The rock face in question is at a pullout just north of Devil's Elbow, a sharp turn as your are driving up to Angwin from St. Helena.
Parking pullout available across the street from the cache.
Geology:
If you've worked on earthcaches at any point in the past, you have probably come to realize that rocks are not "static" that is, they do not stay the same throughout time. In fact, geology is primarily about the process of change that takes place in rocks, based on basic external pressures (water, heat, pressure, wind). To help us understand the inter-relationships between these forces and the rocks they affect, some scientists have created a fundamental "rock philosophy" around something called the rock cycle.
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| A diagram of the rock cycle. Legend: 1 = magma; 2 = crystallization (freezing of rock); 3 = igneous rocks; 4 = erosion; 5 = sedimentation; 6 = sediments & sedimentary rocks; 7 = tectonic burial & metamorphism; 8 = metamorphic rocks; 9 = melting. |
The rock cycle describes the ongoing transitions through geologic time among the three main rock types: sedimentary, metamorphic, and igneous. In the diagram, each type of rock is altered or destroyed when conditions around it change (heat, pressure, wind, water). An igneous rock such as basalt may break down and dissolve when exposed to the atmosphere (wind, water, heat), or melt as it is subducted (pressure) under a continent. Due to the driving forces of the rock cycle, plate tectonics and the water cycle, rocks do not remain in equilibrium and are forced to change as they encounter new environments.
The rock cycle is an illustration that explains how the 3 rock types are related to each other and how processes change from one type to another over time. If you follow the arrows in the diagram, you can see how one factor interacting with a type of rock can produce multiple results and that during extended periods of time, a certain "piece of rock" may have igneous, metamorphic, and sedimentary stages.
Geology of the Angwin/St. Helena Area:
Angwin is located in a series of "hills" formed by a now long extinct volcanic chain (igneous). There are no cones nor visible flows, but basalt, obsidian, and other rocks in the area demonstrate the relative recentness of the volcanic event (Sonoma Volcanics). In front of you is a rock face composed of basalt that is being altered to SOMETHING. It was placed here by igneous action. Your job is to determine the NEXT TWO LIKELY STEPS in the rock cycle that this rock face is likely to go through.
Logging Requirements:
- Give the name and GC# of this cache and list the number of people in your party
- Choose one of the following based on your reading above. The next two steps on the Rock Cycle that this rock face is likely to experience are:
- Sedimentation (Sediments & Sedimentary Rocks) and Tectonic Burial
- Subduction and Tectonic Burial
- Melting/Magma and Crystalization
- Erosion and sedimentation
- (optional) take a picture of you and/or your GPS at the overlook 1/4 mile "downhill" from this rockface. OR
- give the elevation at the GZ and estimate the height of the rock face.
Congrads to Shark Z for yet another FTF
Resources:
Wikipedia "Rock Cycle" article
Alt & Hyndman, Roadside Geology of Northern andCentral California. Mountain Press Publishing. Missoula: Montana. 2000.