  We have experimentally observed a new phenomenon in the surface flow of a
granular material. A heap is constructed by injecting sand between two vertical
glass plates separated by a distance much larger than the average grain size,
with an open boundary. As the heap reaches the open boundary, "soliton-like"
fluctuations appear on the flowing layer, and move "up the hill" (i.e., against
the direction of the flow). We explain the phenomenon in the context of
stop-and-go traffic models, and show that soliton-like behavior is allowed
within a Saint-Venant description for the granular flow.
