Interactive applications such as virtual reality systems have become popular in recent years. A ground surface composed of a granular material can be deformed when it comes into contact with an object, and, in this paper, we propose a deformation algorithm for the ground surface which is useful for such applications. The deformation algorithm is divided into three steps: (1) detection of the collision between an object and the ground surface, (2) displacement of the granular material, and (3) erosion of the material at steep slopes. The proposed algorithm can handle objects of various shapes, including a concave polyhedron, and a texture sliding technique is proposed to represent the motion of the granular materials. In addition, the proposed algorithm can be used at an interactive frame rate.