I: Iron oxide
The same thing that gives the red color to blood is the thing that gives the reddish color to Red Rock Canyon. It’s the mineral iron. Millions of years ago, when the area was a dried basin leftover from a once-rich ocean that flowed all the way into Colorado, it was covered with sand dunes. Windblown sand moved back and forth, leveling dunes and making new ones. The shifting sands became buried by other sediments, and eventually it was all cemented into sandstone with iron oxide and calcium carbonate. In some places the iron has concentrated and oxidized to reveal a rusty red coloring. This is what is now known as Aztec Sandstone, prominently displayed in Calico Hills and the Red Rock cliffs. In areas where the rock is tan to white in color, the iron has either been leached out by water, or it was never deposited there.
Small deposits of iron oxide can also be found as iron concretions, which appear as red spots in Aztec sandstone. Sometimes the sandstone erodes around them, leaving little balls known as Moqui marbles.