This valley branches to the south from the end of the Bardardalur Valley and the Sprengisandur Route lies parallel to it and between it and the eastern branch the Kroksdalur Valley. Valley Mjoidalur extends all the way to the Kidagil Gully near the road. The distance from the southernmost farm of Valley Bardardalur to a spot called Ytrimosar in the Valley Mjoidalur is about 20 km. The northern part of the valley is relatively well vegetated, but the southern part is a victim of severe soil erosion, which brought about the abandonment of the last farm in the valley in 1894. River Mjoadalsa was a difficult obstacle for those who were travelling over the Sprengisandur Route until it was bridged in 1977. Most people travelling through make detours to take a closer look að the Waterfall Aldeyjarfoss and the Kidagil Gully.