The dynamic forces of nature are very evident in this high-oblique, southwest-looking photograph of northern Chad. In addition to producing dramatic expression in wind streaks aligned northeast-southwest, nature has created a unique landscape of well-entrenched, wind-eroded patterns by the predominate wind that begins in the Libyan Desert, blows toward the southwest across the Djourab Sand Region of central Chad, and is funneled between the two more resistant landform features—the Tibesti Mountains and the Ennedi Plateau (both dark colored). The Tibesti Mountains, which are mainly volcanic in origin (northwestern edge of the photograph), are the highest mountains in the Sahara Desert and cover a much broader areal extent than the Ennedi Plateau, a sandstone plateau (eastern edge of the photograph) less resistant to wind and water erosion forces; thus, the plateau is being eroded at a higher rate than the Tibesti Mountains. When wind speeds attain sufficient velocities, sand particles are transported in the lower atmosphere causing a duststorm, such as the one that is pictured here in the Djourab Sand Region of central Chad. Two dark features are barely visible toward the southern horizon—the vague outline of Lake Chad and the vegetative transition zone known as the Sahel (the greening line) where the nonvegetated desert meets the semiarid grassland region of southern Chad.