  We find that circular kinks form on the surface of granular material when the
axis of rotation is tilted more than the angle of internal friction of the
material. Radius of the kinks is measured as a function of the spinning speed
and the tilting angle. Stability consideration of the surface results in an
explanation that the kink is a boundary between the inner unstable and outer
stable regions. A simple cellular automata model also displays kinks at the
stability boundary.
