Valley Adaldalur covers most of the lowland area to the south of Bay Skjalfandafloi, all the way to River Skjalfandafljot in the west and the outcrop north of morland Fljotsheidi. The southern border is Lake Vestmannsvatn, where Valley Reykjadalur continues to the south. The eastern boundaries are River Laxa and Moorland Hvammsheidi to Bruar near the parsonage Grenjadarstadur. The three valleys continuing to the south are Reykjadalur, Þegjandadalur (uninhabited), and Laxardalur.
Most of the valley floor is covered with lava fields between Rivers Laxa and Skjalfandafljot, 100 km. square. The old Laxar lava field (3,500 years old; volcano Ketildyngja) is mostly covered by the younger Laxarhraun (2,200 years old; crater rows Threngsla and Ludentsborgir). The lava fields are mostly covered with bushes and other vegetation. There are also several groups of pseudo craters. Some of the craters are hollow and were used as sheep shelters in the past. The lava fields are dotted with crevice and chasms, where many a sheep has gone lost. Farmer Steingrimur Baldvinsson of farm Nes fell into one of them and it took alsmost three days to find and rescue him.