When the power company replaced its electrical transmission tower here a few years ago, it cleaned up this stretch of St. James Street and uncovered the excellent rock exposure on display above the sidewalk.
Many pieces of rock have fallen from the slope onto the sidewalk border. Pick up a few and inspect them. The light-colored material that makes them up should look familiar. However, the rocks are also full of cracks that are lined with a rusty-brown color. These fractures aren’t considered part of the rock’s color, but they’re the real attraction of this cache.
Notice that there are several large brown flat surfaces in the rocks. These are slickensides. They represent a large fracture, or fault, where the rocks have slipped past each other and polished the surface by the force of friction.
Examine the slickensides carefully. Notice that they are not perfectly flat but slightly corrugated. The linear traces on their surfaces, called slickenlines, are evidence of the direction that the rocks moved against each other. Sometimes it is possible to tell the exact sense of that motion; that is, whether one face of the fault moved left, right, up or down with respect to the other face. Here I don’t think it’s possible, but you can try!
Geologists measure the orientation of slickensides with three numbers. The strike is the compass direction of a horizontal line drawn across the surface. The dip is the angle of the slope going straight down from the strike direction. The rake is the downward angle of the slickenlines, measured directly on the fault surface. Geologists use a specialized compass, plus lots of practice, to measure these numbers accurately, but you can get close enough to earn the find for this cache just by eyeball and the functions on a GPS unit or smartphone.
There’s another much larger slickenside in San Francisco, in Corona Heights at the top of Beaver Street. It’s quite famous among geologists!
To earn the find for this cache, send me your answers to these questions:
1. What material is this rock made of? That is, what type of rock would you call it?
2. What is the strike of these slickensides? A simple compass direction like “northwest” will do.
3. What is the dip of these slickensides? A rough estimate like “45 degrees” is good enough.
4. What is the rake of the slickenlines, approximately?
5. How stable would you say this rock exposure is?