Þjórsárdalur, the easternmost valley of Árnessýsla. There is only one farm in this valley now, Ásólfsstaðir, but ruins of 20 farms have been found. It is believed that the valley was deserted after heavy pumice rain from the eruption of Hekla in 1104. The valley of Þjórsárdalur has been partly revegetated by Land­græðsla ríkisins, the Soil Conservation Service of Iceland, Þjórs­ár­dal­sskógur. Extensive birchwoods in many places. Two rivers, Sandá and Fossá flow through the valley, and the Þjórsá river closes it to the south. National Forest.