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
The clay pit. Note that looking back , several layers of rock were stripped away to reach the clay layer in which the miners were interested. In looking at the upper section that was stripped away, note the alternating layers of sandstone and shale (respectively strong and weak). These connote alternating periods of deposition of sand in river deltas and foreshore environments, interspersed with muddy plumes that deposited fine-grained rock layers with far less strength and smaller clast size (sea bottom environments collecting fine silt and clay-size particles, farther from the coastline).
Sedimentary rocks are the product of the erosion of existing rocks. Eroded material accumulates as sediment, either in the sea or on land, and is then buried, compacted and cemented to produce sedimentary rock (a process known as diagenesis).
There are two major groupings of sedimentary rocks:
Clastic sedimentary rocks
The fragments of pre-existing rocks or minerals that make up a sedimentary rock are called clasts. Sedimentary rocks made up of clasts are called clastic (clastic indicates that particles have been broken and transported). Clastic sedimentary rocks are primarily classified on the size of their clasts.
Clast shape, or the degree of rounding of clasts, is important in differentiating some sedimentary rocks. Clasts vary in shape from rounded to angular, depending on the distance they have been transported and / or the environment of deposition, e.g. rounded clasts are generally the product of long transportation distances and / or deposition in high energy environments (beaches, rivers).
Clasts in sandstone
The degree of sorting of clasts can be an important indicator of a depositional environment. In water, larger clasts are generally not transported great distances, and they settle faster. For example, in a mixture of mud and sand being transported in a river to the sea, the sand (larger clast size, heavier) would begin to deposit as soon as the river's energy dissipated, while the mud (fine, light-weight) would be transported far off shore. Therefore, a well sorted (clasts of approximately the same size), coarse sandstone indicates deposition in a reasonably high energy environment (near-shore) probably close to the source of the sand. Conversely, a mudstone generally indicates deep water deposition (low energy environment, far off shore).
Structures produced during deposition, e.g. bedding and cross-bedding, can give clues as to the depositional environment. So can structures produced by re-working by tidal or storm-generated currents, e.g. ripple marks, rip-up clasts.
Logging Tasks
- How many layers are you able to identify?
- What do the alternating layers indicate?
- What are the two types of layers described in terms of thickness and color?
- Identify the clast size of some of the layers?
References
- “Sedimentary Rocks.” Geology - Rocks and Minerals, University of Aukland, 2005, flexiblelearning.auckland.ac.nz/rocks_minerals/rocks/sedimentary.html.
- The Editors of Encyclopaedia Britannica. “Law of Superposition.” Encyclopædia Britannica, Encyclopædia Britannica, Inc., 2019, www.britannica.com/science/law-of-superposition.
