Basalt dykes, Guernsey

When molten magma flows upward through near-vertical cracks (faults or joints) toward the surface and cools, dykes are formed. Dykes are sheet-like igneous intrusions that cut across any layers in the rock they intrude.

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.