From the South Rim of the Grand Canyon to the top of Hoover Dam: rocks nearly 2 billion years old, concrete that would take 125 years to cool, and a lake that's shrinking.
At the South Rim the ground just stops. Below is a canyon about 1.6 km deep, carved by the Colorado River over 5 to 6 million years, using sand as its teeth.
## Reading the rock
The stripes are layers, oldest at the bottom. The top rock has fossil sea creatures, so this desert was once sea. The bottom rock is nearly 2 billion years old.
## Walking across Hoover Dam
Downriver, I walked across the top of Hoover Dam: calm lake on one side, a 221-metre drop on the other. Built from 1931 to 1936, its concrete was poured in blocks with icy-water pipes, because in one piece it would have taken about 125 years to cool.
## The bathtub ring
A white stripe around Lake Mead shows how high the water used to be. Drought and heavy use have dropped it by tens of metres. When there isn't enough river, who gets the water?