 The plateaus gradually descend toward the northeast to the barren El Djouf, or "Empty Quarter," a vast region of large sand dunes that merges into the Sahara Desert.To the west, between the ocean and the plateaus, are alternating areas of clayey plains (regs) and sand dunes (ergs), some of which shift from place to place, gradually moved by high winds.The dunes generally increase in size and mobility toward the north.