The old word howe was adapted into English from Old Norse haugr, which could mean either an artifical mound or a small natural hill. As used in early English, the word was rather more specific. In this sense a howe described a particular kind of burial mound, a 'bowl barrow', which was a low circular mound with a shape like an inverted bowl. We cannot know whether Tolkien meant the word to be interpreted quite so specifically, but this at least gives some impression of how Snowmane's burial mound might have appeared.