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
Two hundred yards farther along you’ll reach a bridge across Community Ditch1, dug in 1885. Before the bridge turn right on the social trail leading along the north bank of the ditch (no bikes allowed; lock them at the bridge). As you walk west, paralleling the ditch, note that the walls of the ditch are made up of incompetent Pierre Shale. This shale is weak, and it degrades rapidly to dirt, but because the ditch is a relatively fresh excavation, the layers of shale are visible at places in the walls, and they are tilted at the same angle as the Flatirons to the west—in the vicinity of 50 degrees.
Incompetent Pierre Shale - In the Pierre area, the apparent lack of uniform trend of structural features in outcrops suggests that the incompetent Pierre shale has warped and fractured in random directions in response to stresses that might be reflected by folds and faults of a more definite pattern in the more competent rocks at greater depths.
Competent rocks are more commonly exposed at outcrops as they tend to form upland areas and high cliffs or headlands, where present on a coastline. Incompetent rocks tend to form lowlands and are often poorly exposed at the surface. During deformation competent beds tend to deform elastically by either buckling or faulting/fracturing. Incompetent beds tend to deform more plastically, although it is the 'competence contrast' between different rocks that is most important in determining the types of structure that are formed. The relative competence of rocks may change with temperature, such as in metamorphosed limestones, which are relatively competent at low metamorphic grade but become highly incompetent at high metamorphic grade.
Logging Tasks
- Is the soil competent or incompetent?
- Can you see shale exposed in the wall?
- What is the angle compared to the surrounding Flatirons (west)
References
- Fossen, H. (2010). Structural Geology. Cambridge University Press. p. 430. ISBN 978-0-521-51664-8. Retrieved 1 April 2011.
- ^ Cristescu, N. (1989). Rock rheology. Mechanics of elastic and inelastic solids. 7. Springer. p. 1. ISBN 978-90-247-3660-7. Retrieved 1 January 2021.
