red layer between the columnar basalt layers is the interbasaltic
horizon. This is a fossil soil, called laterite, which formed on top
of a basalt eruption, during a prolonged period between eruptions.
The top of these beds are rich in iron and the base in aluminium.
In some places, such as Grenravel in County Antrim, the laterite is
rich enough to have been mined for iron ore and bauxite (aluminium).
An examination of plant pollen in the laterite horizon suggests that
this soil was formed in the Early or Middle Ogliocene, about 20 or
30 Million years ago.