Gravity erosion refers to a series of phenomena mainly under the action of gravity, a single falling rock, detrital flow or a whole soil. The rock mass moves downward along the slope. Based on the above definitions, Which of the following statements are true:. ['The desert areas of Xinjiang, China, are mainly formed by gravity erosion.', 'There are many famous underground caves in Guangxi, China. Underground caves are mainly formed by gravity erosion.', 'A county is facing the sea, and there are many weird reefs along the coast. These reefs are formed by gravity erosion.', 'Landslides occur in a place, and gravity erosion is the main cause of landslide formation.']