Sleeping Stones by Beatrice Wallbank (author)

Gruff and his new friend Matylda live on a small island off the Welsh coast, where legends are beginning to stir. Islanders find themselves irresistibly drawn to the Sleeping Stones, a line of rocks like stepping stones out to sea, and Gruff and Mat soon realise they must risk everything to save each other and their community from a terrifying storm driven by an ancient, magic anger.