Red Rocks Park is just 10 miles outside the city center of Denver, Colorado. These iconic red sandstone structures are home to an outdoor amphitheater that dates back to the 1940s and several hiking trails. The jagged configuration of rocks is known as a fountain formation. It’s believed to be between 290 and 340 million years old. The rocks themselves are red because the iron oxide they are made of has “rusted” or oxidized with exposure to the elements.