
Basalt dykes, Guernsey

In this example from the Channel Islands, two dykes can be seen above the cave roof, with a third further to the right.
These dykes are intruded into metamorphic gneiss. Dating of the radioactive minerals in the gneiss show that it is the oldest rock in southern Britain, at around 2500 million years old. The dykes are much younger.
Basalt dykes, Guernsey, Channel Islands

