Breiðdalur, the biggest of the valleys cutting into the eastern mountains, wide towards the sea but dividing into two valleys, Norðurdalur and Suðurdalur, further inland. The highway goes through the latter. The valley is surrounded by mountains 1,100–1,200m high and extremely colourful because of rhyolite which came from the Breiðdalseldstöð volcano in the tertiary period. Big gravel beds in the middle of the valley are gravel plains, seashores from when the ocean reached further inland. These are quite common in the eastern fjords and their height above the present sea level varies markedly. At the farm Þorgrímsstaðir at the head of the valley there is a small electrical power station for home use, harnessing a stream where the water falls 255m.