From a distance, they look like hay bales covered in snow. But close-up it becomes clear they are hollow. A mystified Ron Trevett photographed them in a field near Yeovil, Somerset. It occurred to him they might be some large-scale prank in the crop circle tradition but for the complete absence of footprints. In fact, snow rolls are a natural phenomenon, but more associated with the prairies of North America than Britain. Certain conditions are necessary for their formation: the right texture of snow, temperature and winds. Strong winds peel off the top layer of snow when it becomes sticky and bowl the roll along until it’s too heavy. The snow doughnuts, as they’re also known, look deceptively solid, as they can distintegrate in an instance.