Whether from water or wind, this is how sand ridges and dunes form and shift: fluid flow across them. Here in the Great Sand Dunes of Colorado, eolian sand from the upper Rio Grande, in the San Luis Valley, has accumulated against the Sangre de Cristos and formed massive dunes amidst the prevailing southwesterlies. However, in this case, northeast outflow winds from earlier mountain thunderstorms blew cold outflow the opposite way, tossing sand off the dune crests. The low angle of sunlight, and its partial diffusion by clouds to soften dynamic range a bit, made a wonderfully textured scene, looking across the top of this dune toward the shadowed side of the next.
18 ENE Mosca CO (7 Jun 3) Looking NNW
37.744, -105.5302