Welcome to Point Reyes National Seashore
The Point Reyes Conglomerate is a classic conglomerate of rock, formed by an underwater landslide. Sedimentary conglomerate is a collection of larger stones and pebbles that are suspended within a fine sandstone matrix (think of chocolate chips in a cookie!). You can read about the underwater landslide that created this formation here! The Point Reyes Conglomerate is a rare rock formation, occupying only a very small area here at the tip of Point Reyes penninsula. However, it appears to be identical to the Carmelo Formation near Point Lobos.
The conglomerate contains clasts (geological chunks) that include purple and pink stones. These stones are visible if you take a look at the rock wall in front of you!
These pink and purple rocks are a type of rock called porphyry tuff. Tuff is an igneous rock that is formed from the concentration and compaction of volcanic ash. Porphyry is simply an adjective indicating that an igneous rock is consisting of coarse-grained crystals. These crystals can be a number of different things; the term porphyry is not specific to any particular crystal. The pink and purple chunks of tuff in front of you contain crystals of feldspar. Feldspar is a very common mineral, but the colors can vary widely. The colors can be influenced by iron, potassium, or calcium, among other things. The bits of tuff also contain clusters of zircon and titanomagnetite grains.
PLEASE DO NOT CHIP AT OR TAMPER WITH THE FORMATION.
To log this cache, please send over the answers to the following questions, with the name of this cache on the first line:
1. How many people are in your group?
2. Locate an example of the purple porphyry tuff. What color are the crystals within the chunk of porphyry tuff?
3. Is the porphyry tuff smooth or rough? Why is this?
Sources:
-https://pubs.usgs.gov/of/2005/1127/chapter9.pdf
-https://s3.amazonaws.com/gs-geo-images/949c28d9-af79-4d2f-9e7c-059b3af6a8f8_l.jpg
-https://noc.ac.uk/news/will-sea-level-rise-lead-more-underwater-landslides
-https://www.sciencedirect.com/topics/earth-and-planetary-sciences/turbidite
-https://geo.libretexts.org/Courses/University_of_California_Davis/GEL_109%3A_Sediments_and_Strata_(Sumner)/Textbook_Construction/Turbidites
-https://en.wikipedia.org/wiki/Clastic_rock
-https://www.britannica.com/science/tuff
-https://en.wikipedia.org/wiki/Porphyry_(geology)
-https://www.britannica.com/science/feldspar