Médanos are continental dunes, as opposed to dunas which are dunes of coastal origin. The term is mainly associated with South America. They may be vegetated or unvegetated, and are notoriously difficult to traverse.
The Swiss naturalist and explorer Johann Jakob von Tschudi provides this 1847 description of Peruvian médanos:
The médanos are hillock-like elevations of sand, some having a firm, others a loose base. The former, which are always crescent-shaped, are from ten to twenty feet high, and have an acute crest. The inner side is perpendicular, and the outer or bow side forms an angle with a steep inclination downward. When driven by violent winds, the médanos pass rapidly over the plains. The smaller and lighter ones move quickly forwards before the larger ones; but the latter soon overtake and crush them, whilst they are themselves shivered by the collision. These medanos assume all sorts of extraordinary figures, and sometimes move along the plain in rows forming most intricate labyrinths, whereby what might otherwise be visible in the distance is withdrawn from the view of the traveller. A plain often appears to be covered with a row of médanos, and some days afterwards it is again restored to its level and uniform aspect. Persons who have the greatest experience of the coast are apt to mistake their way, when they encounter these sandhills.