The Flatirons are Boulder‘s favorite portion of the Fountain Formation, a geological formation of mostly red sandstone. This sandstone, more appropriately called “arkose” gets its color from the pinkish feldspar contained it it. This red rock makes up the Red Rocks of Morrison and the Garden of the Gods in Colorado Springs.

The Fountain Formation stretches from Colorado Springs all the way to Wyoming. The reddish color is due to the concentration of feldspar in the sandstone, which is pinkish in color by itself. Other chemicals present in the rock can shade it from pink, to red, and as far as purple. But where did all this rock come from?

Well, geological theories have said that Boulder Colorado and the rest of the Fountain Formation was the shore of an inland sea about 300 million years ago. This sea deposited clay minerals over eons and formed the basis for the Flatirons. Over time, the clay solidified into feldspar. But that means the red rock formed at sea-level. How did it get into the air over Boulder?

Photo by Local Photographer: Gerry Morrell