Rising out of the Arizona desert, the Camelback Mountain has long been an icon in the Phoenix valley. The mountain is named for its signature two humps and the head of a camel. It is the "head" of this "camel" that has geologists stumped.

The granite that makes up part of the head of the mountain, dates from the Pre-Cambrain era of 1.5 billion years ago. The puzzle to geologists, though, is that most of the mountain, including the hump, is made up of red sandstone from a measly 30 million years ago. As pictured here, there are spots on the mountain where the billion and a half years older and much, much heavier granite lies right on top of its younger, lighter neighbor. How exactly it got there, though, nobody yet knows.

The summit of the mountain is 2,704 feet, but hiking trails only go up to 1,280 ft. Echo Canyon Recreation Area occupies the head of the mountain and has many hiking trails. Caves on the north side of the mountain signify that it was likely used by Native Americans before European colonization. A warning to hikers: there are rattlesnakes on the mountain.