Dolly Tubs are erosive forms produced by flowing water. A river with high energy (steep downward slope) transports sand, gravel and rocks. This hard material erodes the walls and the floor of the river bed and produces rounded forms. Maelstroms in the water, locations were the water swirls around on the same place for a long time, the gravel and sand starts to drill a hole into the floor. This are the dollytubs.
As variegated as the swirls is the form of the dolly tubs. It is typically round or oval, normally it has a steep rim. Very characteristic are deep boreholes, much deeper than their diameter.
Depending on their formations this kind of holes is found everywhere where water flows. Gorges and river caves are a good place to look for them. Dolly tubs are common in caves, as many caves or at least parts of cave systems were formed by flowing water. But they are not restricted to caves and they are not even typical karst features. Dolly tubs and rockmills are found in all kinds of rocks and everywhere on earth.
Many are found at the mouth or the former location of glaciers, where centuries of flowing melting water formed the most extraordinary rockmills or glacier mills. They are also sometimes dubbed kettles or giant’s kettles. Famous samples of this kind can be found in Switzerland in the Canton Wallis, about a metre in diameter and 10 or 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.
Often the bigger rocks which are turned and swirled around by the water and helped to drill the hole cannot leave the tub again and are now lying on the bottom. They were rounded by the movement and have a spherical form. They are sometimes called millstone or drillstone, or simply pebbles.