In local legend, the Halls are fairly old, with stories of people disappearing going back for hundreds of years, said to be taken by the Faeries. In actuality, the Halls are much older.
To purify the water from a certain river the monks have carved a section of the river that goes over a stone bed. Through holy carvings of gods, godesses, and holy symbols and then the river flowing over them, it purifies the water making it safe for drinking and/or makes it holy water. For rivers that do not have natural rock bottoms, large smooth stones can be carved and added to the water to fully cover the bottom.