Talus, Scree, and Angle of Repose:
Talus is another name for the rock debris caused by a rockfall. It is formed when chunks of rock fall away from the bedrock. Rockfalls can be caused by a number of ways. Triggering mechanisms like water, ice, earthquakes, and vegetation growth are among the final forces that cause unstable rocks to fall. We will focus on two processes that causes rockfalls in this earthcache lesson: frost wedging and degradation. Frost wedging is the process in which water fills cracks and crevices in the rock, and then freezes. This fluctuation in temperatures causing the water to freeze can break off chunks of the rock. Water expands by approximately 9% when frozen, so this widens cracks, and eventually pushes the rock away from the rest. It then plunges to the ground below. Another way rockfalls can occur is simply by the degradation of the rock bonds being to much for the cliff. It is not the weight of the rock that triggers it to fall, but rather the progressive degradation of the bonds (physical or chemical) that hold the rock in place. This degradation can occur from water, heat, or just sub-critical crack growth. But it is the degradation that is the trigger, not the weight of the rock. Some valleys that have been cut by glaciers (like Yosemite Valley) can be carved to steeply. When the ice has melted, the sheer cliff cannot support the weight of some rocks at the top, and the cliffs crumble from lack of support.
Once the rocks fall, they form talus slopes, which are the evidence of rockfalls you see today. Talus will continue to pile up to the angle of repose. The angle of repose is the steepest angle a rock pile or sand dune can achieve. The angle is determined by the size of rocks or grains. Naturally, larger chunks will have a higher angle of repose. In general, talus slopes can form at the base of mountains, cliffs, valleys, and even volcanoes!
Scree slopes are smaller rocks, making them have a smaller angle of repose. Slopes with rocks less than one foot in size are referred to as scree slopes. If you have ever tried to climb a talus or scree slope, you will notice talus is much easier to scramble over than scree. The large rocks (usually) hold their position (higher angle of repose) than the small rocks of a scree slope.
Logging this cache:
Please e-mail with the following:
•The name of this Earthcache on the top line
•Number of people in your party
•Is this slope seen here a talus slope or a scree slope? Why do you think that?
•Do you think the pile has reached it's angle of repose?
•Was the process of rockfall frost wedging, degradation, or neither?
Most importantly, have fun with this earthcache.
Sources:
1) http://en.wikipedia.org/wiki/Scree
2) http://www.geocaching.com/geocache/GC2M28R_merced-river-talus-slope?guid=896b17c4-34e7-4645-975e-ecbc9005cf82
3) http://geology.campus.ad.csulb.edu/people/bperry/Mass%20Wasting/RockFallDiagramAfterS.GIF