The famous steel mills of Pittsburgh left their mark on the Pittsburgh area. Pittsburgh once was the largest maker of steel in the world. When steel is made, unwanted impurities remain. This material is slag. Slag is usually made up of dirt and limestone or dolostone.
Steel is made by mixing iron ore with limestone or dolostone .
When the newly mixed steel is hot, the more pure steel sinks to the bottom of the container and the slag floats on top. The pure steel is drained out of the container from the bottom leaving the slag. The steel mills made milions of tons of steel a year, which meant that a great deal of slag was also made and dumped in huge piles. A large slag dump is located along Nine Mile Run near the Squirrel Hill Tunnel. Another large slag dump, shown in the following photos, is located behind Century Three Mall.