Information: Esja was built up at the end of the Pleistocene with the beginning of the Ice Age. During the warm periods, lava flowed and in the cold periods tuff-stone ridges were built under the glacier. The western part of the mountain range is the oldest (about 3.2 million years) and the eastern part is the youngest (about 1.8 million years). The movements of the plate boundaries cutting diagonally through Iceland are pushing the stratums to the west and away from the active volcanic zone. Intrusions, i.e. big magma channels coming from the old central volcanoes at Kjalarnes and Stardal, introduced themselves into the stratums. Big lava fields were the result, one above the other, which the Ice Age glacier ground down.