level. There is significant difference in hardness and tenacity of the igneous rock compared to that of the mudstones which have been metamorphosed to slates. Consequently, during the Caledonian period of mountain-building, the dyke has been fractured and injected with mineralising fluids. Together with its green colouration, this renders it highly visible - though sadly the rock is worthless and will simply go to the tip. The history of deformation allows us to attribute an age to the dyke, which pre-dates the Caledonian and is therefore probably Ordovician and associated with the volcanism which is pervasive in North Wales at that time. Other dyke exposures nearby are highly weathered and reduced to a brownish earth.