A brat is a square of sackcloth about two metres by two that is used to carry stuff - usually hay or straw. You just pick up the corners and fling it over your shoulder - it's not used to carry stones but it is the (fairly recent) source of your cairn placename. There are plenty of those cairns, but unexcavated, in the North Pennines. It's often surprising how similar in size and shape the collected stones are.