More Fins Than a School of Fish! EarthCache
More Fins Than a School of Fish!
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This EarthCache is located in Red Rock Canyon Open Space, one of the newer parklands of the City of Colorado Springs. You will be taking the Contemplative Trail during this hike, and will be walking through several fins of Fountain Sandstone, also known as Fountain Arkose. The aim of this EarthCache is to get an up-close view and feel of this interesting type of sandstone. Please bring a ruler or other measuring device and plan on getting a little dirty.
38 51.143 104 53.048 Texture
The fins of rock before you, and through which you will be walking on this hike, are made up of Fountain Sandstone, the oldest type of rock found in this area of the park. Fountain Sandstone, also known as Fountain Arkose, is a conglomerate made up of coarse-grained sediments. About 300 million years ago, the Ancestral Rocky Mountains were eroding and the sediments were being deposited as stream gravels and alluvial fan deposits. The Ancestral Rocky Mountains were formed from granite, like much of today’s Rocky Mountains, and the erosion of this granite is where the feldspar in the Fountain Arkose came from. An arkose, by definition, is a rock formed from at least 25% feldspar, and the presence of the feldspar is what gives these fins their pink hue.
Question 1. From these coordinates, walk over to the nearest rock wall and run your hand down the rock. Is it smooth or rough? What is the reason for your answer?
38 50.963 104 53.130 Size Matters
Rocks formed from alluvial fan deposits often have a large differential in the particle size due to the nature of this type of deposition. Alluvial fans are formed from material which is carried in streams which travel through restricted areas such as canyons or narrow washes. When the water reaches the end of the restriction, the water spreads out and the energy of the water is dissipated. With the reduction of energy in the water, the material which was carried along drops out, with the larger material dropping first. Weather events have a large effect on the erosion of this material. Storms and spring snowmelt often cause larger particle sizes to be moved in the streams (boulders and gravels) versus smaller material (gravels, sands, and mud) which is moved during dry seasons.
Question 2. A good deal of Fountain Sandstone was formed from alluvial-fan-deposited material. From this location, look at the wall to the east and view the many different sizes of the particles making up the sandstone. Measure the largest and smallest particles that you can find in this wall.
Question 3. Looking at the sizes of the particles in the rock, do you think it was a fast-moving stream or slow-moving stream that deposited this material? Explain your answer.
38 50.867 104 53.217 The Overlook
From the overlook at this location, look north toward the Garden of the Gods Park. You can see how the rows of fins head northeasterly, then turn to the north and disappear from view. The fins of rock are almost vertical, but they didn’t start out this way. After the alluvial deposits were laid down horizontally about 300 million years ago, several successive layers of sediments covered this material until it lithified, or turned to rock. It is thought that these layers were about a mile deep when the Laramide Orogeny began about 65-70 million years ago. This is the mountain building episode that pushed up the current-day Rocky Mountains. As the mountains pushed upward, the layers of rock on the edges were uplifted and pushed into a vertical position. After erosion of much of the top-most material, the ridges of Fountain Sandstone were exposed.
Question 4. Follow the fins of Fountain Sandstone as they are exposed northward into the Garden of the Gods Park. Estimate the distance that you can see them before they disappear.
38 50.688 104 53.318 Crossbedding
As successive layers of alluvial material are deposited, each makes its own distinctive pattern. Some of the layers are made of material which contains the same sized particles, and other layers contain material with a large differential in particle size. These differences in particle size give an idea of the force of water that was active when the deposits were laid down. After lithification, these layering patterns are frozen into the rock.
Question 5. The layering at this location shows a crossbedding pattern from successive deposition of sands and gravels. Its easy to see each layer and the material that was deposited from each event. Using your ruler, measure off 6” (15cm) perpendicular to the bedding and count the number of layers.
To log this earthcache: Send me an email answering each of the 5 questions from the 4 stops. These are listed as follows:
1. Is the rock at the first stop smooth or rough? What is the reason for your answer.
2. Measure the largest and smallest particles on the wall at stop 2.
3. Looking at the particles of rock at the second stop, do you think it was a fast-moving or slow-moving stream that deposited this material? Explain your answer.
4. From the overlook, estimate the distance north that you can see the fins of Fountain Sandstone.
5. Count the number of layers in a 6” (15cm) length perpendicular to the layering at the fourth stop.
Please consider posting photos of yourself, or the local geology, when you log this EarthCache. Photos can be an additional rewarding part of your journey, but posting them is not a requirement for logging this EarthCache, and is strictly optional.
The above information was compiled from the following sources:
2007. Chronic H., and F. Williams. Roadside Geology of Colorado, 2nd Ed.
2004. Hopkins, R.L., and L.B. Hopkins. Hiking Colorado’s Geology
Colorado Springs City Webpage: Garden of the Gods Park. (visit link)
Colorado Springs City Webpage: Important Events in Garden of the Gods Geology. (visit link)
Colorado Springs City Webpage: Red Rock Canyon Open Space: (visit link)
Geologic Map of the Red Rock Canyon Area: (visit link)
Thanks to the City of Colorado Springs and the Parks, Recreation & Cultural Services Department for allowing placement of this EarthCache!
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