Millions of years ago most of this very arid region in northwest Mauritania was made up of large sandstone plateaus that looked like the brownish sandstone tabletop terrain that exists in the area today. This entire region has undergone a dramatic climatic change. It is apparent when looking at the existing landscape that substantial rainfall altered the former landscape into one of broad canyons bounded by craggy, vertical cliffs. The area now averages approximately 4 inches (10 centimeters) of rainfall annually, sometimes decreasing to as little as 1 inch (25 centimeters). The base of these cliffs is hidden by numerous talus slopes visible even at this small scale. The Amojjar Pass, near the town of Atar, is in the canyon just east of the large flat-topped plateau (center of the photograph). The pass is an important ground transportation link because it provides a somewhat protected roadway connecting the two Mauritania cities of Fdérik and Nouakchott, the capital. The effects of wind erosion on the landscape are evidenced by the northeast-southwest orientation of the sand streaks.