Geo Lab - San Gregorio Fault EarthCache

Along the Big Sur coastline, just an easy walk from the highway, lies an easily accessible fault to explore!

The Big Sur coastline is a perfect example of an emerging coastline. Along this coastline, the Pacific and North American plates once collided together to form the inland Sierra Nevadas and the local Santa Lucias. Since geology works on extremely long time scales, this once converging plate boundary has now changed to one that slips past one another, also known as a transform plate boundary. These places between plates are called faults. The most famous fault in California is the San Andreas Fault.

At this Earthcache site, one can get up close and touch an actual fault. This fault, called the San Gregorio Fault, runs parallel to the San Andreas Fault. It runs from south of Big Sur, across Monterey Bay, and converges with the San Andreas Fault north of San Francisco near Bolinas Bay.

Typically, the easiest way to identify a major fault is to look for offset features. Another way is to look for crushed or pulverized rock between two relatively strong masses. If damp enough, you can easily form balls of clay from the material. Lastly, if one encounters two different rock types relatively close to one another, more than likely a fault is nearby.

To log this visit, send me an email with:

• "San Gregorio Fault - GC2YTK1" in the first line
• To identify the San Gregorio Fault, look for two different rock types. This can be difficult due to local conditions, but look for adjacent rocks with totally different texture. Can you identify the two different rock types?
• Identify the type of man made structure found near the fault. Is this a smart place to construct this structure? Why or why not?
• The cache site seems to have tapped a source of water hidden in the cliff side. Why might faults expose hidden water sources?