A large continental block existed with oceanic crust subducting underneath it. In front of the block and above the subducting oceanic plate are volcanoes emitting a range of varied deposits that give a spectrum of signatures (basaltic eruption, intermediate eruptions and acidic eruptions, as the material melting and rising changes over the period of the subduction). These volcanic deposits get mixed up with the sediment from the continent (now in the form of a back arc basin). These are the volcaniclastic deposits that we can see today at places such as The Wrekin.