In the the eastern fjörds are found some of the oldest rocks, for this part of the country has drifted steadily away from the active volcanic zone over the past 13 million years. As a result the landscape has suffered much repeated erosion and there are deep valleys. The highly altered rocks around extinct central volcanoes are sometimes exposed in such valleys.
The picture on the left shows the deeply dissected Tertiary basalt plateau of the eastern fjörds, viewed from the younger rocks closer to the active volcanic zone. The Lagarfljót Lake has its monster! On the right are highly altered rocks around an extinct eroded volcano.