I was told this by an old man many years ago. There was an old grist mill on a river. To make a channel through rock to carry water tothe mill, they first blocked the river off the proposed route across the rock. They then put a lot of wood on the route burned it most of a day. They had a low dam across the river. After the fire was out they turned the water onto the hot rock which washed a lot of rock chips away. Don't know how often they repeated the process.