Mile 65.2 - Built in the cliffs is a lime kiln. They would pour alternating layers of coal and stone from the top of the mill and start it from the fireplaces underneath.
After the coal was all burned out, the stone would be changed to quicklime which is used for making cement. You can watch a video of a home made lime kiln in action here and it'll give you a pretty good idea of how this worked.
A close up of the fireplace underneath where the fire was started.