Areas on a geological map usually are drawn with reference to the ages of the rock. For example, in the map below, the red areas are Tertiary in age i.e. the rocks which underlie the soil in that area were laid down between 2 and 65 million years ago. Those rocks are tertiary lavas which cooled to form basalt. However some of the granites (in the Mournes for example) are also tertiary in age and, towards the end of the Tertiary period, clays were laid down around the south of Lough Neagh. In this map we tried to show distinct rock types, rather than geological periods, although in some areas a number of different rock types can be found very close together, and this could not be shown here as the scale of the map made it difficult to include fine detail. I would recommend the excellent Solid Edition of the Geological Map of Northern Ireland (1:250,000), published by the GSNI.
