Giants kettle

Giants kettle, also known as giants cauldrons or potholes, are cavities or holes which appear to have been drilled in the surrounding rocks by eddying currents of water-bearing stones, gravel and other detrital matter.

The size varies from a few inches to several feet in depth and diameter. The most common occurrence is in shields, where there are ancient rocks (granite, gneiss) with different resistance to erosion, so strong pebbles fall in a small cavity and swirl making this cavity wider and deeper. In areas where there are diamonds and quartz rocks, the hardness of these rocks cave potholes and sometimes remain trapped in the bottom of it. Hardness of pebbles must be the same or higher than the bottom of stream where the kettle is forming. That is why these potholes are a good place to find diamonds (the hardest rock in Earth) and also explains the name of some potholes in the Blyde River Canyon of South Africa (Bourke's Luck Potholes) as the image shows. A famous locality for giants kettles is the GletscherGarten of Lucerne (Switzerland), where there are 32 giants kettles, the largest being 8 m (26 ft) wide and 9 m (30 ft) deep; they are also common in Germany, Norway, Sweden ("jättegryta"), Finland ("hiidenkirnu"; hiisi's churn) and in the United States. It appears that 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, are finally cast 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. Similar holes are met with in riverbeds at the foot of cascades, and under some other circumstances. The term pothole is also sometimes used synonymously with swallow-hole.