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. All of the stops are listed as waypoints at the bottom of this page. You do not have to do them all. It is just for convenience. If you park in the parking lot at the trailhead there is a fee for non-residents of Boulder County.
Earth Science Lesson
You should start at the Doudy Draw parking lot. Note the overall landscape around you. Consider the slope to the east; it consists of a thin cover of colluvium over the bedrock of Pierre Shale, sloping up at the angle of repose. The level area at the top of the slope has been protected by a cap of gravel known to geologists as the Verdos Alluvium (the Verdos constitutes the terrace just below, and a little younger than, Rocky Flats Alluvium), which was deposited 640,000-680,000 years ago (Birkeland et al. 1996; Chadwick, Hall, and Phillips 1997). This capping layer of gravel resists erosion, except at the edge of the slope. When it is undercut, gravel at the edge slides down the incline and forms the colluvium on the slope itself and at the base, where you are standing.
Angle of repose - The angle of repose, or critical angle of repose, of a granular material is the steepest angle of descent or dip relative to the horizontal plane to which a material can be piled without slumping. At this angle, the material on the slope face is on the verge of sliding. The angle of repose can range from 0° to 90°. The morphology of the material affects the angle of repose; smooth, rounded sand grains cannot be piled as steeply as rough, interlocking sands. The angle of repose can also be affected by additions of solvents. If a small amount of water is able to bridge the gaps between particles, electrostatic attraction of the water to mineral surfaces will increase the angle of repose, and related quantities such as the soil strength.
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.
Colluvium - Colluvium (also colluvial material or colluvial soil) is a general name for loose, unconsolidated sediments that have been deposited at the base of hillslopes by either rainwash, sheetwash, slow continuous downslope creep, or a variable combination of these processes. Colluvium is typically composed of a heterogeneous range of rock types and sediments ranging from silt to rock fragments of various sizes. This term is also used to specifically refer to sediment deposited at the base of a hillslope by unconcentrated surface runoff or sheet erosion.
Logging Tasks
- Are you able to identify the colluvium?
- Can you see evidence of the underlying Pierre Shale?
- Estimate the angle of repose based on the location of the rocks?
- Based on the angle where are most of the rocks on the hillside (top/bottom)?
References
- Canadian Fossil Discovery Centre. "All About Bruce". Archived from the original on 2014-11-06.
- Mehta, A.; Barker, G. C. (1994). "The dynamics of sand". Reports on Progress in Physics. 57 (4): 383. Bibcode:1994RPPh...57..383M. doi:10.1088/0034-4885/57/4/002.
