This cache is located at a State Beach in Santa Cruz. It cost $10 to park in the parking lot listed in the waypoints. This cache is only available during park hours.
Natural Bridges Earth Cache
About the Natural Bridges State Beach
The Natural Bridges State Beach in Santa Cruz is the location of a unique geological structure. It is the remainder of what once was a natural bridge composed mostly of Santa Cruz mudstone. This structure started out as a cliff jutting out into the Pacific Ocean that eroded into sea caves and then into arches that composed the natural bridge. There were originally three arches to this structure that have seen significant change from the 1900s to the present. The changes have been photographed over the years and can be seen in the photo below. People used to be able to walk, climb, and even drive on the natural bridge that once stood here, but are now closed to the public to preserve the structure as long as possible. As you can see at the GZ, there is only a small portion of the once large structure that still remains standing today. Unlike other geological structures all over the world that can take thousands and sometimes over a million years to form(i.e mountain ranges, deserts, canyons), this structure formed much more rapidly. So how did this marvelous structure come to be?
Santa Cruz Mudstone Makeup
Santa Cruz mudstone was previously considered to be part of the Monterey Formation, but it is now a standalone geological formation. The layer of mudstone is sandwiched between two other geological formations found in this part of Santa Cruz. It lies on top of Santa Margarita sandstone and lies under the Purisima Foundation. The minerals that it consists of include quartz, feldspar, mica, silica, and mostly consists of clay minerals. It dates back to the late Miocene period and consists of “thick bedded siliceous mudstone with bituminous sandstone interbeds.” The mudstone was dated by a man named J. A. Barron and he was able to do so based on the benthic foraminifera(a type of zooplankton) and diatoms(a specific type of algae characterized by having a symmetrical body and a thick siliceous covering) that are present in the mudstone. This mudstone also preserves bivalve and echinoid fossils as well as vertebrates of Parabalaenoptera baulinensis(a prehistoric species of baleen whale) and otodus megaladon(prehistoric species of giant mackerel shark). So why do I mention the geological makeup of Santa Cruz mudstone? Let’s go to the next section to see why it is relevant.
Coastal Erosion
Many coastlines around the world are often the same over a person’s lifetime of visits while others can change quickly during a person’s lifetime. There are many different types of rock types that make up the different coasts. Some rock types are very resistant to erosion (i.e. granite, basalt) while other rock types are more susceptible to coastal erosion (i.e. limestone and sandstone). In addition to various rock qualities like hardness and susceptibility to erosion, there are also often structural weaknesses or irregularities that cut through and penetrate the rocks composing coastal cliffs or bluffs. These weak areas can be created by different things such as faults, joints, fractures, or other features in the earth’s surface. Waves from the ocean have a way of finding these weak areas and eroding away at the coastline. This erosion eats away at these weak points creating caves, tunnels, natural bridges, and arches. The Pacific Ocean creates the strongest waves in the world; just imagine those waves during a strong winter storm. A storm such as this occurred on January 10th, 1980. During this storm, the inner arch at Natural Bridges State Beach fell. The outermost arch fell sometime during the early 1900s(approximately 1905-1906).The waves off the Santa Cruz coastline range between 10-12 feet high, during this storm they were more rough and higher than average. With an ocean this powerful and the rock composition of the natural bridge and the surrounding coastline, the average rate of erosion is higher at this location in comparison to other beaches. Most beaches have an average rate of erosion that is equivalent to a few inches to about a foot per year. Here in Santa Cruz we see cliff failure occur regularly with large slabs of the cliffs falling to the beach or structures like natural bridges and arches catastrophically collapse. Some natural bridges and arches may last a century or longer (i.e. Owachomo Natural Bridge aka “The Old Man”) while others have appeared and disappeared within a few decades. While we only have one of the three original arches that were present at this location, we can appreciate the last remaining arch while it stands and remember this beautiful geological structure even after the last arch is long gone.
To log this Earth Cache, you must answer the following:
Have the name “Natural Bridges” and the GC number at the top of your message.
The number of people in your party.
What unique combination allowed the natural bridge and arches to form at this location? How do you think this location will look 100 years from now?
How long and tall do you think the structure is? I am just looking for an approximation.
Which arch from the original three that were present here are you observing? How did it become the last remaining structure here today?
Describe the colors you see on the remaining structure. What do the colors tell you about the mineral makeup?
Optional (but highly encouraged): Post a picture of you or a personal item in front of the remaining arch.
It is okay if your answers are not correct! Earth caches are all about learning!
Sources
https://en.wikipedia.org/wiki/Natural_Bridges_State_Beach
https://coastalcare.org/2017/06/the-natural-bridges-of-santa-cruz-county-by-gary-griggs/
https://ngmdb.usgs.gov/Geolex/UnitRefs/SantaCruzRefs_11774.html