Beoseotbawi, also known as Mushroom Rock, is exposed along the slopes of east coastal road (Taegeuk Road) located in Seo-myun, Ulleung-do. It is given the name as its shape resembles the curved surface of a mushroom. Beoseotbawi is composed of lapilli-tuff layers which are accumulations of fine-sized pyroclastic particles (fine dust and fragments ejected during eruptions) produced during an underwater volcanic eruption. The formation of the mushroom-shaped rock is caused by differential erosion in the lapilli-tuff layers. Differential erosion results from a graded-bed formed of pyroclastic material. A graded bed is a sedimentary deposit characterized by a systematic change in size or density of particles from the base of the bed to the top. In other words, gradual deposition of particles, first larger and denser particles and then finer and less dense particles, forms beds with graded bedding. Differential erosion is caused by the differences in the sizes and density of particles in strata. Hard layers, formed of larger particles, are more resistant to erosion and remain in shape whereas the softer layers, formed of smaller particles, are eroded and washed away.