Triangular avalanches and uphill instabilities
  Recent experiments show that an avalanche initiated from a point source propagates downwards by invading a triangular shaped region. The opening angle of this triangle appears to reach 180$^o$ for a critical inclination of the pile, beyond which avalanches also propage upwards. We propose a simple interpretation of these observations, based on an extension of a phenomenological model for surface flows. 
Jean-Philippe Bouchaud (Saclay), Michael Cates (Edimburg)