The most northwesterly point of mainland Britain is called Cape Wrath. Why?

The name Cape Wrath is derived from Old Norse hvarf ("turning point"). The Vikings apparently used the cape as a navigation point where they would turn their ships. There are Viking settlements at many points along the western coasts of the British Isles: if you are heading for those and you miss the turning at Cape Wrath, then you end up in the great wilderness of the Atlantic Ocean.