Barry Eckert, age ltd, of Strasburg,Penn.;`

 What is an alluvial fan?

An alluvial fan is a fairly common formation where streams run down uneven slopes. Fast‑running water can carry along more debris than slow‑running water. Down a high slope, a stream may sweep along a great load of silt and gravel. Suddenly the slope meets a flat plain or the angle changes to make it more gradual. The stream is forced to slow down and here the running water must dump some of its debris.

The shape of the ground and the natural forces of moving water build the growing deposit into a fan shape. The formation looks as though Paul Bunyan has spilled a pile of dirt through a crack in the side of a wall.