Tjörnes, the peninsula between the Skjálfandaflói gulf and Öxarfjörður. There is a high mountain range along it, and an old footpath and horse–trail across Tunguheiði moor between the farms Syðri–Tunga and Fjöll in the Kelduhverfi district. On the west and north there is a good deal of flat land with rivers running into the Skjálfandi gulf, as well as some high sandstone ridges by the sea, 400–500 m thick, dating from the Tertiary period and the Ice Age. Towards the south there are alternate shell and lignite layers, showing big changes in sea level. In the southernmost and oldest of these are shells of creatures that now live only in warmer sea along west–European coasts. In younger layers there are cold–sea shells and at Breiðavík arctic shells. During World War I there was a lignite mine on Tjörnes in the land of the farms Hringver and Ytri–Tunga.