Slag is a waste material that was created when iron was made using the blast furnace. It had very minimal use, and more slag was produced than actual iron. In the 1600s, most of the slag was thrown into the river, creating the slag pile. Over time, grass has grown over the slag, giving the pile an appearance similar to that of a small natural ridge.
When archaeologists began to work on the site in the 1940s, they knew that an iron works had been here, but they did not know exactly where each building was. The remnants of foundations, water wheels, and other artifacts had been buried over time. The slag pile provided them with a great clue as to where to start digging. Since it is a long, straight line, they knew that if they followed it back, they would find the blast furnace where the slag was produced. Sure enough they found remnants of the blast furnace very early on in the excavation.