Big Thompson Canyon - Dipping Into The Narrows EarthCache
Hidden : 12/3/2008
Difficulty:
2.5 out of 5
Terrain:
1 out of 5

Size: Size: other (other)

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Geocache Description:


Parking for this EarthCache is by the Forest Service sign at a paved pullout on the right side of the road as you head upstream. Please take care at this location, as vehicles are moving fast and lines of sight are shortened by curves in the road. You will be measuring the angle and direction of rock beds for this EarthCache, so have a clinometer, or use the clinometer portion of a compass to answer the questions.

At this site, you are in an area of the Big Thompson Canyon called The Narrows. This is an area about 2 miles long where the width of the canyon is noticeably thinner than that to the east or west, and where the river makes several tight turns on its way out of the mountains towards the plains.

To the east, and across the Big Thompson River, you can see steeply dipping beds of rock. These are layers of metasedimentary rocks which contain sills of light gray trondjemite. The term “metasedimentary” means that these are sedimentary rocks which have undergone metamorphism at some time in the past. Trondjemite is an intrusive igneous rock.

These rocks are ancient, and on the order of 1.75 billion years old. The material forming the sedimentary layers was laid down in horizontal beds. Over time, these layers were covered over and lithified into sedimentary rocks. Still later, there were several stages of metamorphism and deformation which raised the beds to the form in which you now see them.

Dip and Strike

Dip and Strike are measurements that define the orientation of a geologic feature. The dip gives the angle below the horizontal of a tilted feature. Dip is measured as an angle and a direction, and can vary from 0 degrees (horizontal) to 90 degrees (vertical). The strike is a line that intersects the feature with the horizontal, and is usually given as a compass bearing. For example, if you had beds of sedimentary rock dipping at an angle of 45 degrees to the south, the dip would be 45 degrees to the south, and the strike would be 90 degrees east, or 090.

To receive credit for this Earthcache, complete the following: Determine the Dip and Strike of the beds of metasedimentary rock east across the river from the Forest Service sign and send this to me in an email.

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:

Wikipedia. Strike and Dip. (visit link)

Strike and Dip. University of Saskatchewan, Department of Civil and Geological Engineering. Online at: (visit link)

1976. Braddock, W.A. Road Log, Precambrian Geology of the Northern and Central Front Range, Colorado; First Day, Northern Front Range Professional Contributions of the Colorado School of Mines, Studies in Colorado Field Geology. No. 8, P. 1-8.

1987. Hutchinson, R.M., and W.A. Braddock. Precambrian Structure, Metamorphic Mineral Zoning, and Igneous Rocks in the Foothills East of Estes Park, Colorado. Geological Society of America Centennial Field Guide – Rocky Mountain Section, 1987.

Thanks to the Roosevelt National Forest for allowing placement of this EarthCache!

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