Sandstone cliffs tower over Lake Powell, a 300 kilometre-long reservoir that straddles the border of Utah and Arizona, USA. The red color found in many sandstone formations is caused by iron oxide found in various amounts throughout the rock and is due to the same chemical reaction that causes rust. Sandstone originates from the breakdown of older rocks through various processes of erosion. Lake Powell is one of the largest man-made reservoirs in the world and provides water for 20 million people living in the drought-prone American Southwest. In addition to be a reservoir it is a mecca for recreational boaters. The reservoir fills Glen Canyon, a vast area that was once home to ancient Native Americans and where numerous archaeological sites are now submerged.