Arkose & Alluvial Fans

Today was my second field trip with my Sed Strat class. We went up to Settlers’ Park to look at some exposed facies there. (A Facies is a group of sedimentary structures you see in a rock that points to a particular environment that the sediment was deposited in.) If you’re ever in the Boulder area and up for a little bit of an uphill hike, I recommend it. The facies we looked at belonged to the Fountain Formation and the Lyons Formation.

The Fountain Formation is pretty famous, at least locally. A formation is a unit of rock that is geographically contiguous (it’s all connected) and clearly seperate from the formations above and below. Sometimes this seperation is due to a change in rock type, since formations are usually of a single lithology – which is to say composed of just one type of rock. However, sometimes formations are seperated by unconformities, which are boundaries caused by erosion and other events.

So calling it the Fountain Formation means that it’s a big unit of a single kind of rock that covers a definite geographical area. (In this case, a broad swath at the feet of the east face of the Rocky Mountains.) Fountain is the name of the formation. It is composed of sandstones and Arkose; the Arkose is the most famous and gives it its beautiful color. Arkose is a particular kind of sedimentary rock. Arkose is normally primarily quartz, but it has at least 25% Feldspar in it. This will give the rock a definite pink cast, or if its been exposed to any weathering, the Feldspar will cause iron oxide (remember: rust is iron oxide) that stains the rock anywhere from orange to a beautiful, deep red.

A large portion of the Fountain Arkose was deposited by alluvial fans. Alluvial fans are a phenomena found at the base of mountains. What happens is that there are canyons through the mountains – formed by rivers. During the spring melt (or intense storms), massive amounts of water will flood through these canyons, picking up lots and lots of sediment along the way. These canyons let out at the base of the mountains, and the water suddenly spills out in a characteristic fan-shape. (To visualize this, turn on a hose that’s laying on the ground. Notice how the water spreads out in a fan at the end of the hose.)

While the water is shooting through the canyon, it’s going very, very fast. This translates to the water having a lot of energy – and the more energy water has, the bigger rocks it can carry. As the water spills out of the canyon, it loses a lot of that velocity because it’s no longer directed in a channel formed by the canyon walls. So it drops everything that it was carrying.

Alluvial fan deposits are very interesting to look at. They’re composed of layer after layer of different kinds of mudstones, sandstones, and conglomerates. When the water first comes out of the canyon, it drops all of the big rocks that it picked up – anything from coarse sand to even boulders! The rocks formed from that are conflomerates – there’s a wide range of how big the clasts (the bits of rock that the river dropped) are, and some of them are very large. At other times, the water wasn’t moving fast enough to carry large rocks, and it will just drop sand, or even mud. So you will layers with all different clast sizes in them. Mudstones are often far darker than the layers above and below them, so you will see stripes running through the formation.

The Fountain Arkose formed from the erosion of the Ancestral Rocky Mountains – the mounains that existed in the past before today’s Rockies. They were worn completely down, and then a new session of mountain building brought today’s Rockies up. The Ancestral Rockies were made of granite as well – that’s where the Feldspar in the Arkose comes from. Feldspar is an “unstable” mineral. It is subject to chemical weathering, and because of its physical properties, it breaks into tiny pieces easily. So large deposits of Feldspar are normally found close to their source. If they’re buried quickly, they can’t be weathered away!

If you want to see the Fountain Formation, there are many good places to see it in Colorado. In Boulder, you can go to Settlers’ Park, where its been uplifted into a hogback – the originally flat layers of rock are standing vertically. Also in Boulder, the Flatirons are part of that formation. Red Rocks Amphitheatre is built in another exposure of the Fountain Formation. It can also be seen in Garden of the Gods. If you ever have a chance to go to any of these places, I recommend it. They’re beautiful, and there’s some good hikes in those areas along with great geology!

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