 A short tunnel—the first one in Anglesey—pierces the low hills which extend along the west side of Malltraeth Marsh before reaching Bodorgan railway station.Above Malltraeth lies the extensive Common of the marsh, which is bounded on the northwest by the parishes of Llangadwaladr, Trefdraeth, and Llangristiolus; on the northeast by Llangefni and Llanffinan.Cors Ddyga (Tygai's Marsh) lies in the commote of Rhosyr .