from the Author's Note:
The heath in this story refers to a vast expanse of meadow or moorland once common in rural England. ... Reddle is the name of a red clay, dug from the ground, and sold to sheep farmers, who used it for marking sheep. ... The last reddle-seller died around 1920.
Thus, Lewis has kept alive a bit of cultural history and wrapped it in a Christmas tale. I just wish he had let the children linger on the heath for a page or two, thus developing a stronger sense of place.