During the first half of "The Free State Period" (930-1262), the settlement of this green valley flourished until the 1104 eruption of Mt. Hekla devastated it. In 1939, Nordic archaeologists excavated a few farm ruins, among them the farm Stong. This ruin was protected with a roof and is open to the public. A replica of this farm was built in the mouth of the valley in 1974 to commemorate the 1100 years of inhabitancy of the country. The lava fields and pseudo craters in the valley have their sources in the central highlands, far away in the northeast.

Both the Forestry Service and the Energy Authority have protected the remainders of the forests and planted tens of thousands of trees and sown grass and fertilizer to turn the valley green again. The Energy Authority even built an open-air swimming pool near the only thermal area.

The second highest waterfall of the country, Haifoss (122 m), with its neighbour Granni are at the end of the valley. Down by the confluence of the Fossa and Thjorsa Rivers is the waterfall Hjalparfoss. More waterfalls are in the beautiful, green oasis Gjain near the archaeological excavation at Stong.