The Flatirons: Two Hundred and Eighty Million Years in the Making

Those angled slabs of reddish-brown rock pressed against the mountain front to the west are called the Flatirons, and they require a certain patience to understand. They are not the mountains themselves. They are older than the mountains as they now stand.

The rock is sandstone and conglomerate, sediment laid down roughly two hundred and ninety million years ago, during the Pennsylvanian Period. At that time, a range of mountains called the Ancestral Rockies rose about thirty miles west of here and began eroding. Coarse sand and gravel washed eastward off those peaks, accumulated in thick beds across the lowlands, and slowly compacted into stone. The Fountain Formation, geologists call it, for Fountain Creek to the south.

Those layers originally lay flat and horizontal. They did not tilt until the current Rocky Mountain uplift, the Laramide Orogeny, began shoving the crust upward between roughly eighty and thirty-five million years ago. As the mountains rose, the edge of the ancient sedimentary layers tilted with them, pivoting up at a steep angle until they reached their current inclination of somewhere between fifty and sixty degrees.

Then water and wind went to work on the softer rock around them, stripping away material and leaving the hard Fountain Formation sandstone standing exposed against the sky like the prows of ships run aground on the mountain front.

The five main Flatirons visible from town are among the most photographed geological formations in Colorado. Every one of them is made of the same ancient river gravel, stone that began as mountains, washed to a plain, and then rose again into something that looks, from the right angle, like cathedral windows.