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In the southeastern part of Utah, USA, the San Juan River intersects a mountain ridge called the Comb Ridge, on whose slopes flatirons – zig-zag- formations of rocks – can be discerned.

Flatirons - carved in rock

By Angelika Jung-Hüttl

Geology illustrated | October 2018


Rugged multilayer fault-block rocks loom high next to each other like gigantic spikes. There are thousands of these along the 130 km long narrow mountain ridge called the Comb Ridge, which stretches from Utah to Arizona on the famous Colorado Plateau in the southwest of the USA.

They mostly consist of sandstone. Traces of iron minerals in the different rock layers tinge it red, brown or even violet.

  • Flatirons of red sandstone .....
  • ... and ochre-coloured flatirons of the Comb Ridge.

Legend has it that the wives of the first American settlers gave these strange triangular rock formations the name of flatirons. Geologists have taken over this name and today these rock formations are known all over the world as flatirons.

Very distinct stone flatirons ‘can also be seen in Death Valley National Park,  California, which is situated 400 km away from the Comb Ridge.

The flank of the Panamint Range in Death Valley, California.

One finds them on the western flank of the valley, on the slopes of the 3000m high Panamint Range. There the flatirons consist of cream, brown and green lime stone layers, which had been deposited in an ancient sea already in the Precambrian era (over 500 million years ago) and later tilted during the orogeny.  

How have Flatirons been formed?
This process requires gigantic masses of stone in the earth crust, in which softer and harder stone layers are piled upon each other like the layers on a cake. Then comes a orogeny during which such stacks of rock are exposed to an enormous pressure on each side until they fold like a thick rug. At the same time the rock layers bulge in a barrel-like shape.

Flatirons in the Panamint Range separated by deep drainage channels.

After the process of bulging, climate, weathering and erosion come into play. Flatirons can only be formed in dry areas devoid of vegetation, in which it seldom rains, but where precipitation is hefty. Rainwater falling on the tilted rocks, flows down extensively to form streams, which after a while flush out deep channels in the slant flanks of the gigantic rock bulges.

With the right tilt of the different rock layers, the right amount of rainwater as well as its runoff, triangular rock formations, namely flatirons, come into being.  The whole process, from the sedimentation of the rock through the orogeny to the formation of flatirons, lasts millions of years.

Where does one find flatirons? Remarkable flatiron formations can be seen in all arid regions of our planet – in desert regions as well as in the drier mountain regions in more moderate latitudes.

  • The Damara Mountains, Namibia
  • The Quebrada el Jardin, Chile

For example, in the Damara Mountains, Namibia: The gigantic ‘cake’ made of differently coloured layers of rock was deposited between two continental plates - the Congo and Kalahari Cratons.  Approximately 750 million years ago, when both continental plates were drifting toward one another, the marine sediments got trapped in between and pushed upwards to form the Damara Mountains. The mountain range bonded the two continental plates thus forming the southern part of the present-day African continent.

For example, in South America in the Quebrada el Jardin: Quebrada el Jardin, Garden Canyon, is located in the Codilliera de Domeyko, a 600 km long mountain range in Chile, which separates the main ridge of the Andes from the Atacama Desert. The blue layers contain sediments rich in minerals, which were once disgorged by an erupting volcano and are visible in the present-day flatirons as bands of bluish colour.

Flatirons can also be found in Europe….

Lasieso Valley, Sierra Sabocos, The Pyrenees

…… for instance in the European Pyrenees:
The flatirons of the Lasieso Valley in the Spanish Pyrenees are a particularly beautiful example. They consist of lime stone layers which deposited in a shallow ancient sea between the small Iberian continental plate (Spain and Portugal lie on it today) and the big Eurasian continental plate. As the Iberian plate moved toward Europe between 80 and 20 million years ago to finally collide with it, the lime stone layers were pressed together and pushed upwards in some places to form gigantic barrel-like vaults, weathering and erosion eventually turned into flatirons.

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