  We have numerically studied a model of granular landscape eroded by wind. We
show the appearance of labyrinthic patterns when the wind orientation turns by
$90^\circ$. The occurence of such structures are discussed. Morever, we
introduce the density $n_k$ of ``defects'' as the dynamic parameter governing
the landscape evolution. A power law behavior of $n_k$ is found as a function
of time. In the case of wind variations, the exponent (drastically) shifts from
2 to 1. The presence of two asymptotic values of $n_k$ implies the
irreversibility of the labyrinthic formation process.
