Parry Channel is a sea passage running east to west through the arctic islands. Named for explorer W.E. Parry, it begins at Lancaster Sound, passes through Barrow Strait, leads into Viscount Melville Sound, finally reaching the Beaufort Sea through M'Clure Strait. The permanent pack ice in M'Clure Strait is an impassable obstacle to further navigation through the Parry Channel, forcing ships making the Northwest Passage to detour far to the south. The straight, parallel coastlines and great depths found in Parry Channel suggest it is a deep, submerged trough caused by past intense movements of the Earth. It marks the geological divide between Precambrian rocks of the Canadian Shield to the south and sedimentary rocks forming the arctic islands to the north.