Doudy Draw EarthCache Trail
The EarthCaches in this series can all be done independently but if you are going on the hike you might as well check out all of the stops. These EarthCaches are not meant to be tricky however weather can cause difficulties. This series is meant to be fun. No one should ever have to DNF an EarthCache. Do your best and enjoy the view. If you park in the parking lot at the trailhead there is a fee for non-residents of Boulder County.
Earth Science Lesson
Approximately 300 yards along the trail to the right of the bridge, you can see and hear a dam in the ditch. Walk over to the edge where you can inspect it (Figure DD-WP4). Here a stronger sandstone layer in the Pierre Shale resists erosion by the running water far more than does the enclosing shale, so it has formed a dam. It also clearly demonstrates the angle at which the shale is uplifted—just the same angle as the Flatirons and the Dakota ridge a little farther west—a little over 50 degrees. This dam is formed by the Hygiene Sandstone member of the Pierre, which forms a small ridge at a number of locations to the northeast in Boulder County, particularly near the town of Hygiene, after which it is named.
Dam geology - A dam is a barrier that stops or restricts the flow of water or underground streams.
Pierre Shale - The Pierre Shale /pɪər ʃeɪl/ is a geologic formation or series in the Upper Cretaceous which occurs east of the Rocky Mountains in the Great Plains, from Pembina Valley in Canada to New Mexico.The Pierre Shale was described by Meek and Hayden in 1862 in the Proceedings of the Academy of Sciences (Philadelphia). They described it as a dark-gray shale, fossiliferous, with veins and seams of gypsum, and concretions of iron oxide. The Pierre Shale is about 700 feet (210m) thick at the type locality. It overlies the Niobrara division and underlies the Fox Hills beds. It was named for an occurrence near Fort Pierre on the Missouri River in South Dakota.
Sandstone is a sedimentary rock composed mostly of quartz sand, but it can also contain significant amounts of feldspar, and sometimes silt and clay. Sandstone that contains more than 90% quartz is called quartzose sandstone. When the sandstone contains more than 25% feldspar, it is called arkose or arkosic sandstone. When there is a significant amount of clay or silt, geologists refer to the rock as argillaceous sandstone. The color of sandstone varies, depending on its composition. Argillaceous sandstones are often gray to blue. Because it is composed of light colored minerals, sandstone is typically light tan in color. Other elements, however, create colors in sandstone. The most common sandstones have various shades of red, caused by iron oxide (rust). In some instances, there is a purple hue caused by manganese. Sandstones make up about 20 to 25 percent of all sedimentary rocks. Most sandstone is composed of quartz or feldspar (both silicates) because they are the most resistant minerals to weathering processes at the Earth's surface, as seen in the Goldich dissolution series.
An orogeny is an event that leads to both structural deformation and compositional differentiation of the Earth's lithosphere (crust and uppermost mantle) at convergent plate margins. An orogen or orogenic belt develops when a continental plate crumples and is uplifted to form one or more mountain ranges; this involves a series of geological processes collectively called orogenesis.
Logging Tasks
- Using the attached diagram, label the rocks. Which is #1? Which is #2?
- Which rock shows an example of uplift?
- Explain why the sandstone layer resists erosion better.
References
- Tony Waltham (2009). Foundations of Engineering Geology (3rd ed.). Taylor & Francis. p. 20. ISBN 978-0-415-46959-3.
- Philip Kearey; Keith A. Klepeis; Frederick J. Vine (2009). "Chapter 10: Orogenic belts". Global Tectonics (3rd ed.). Wiley-Blackwell. p. 287. ISBN 978-1-4051-0777-8.
- “Sandstone.” Minerals Education Coalition, 3 Apr. 2017, mineralseducationcoalition.org/minerals-database/sandstone/#:~:text=Sandstone is a sedimentary rock,called arkose or arkosic sandstone.
- “Sandstone.” Wikipedia, Wikimedia Foundation, 19 Jan. 2021, en.wikipedia.org/wiki/Sandstone.
- Boggs, Sam (2006). Principles of sedimentology and stratigraphy (4th ed.). Upper Saddle River, N.J.: Pearson Prentice Hall. pp. 119–135. ISBN 0131547283.
