Glacial Mill

Moulin - Glacier Mill

Glacial Mills are glacial potholes, typically large and cylindrical, drilled into solid rock underlying a glacier. They are a special form, a subset of potholes, which was formed by the melting water of glaciers. They are also called kettles or giant’s kettles, giant’s cauldron, or moulin pothole.

It is quite unwieldy that this geological phenomenon has so many different names. Some names are from different regions, so they were simply named differently in Austria, Switzerland, northern and southern Germany. Most of the names originated from local colloquial terms. Even more unpleasant is the fact that they are often used as proper names and It is simply wrong to refer to them as glacier mills. But this is actually wrong, the glacier mill is a shaft in the ice of the glacier. It can create a glacial pot, but it disappears once the glacier has melted. To minimise the confusion, we have only used the term Glacial Mill on showcaves.com.

If the rock surface is covered by a glacier, the ice often melts on the surface of the glacier in summer. 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 pothole in a river bed or gorge. And if the shaft moves, because the glacier is moving, series of such holes are drilled.

Very impressive specimens can be admired in Switzerland in the canton of Valais, which are around one metre in diameter and 10 to 15 metres deep. The Gletschergarten (glacier garden) in Lucerne is also worth a visit. Of course, the most spectacular ones can be found in Scandinavia and Canada.