The coordinates here will bring you to a great viewing point of the rare Alcatraz terrane. In short, a terrane is just a fault-bounded area of rock that have unique geologic features and history than the other large areas of rock surrounding it.
The Alcatraz terrane is comprised of greywacke (or graywacke) turbidites that contain fossils! The presence of fossils helps determine that the terrane sediments were deposited around 130-140 milion years ago. A turbidite is sediment that has been deposited as a result of turbidity flows, which can be pictured as an underwater landslide due to seismic activity (earthquakes) or gravitational forces (collapsing slopes, etc).
Greywacke is sandstone that is full of a lot of other angular rocks, leading it to often endure being called “dirty sandstone.” Also greywacke is know for its poorly sorted angular particles and impurities, it is actually a very hard rock. It comes in at around a 6-7 on the Mohs Scale.
Lastly, lets cover where exactly the Alcatraz terrane exists. For starters, Alcatraz Island is indeed made up of the greywacke in the Alcatraz terrane. This particular terrane also makes up a fair amount of the hills that give San Francisco its unique shape. Although most of these hills have been heavily developed, this Earthcache will take you to one of the best viewing opportunities here at Telegraph Hill. The Alcatraz terrane runs in an imperfect diagonal line from the south side of the Golden Gate Bridge to the edge of the China Basin neighborhood.
To log this earthcache, please send a note with the following:
1: Name of this cache and number of people in your group.
2: Since greywacke is a sandstone full of angular rocks of different shapes and sizes. As you approach the outcropping to get a closer view (coordinates listed as waypoint), do you see any irregularities? If so, describe the particle size distribution of the different material you see (in other words, describe the different types of particles and their size as they relate to each other).
3: The Alcatraz Terrane covers a relatively small section of ground. Considering the process of turbidities, explain why this rock feature occupies a relatively limited space (more than one possible answer).
4: Considering San Francisco's history with earthquakes, explain why having greywacke bedrock is beneficial to the City.
Have fun and pictures are welcome!
Sources:
-https://pubs.usgs.gov/bul/b2188%202/b2188ch3.pdf
-https://www.sciencedirect.com/topics/earth-and-planetary-sciences/terrane
-https://www.sciencedirect.com/topics/earth-and-planetary-sciences/turbidite
-https://www.e3s-conferences.org/articles/e3sconf/abs/2019/59/e3sconf_ag2019_02003/e3sconf_ag2019_02003.html
-https://www.sciencedirect.com/topics/earth-and-planetary-sciences/graywacke
-https://www.geocaching.com/geocache/franciscan-complex-alcatraz-terrane
-https://rocks.comparenature.com/en/properties-of-greywacke/model-41-6
-https://www.geologictrips.com/sf/sfpal.htm
-https://blogs.egu.eu/divisions/ts/2020/11/30/san-francisco-where-the-plates-meet/
-https://oceanservice.noaa.gov/facts/turbidity.html