Transverse dunes cover the valley floor of Nirgal Vallis, a 670 km long valley network at 318¿E and 29¿S. The dunes are superposed by small undeformed impact craters, which implies that the dunes are inactive under present atmospheric conditions. The last active phase of dune movement (absolute age) can be determined by crater size frequency distributions. The derived absolute ages from high-resolution Mars Global Surveyor Mars Orbiter Camera imagery range from approximately 300,000 years to 1.4 Myr B.P. (error of factor of 2). The dunes are also superposed by gully deposits (aprons), which implies a younger age of the gullies. These superposition relationships show that the gullies in this midlatitude region are at the most 3 Myr old but might be even younger than 300,000 years.