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. The Flatirons on Green Mountain are within the City of Boulder Open Space and Mountain Parks system. They are popular destinations for hikers and rock climbers, with rock grades ranging from easy (5.0) to world-class (5.14). [Source]

Unfortunately a search on TinEye and scraping of comments on Reddit did not reveal the photographer of this wonderful shot. If you know the original source, please advise in the comments.