Some water spirits were less than benevolent. A man desperate to reach his sick wife but despaired because the Luib Bridge over the Don had been swept away in a flood, accepted an offer from a very tall individual to carry him across. In the mid-river the kelpie, for such the stranger was, tried to drown the man, who only escaped after a fierce struggle. When he reached the bank the fustrated creature threw a boulder at him. Passers-by added stones to the boulder until it became known as Kelpie’s Cairn.
W. Gregor
The Witch. (Stories From Congarff)