Giant's kettle

A giant's kettle, also known as either a giant's cauldron, moulin pothole, or glacial pothole, is a typically large and cylindrical pothole drilled in solid rock underlying a glacier either by water descending down a deep moulin or by gravel rotating in the bed of subglacial meltwater stream. The interiors of potholes tend to be smooth and regular, unlike a plunge pool.

Source: Wikipedia