The mill pond stores the water that drives
the water wheel.
When it was dug, over 250 years ago, they had to build a dam
across the stream to hold the water back. The pond was lined with clay
to make it waterproof. What would you use today to make a waterproof
pond liner? If a material is waterproof it is said to be impermeable.
If water can pass through it, it said to be permeable.