One sedimentary process will be studied in this earth cache – the formation of a scree and/or talus observed on the side of the cliff away from the road. Do not attempt this EarthCache if you are not capable.
The scree is a collection of broken rock fragments at the base of the cliff side that has accumulated through periodic rockfall from the adjacent cliff faces. The landform associated with these materials are often called talus deposits. Talus deposits typically have a concave upwards form, while the maximum inclination corresponds to the angle of repose of the mean debris size. The formation of a scree (in general) is from the result of physical and chemical weathering and erosion acting on the rock face. The predominant processes that degrade a rock slope depend largely on the regional climate (temperature, amount of rainfall, etc.). Examples are:
- Physical weathering by ice
- Chemical weathering by mineral hydration, dissolution, and salt deposition
- Thermal stresses
- Topographic stresses
- Biotic processes
Scree slope – A portion of a hillside or mountainslope mantled by scree and lacking an up-slope rockfall source (i.e., cliff).
Talus – Rock fragments of any size or shape (usually coarse and angular) derived from and lying at the base of a cliff or very steep rock slope. The accumulated mass of such loose broken rock formed chiefly by falling, rolling, or sliding.
Talus slope – a portion of a hillslope or mountainslope mantled by talus and lying below a rockfall source (e.g., cliff).
The recognition and characterization of talus slopes is often important in determining the potential for mass movements (landslides, etc.). Movements occur whenever the talus slope exceeds the critical angle. The exact angle at which failure takes place depends upon the materials (e.g., rock type and shape), rock size, moisture content, but dry homogenous materials in a pile generally experience slope failure when the angle of repose (the resting slope angle) exceeds 33–37°. The critical angle lowers as materials become less intrinsically cohesive or when friction between particles is reduced by rain or other forms of moisture. Moisture also adds to the overall mass of the slope and thus increases the gravitational force on the slope.
We are now ready to interpret what we observe at GZ. This will be done by using the questions below and the text above.
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Questions:
1. Based on the observations you make from the top of your climb – which of the scree forming weathering processes above do you believe is most important (name one or two).
2. Study closely the rock in the scree, you do not have to walk into the scree. Based on your observation of the rock; do you belive this is a rock resistant to weathering or a rock that easily will weather; explain your reasoning.
3. Go to the edge of the scree field and estimate the angle (in degreees – 90 degress is vertical) of the average scree surface relative to the horizontal plane.
4. Are you able to locate any talus? How can you distinguish it from the scree?
5. In your best estimate - what would happend if the location was steeper?
6. On level ground you can go up or down. There is a difference in the two slopes. Can you identify each as a scree or talus slope?
7. REQUIRED Take a picture of yourself, or a proxy, at the site without revealing any of the answers to the questions above.