Giant's kettle

A giant's kettle, also known as a giant's cauldron or pothole, is a cavity or hole which appears to have been drilled in the surrounding rocks by eddying currents of water bearing stones, gravel, and other detritus.

The interiors of potholes tend to be smooth and regular, unlike a plunge pool. An example is the large pothole found in Archbald, Pennsylvania in Archbald Pothole State Park.

Potholes vary in size from a few inches to several feet in depth and diameter. They most commonly occur in shields, where there are ancient rocks (granite, gneiss) with different resistances to erosion, so strong pebbles can fall into small cavities in stream bottoms and swirl, making the cavities wider and deeper. The hardness of the pebbles must be the same or higher than the rock at the bottom of stream where the kettle is forming. In areas where there are diamonds and quartz pebbles, the hardness of these carve potholes and sometimes remain trapped in the bottom of them.

Giant's kettles can also be formed while the surface is covered by a glacier. Water, produced by the thawing of the ice and snow, forms streams on the surface of the glacier, which, having gathered into their courses a certain amount of morainic debris, finally flow down a crevasse as a swirling cascade or moulin. The sides of the crevasse are abraded, and a vertical shaft is formed in the ice. The erosion may be continued into the bed of the glacier; and, the ice having left the district, the giants kettle so formed is seen as an empty shaft, or as a pipe filled with gravel, sand, or boulders. Such cavities and pipes afford valuable evidence as to the former extent of glaciers.

...
Wikipedia