Boulder’s Flatirons are more than a spectacular backdrop. They are the visible result of hundreds of millions of years of deposition, mountain building, faulting, erosion, and an unusual degree of resistance to weathering.
The story begins roughly 300 million years ago.
Born From an Ancient Mountain Range
The Flatirons are made primarily of rock belonging to the Fountain Formation, a thick sequence of sandstone and conglomerate deposited as the Ancestral Rocky Mountains were being eroded.
Those mountains were not the Rockies we know today. Rivers and streams carried enormous quantities of sand, gravel, and pieces of older crystalline rock eastward from the ancient highlands. Over time, those sediments were buried and hardened into rock…