The document discusses various types of sand dunes and how they form. It describes: 1) Barchan dunes, which are crescent shaped and form in areas of sparse sand with winds blowing in one direction. 2) Transverse dunes, which are long ridges that form perpendicular to the wind in areas with more available sand, often near coasts. 3) Longitudinal dunes, or "seifs", which are exceptionally large ridges over 100 meters high and 100 km long that parallel the dominant wind direction.