Figure 19.10Sand dunes may form in the lee of a rock or other obstacle. (a) By separating the wind streamlines, the obstacle creates a wind shadow in which the eddies are weaker than the main flow. Windborne sand grains are thus able to settle in the wind shadow, where they pile up in drifts that eventually coalesce into a dune. (b) Sand drifts, Owens Lake, California.