The Flatirons are rock formations near Boulder, Colorado. There are five large, numbered Flatirons ranging from north to south (First through Fifth, respectively) along the east slope of Green Mountain, and the term "The Flatirons" sometimes refers to these five alone. Numerous additional named Flatirons are on the southern part of Green Mountain, Bear Peak, and among the surrounding foothills.
The Flatirons consist of conglomerate sandstone of the Fountain Formation. Geologists estimate the age of these rocks as 290-296 million years; they were lifted and tilted into their present orientation between 35 and 80 million years ago, during the Laramide Orogeny. The Flatirons were subsequently exposed by erosion. Other manifestations of the Fountain Formation can be found in many places along the Colorado Front Range, including Garden of the Gods near Colorado Springs, Roxborough State Park in Douglas County, and Red Rocks Amphitheatre near Morrison. (Wikipedia)