are predominantly silicates, composed of 80 to 90% quartz and the remainder feldspar, whose iron oxide-coated grains color the sands orange, purple, and red.

A corridor of sandy terrain known as the Ad-Dahna desert connects the An-Nafud desert (65,000 km² or 40,389 square miles) in the north of Saudi Arabia to the Rub' al-Khali in the south-east. The Tuwaiq escarpment is an 800 km arc that includes limestone cliffs,