Definition of "stoneboat"

They looped ropes around the stiff legbones of the first cow and pulled her body toward the stoneboat, resting between spurts of hauling.

1984, Lois Phillips Hudson, The Bones of Plenty, page 204

Each night the builder and the horse and the empty stone-boat left for the mountain.

2017, Neil Gaiman, Norse Mythology, Bloomsbury Publishing, page 59