A Geological Tour of Denver, Golden, and Colorado's Front Range

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Geologic Time Period

The Flatiron Formations

Dakota Hogback

Dakota Hogback, on the left, with one of the red rock flatirons of the Fountain Formation at Red Rocks Park.

Hogbacks and Flatirons...

By Jack Barkstrom

Hogbacks - sharp, narrow ridges - such as the Dakota Hogback of Dinosaur Ridge, are said to resemble the ridged back of the razorback, the wild hog of the southern U.S., perhaps better known as the namesake of  the Arkansas Razorbacks, the state's legendary football team. Geologically, hogbacks are relatively young ridges of sedimentary formations, tilted up at a steep angle.  Flatirons, rounded and somewhat triangularly-shaped, such as the red rocks of the Fountain Formation, are basically the eroded remains of what once were hogbacks.[1]

Just before the rise of the present-day Rocky Mountains 70 million years ago, the various strata lay flat, one on top of another, on the ocean floor of the Western Interior Seaway.  The red sandstone of the Fountain Formation was at the very bottom.  Above it lay the Lyons sandstone, the Lykins limestone, and the claystone and limestone of the Morrison Formation. When the uplift of the Laramide Orogeny began, the overlying formations rode up with it (or on top of it).  While their steep angle suggests that some massive rock slide or movement took place, they are mostly the result of downslope erosion.  The red rocks of the Fountain Formation, as well as the ridges of the Dakota hogback, are the eroded remnants of layers of sediment which extended to the peaks of the current Rockies.[2]

At times, the term "Flatirons" has been applied loosely to include both the hogbacks and the red sandstone identified specifically as "one of"  the flatirons of the Fountain Formation. Any of the angled formations found along the Front Range between Fort Collins and Colorado Springs are considered to be part of the Flatirons.  The whole Front Range, in that context, is considered a region of flatirons.[3]


(1) Ralph Lee Hopkins and Lindy Birkel Hopkins, "Hiking Colorado's Geology," The Mountaineers, (Seattle, WA 2000), pp. 81-82;
(2) Hopkins, "Hiking Colorado's Geology," pp. 81-82.
(3) Hopkins, "Hiking Colorado's Geology," p. 82.