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
SLUMPS Junction of Spring Brook south and north branches. Turn right on the north branch. The trail now climbs towards the mesa top and is cut primarily into the Pierre Shale bedrock. If you examine the slope ahead of and behind you, you’ll see examples of past slumps here and there on the slope. (The shelf just below the trail is an old railroad grade, not a geomorphological feature.) Examine the trail cut as you climb up this section. You’ll see places where the Pierre Shale is quite visible, with small (¼- to ½-inch), platelike fragments visible in the cut. When wet, the shale turns to a slippery, gooey, sticky mass. (It has a high clay content.)
The geomorphology of the slumps is that when heavy precipitation or melting snow saturates the slope, the weight of the saturated material causes it to slide, and material moves downslope. This undercuts the material above, which can slide in its turn. Ultimately, the edge of the gravel cap is undercut, and that material rolls down the slope, forming the thin layer of colluvium at the slope surface. Evidence of many past slumps is everywhere on these slopes, which are all at the steepest angle that the bedrock will support (the angle of repose). They are basically falling downhill over time. The implication of all this is that the trail cut into this slope is not very sustainable. It will suffer from slumps, and their timing will be determined by precipitation patterns. The vegetative cover on the slopes can shed moderate amounts of rain, but the only plants that can grow on the slopes have shallow root systems. They can’t effectively stabilize the slope, because it is formed by weak and impermeable Pierre Shale. Under conditions when the slope becomes saturated with water it will slump, and sections of the trail will fall down the hill.
A slump is a form of mass wasting that occurs when a coherent mass of loosely consolidated materials or a rock layer moves a short distance down a slope. Movement is characterized by sliding along a concave-upward or planar surface. Causes of slumping include earthquake shocks, thorough wetting, freezing and thawing, undercutting, and loading of a slope.
Translational slumps occur when a detached landmass moves along a planar surface. Common planar surfaces of failure include joints or bedding planes, especially where a permeable layer overrides an impermeable surface. Block slumps are a type of translational slump in which one or more related block units move downslope as a relatively coherent mass.
A rotational slump occurs when a slump block, composed of sediment or rock, slides along a concave-upward slip surface with rotation about an axis parallel to the slope.[3] Rotational movement causes the original surface of the block to become less steep, and the top of the slump is rotated backward. This results in internal deformation of the moving mass consisting chiefly of overturned folds called sheath folds.
Slumps have several characteristic features. The cut which forms as the landmass breaks away from the slope is called the scarp and is often cliff-like and concave. In rotational slumps, the main slump block often breaks into a series of secondary slumps and associated scarps to form stair-step pattern of displaced blocks. The upper surface of the blocks are rotated backwards, forming depressions which may accumulate water to create ponds or swampy areas. The surface of the detached mass often remains relatively undisturbed, especially at the top. However, hummocky ridges may form near the toe of the slump. Addition of water and loss of sediment cohesion at the toe may transform slumping material into an earthflow. Transverse cracks at the head scarp drain water, possibly killing vegetation. Transverse ridges, transverse cracks and radial cracks form in displaced material on the foot of the slump.
Logging Tasks
- How many slumps are you able to identify?
- What characteristics of a slump did you use in your identification?
- How do you believe this slump occurred?
References
- Tarbuck, E.J.; Lutgens, F.K. (1998), Earth, an introduction to Physical Geology (6th ed.), Prentice Hall, pp. 219–220, ISBN 978-0139741227
- Girty, G. H. (2009), "Landslides" (PDF), Perilous Earth: Understanding Processes Behind Natural Disasters, Montezuma Publishing, pp. 1–17
- Easterbrook, D. J. (1999), Surfaces Processes and Landforms (2nd ed.), Prentice Hall, ISBN 978-0138609580
- Hansen, M. C. (2000), Earthquakes and Seismic Risk in Ohio, Ohio Department of Natural Resources, Division of Geological Survey
