Gneiss Views: Braving the Sediments

The Flatirons, the massive tilted slabs of sandstone that dominate the Boulder skyline, are a strange sight.

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The 3rd #flatiron, from atop the 1st flatiron in #Boulder

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Growing up in New England, I learned to hike and climb on the granite domes and metamorphic cores that formed during the cyclical collisions of the tectonic plates and then were shaped by most recent ice age. And though I've seen occasional outcrops on field trips, my training (and research) in geology has kept me well away from sedimentary rocks. So being surrounded by (and climbing on!) all this sandstone is a new experience for me.

The Flatirons are a conglomerate sandstone (the Fountain Formation) and are home to some classic multipitch rock climbing routes. In addition to their verticality, the Flatirons are conducive to climbing because the sandstone is particularly hard, making it resistant to erosion by the elements and by climber-hands. Just north of the Flatirons, the sandstone exposed in the Red Rocks outcrop (different from the famous Red Rocks Ampitheater) is an example of the type of sandstone that you don't want to climb. Though no less dramatic, the rock is crumbly and holds can easily break off in your hand.

Looking North to Red Rocks (Boulder)

The sandstone beds that make up Red Rocks initially formed as sediment was deposited in horizontal layers at the bottom of an inland sea that extended over much of the western US (link). But since then, the beds have tilted 90 degrees, resulting in the vertical towers as the softer sections eroded away. While hiking up to the outcrop I found myself wondering which way the rock was tilted - are the rocks to the east or the west younger? At this point I didn't really have a sense for the regional geology, but I happened across a clue to the answer.

Along the main trail to Red Rocks from Settler's Park, there are some smaller outcrops with some nice cross bedding. Based on the truncation of the beds, I'd say the sandstone towards the east is younger and used to be on top before being tilted to the current position.