Glacial Mills are glacial potholes, typically large and cylindrical, drilled into solid rock underlying a glacier. They are a special form of dolly tubs. If they are formed by the melting water of glaciers the dolly tubs are called kettles or giant’s kettles, giant's cauldron, or moulin pothole. Famous samples of this kind can be found in Switzerland in the Canton Wallis, about a metre in diameter and 10 m to 15 m deep.
When the rock surface is covered by a glacier, the ice is often melting on the surface of the glacier, during summer for example. The melting water flows on the surface, reaches a shaft and enters a glacier cave. Typically, the water melts a shaft into the ice down to the bedrock, where it starts to flow on the surface downhill through a glacier cave. This kind of mill is formed at the point where the water reaches the bedrock after falling down the shaft. It has a lot of energy and starts to form a hole, later rocks are moved around the hole, and it works like a drilling machine. The result is a hole with rounded forms, which is actually the same as any dolly tub in a river bed or gorge. And if the shaft moves, for example by the moving glacier, series of such holes are drilled.