So he made a quick sketch of the landscape, the canyon mouth, the alluvial fan, and then drew the trench.

Above Curiosity's landing site, orbiters have photographed an alluvial fan (below) -a fan-shapedarea where debris once spread out.

The giant shale seemed to melt away into this alluvial fan of snow like a turbulent stream feeding into glassy, slow water.

The enemy's position consisted of a line of sangars blocking the roads from the river up to the alluvial fan on which they were placed.

And the alluvial fans are no good either-flashflooding.