No single scientific definition of the term exists, and it has been applied to a variety of rock features that fall into one of four general categories: - A glacial erratic is a boulder that was transported and deposited by glaciers or ice rafts to a resting place on soil, on bedrock, or on other boulders. It usually has a different lithology from the other rocks around it. Not all glacial erratics are balancing rocks; some are firmly seated on the ground. Some balancing erratics have come to be known as rocking stones, also known as logan rocks, logan stones, or logans, because they are so finely balanced that the application of just a small force may cause them to rock or sway. A good example of a rocking stone is the Logan Rock in Cornwall, England, United Kingdom; another is the Trembling Rock in Brittany, France. - A perched block, also known as a perched boulder or perched rock, is a large, detached rock fragment that most commonly was transported and deposited by a glacier to a resting place on glacial till, often on the side of a hill or slope. Some perched blocks were not produced by glacial action, but were the aftermath of a rock fall, landslide, or avalanche.