When we tour the San Andreas Fault, our guests always ask us what lives in the holes. Well, the photo below shows a chuckwalla living in one of them. (The Chuckwalla is the largest non-venomous lizard in the United States!) The reptiles, birds, and animals do not make the holes. They are formed when the hot sun hits the rocks and boulders embedded in the sandstone. The heat of the sun causes them to expand during the daytime and then contract at night when the temperature drops. After thousands of years of this expansion and contraction, the rock becomes lo0se and pops out. The impression left is fair game to all the critters that live on the fault! They eventually make the hole larger by digging or scratching.