Merchant Cemetery Ball

A large ball-shaped stone sitting on a grave marker has been rotating since jump street.
The best explanation I’ve seen is that of uneven thawing of ice. That is, ice forms at the base, thereby lifting the ball slightly (due to the fact that water increases in volume upon freezing), and then melts unevenly, as the sun is on one side of the stone, thereby causing the stone to roll (ever so slightly). (BTW, There is a famous poem by Robert Frost titled ‘Fire and Ice’, which can be taken as an unintended ironic reference to the Merchant Cemetery Ball. The poem discusses the end of the world, whereas the stone ball marks the end of a life.life. There is also a poem by A. E. Housman referencing fire and ice, regarding the living.)
Here is the link to one of the sites discussing this phenomenon.