Chiquita Canyon Fossils - Rancho Mission Viejo, CA
Posted by: 
bluesnote
N 33° 32.003 W 117° 36.420
11S E 443639 N 3710585
Sandstone beds full of shell fossils, most of which are preserved intact.
Waymark Code: WM13TKH
Location: California, United States
Date Posted: 02/17/2021
Views: 1
While Geocaching in the area, I came across this unique geologic location. A large piece of sandstone full of carbonate rich shell fossils. Pretty cool spot for the kids who want to see really cool fossils! So, let me explain the type of rock and fossils are here.
At the above coordinates, you will see a large piece of sandstone which is a type of sedimentary rock. A sedimentary rock is made of smaller fragments from other types of rocks (could be other sedimentary rocks, or it could be igneous or metamorphic). In this case, sand grains make up the rock. There are two types of sandstone deposits. A marine sandstone deposit was formed close to the shore (or part of the shore like a beach). These often contain shell and other marine fossils. A terrestrial sandstone deposit formed from a large desert sand dunes. Some examples include the large sandstone deposits of Arizona and Utah. These contain no marine fossils and are much thicker in size.
The sandstone was buried under more and more sediment where it was compressed and heated to fuse the sand grains together. This left imprints where the shells once were, leaving behind molds of fossils. This rock was likely sourced locally as this entire part of Orange County was under water about 10 million years ago.
Now for something a little more advanced. For certain fossils, geologist can tell the direction in which the rock formed in. For example, is the rock right-side up? Or is it upside down? Simple fluid mechanics can help answer this question. Think of a concave fossil with a "bumb". If an animal with a concave shell dies, its shell will fall to the ocean floor (in this case near a sandy beach). The ocean waves will move that shell to a more stable position. For example, a shell is more stable and less willing to move if the shell has its "bump" facing upward (concave down). If a geologist sees an impression of a shell that has its bump pointing a certain direction, a geologist can almost certainly tell which way the rock originally had formed. This will help geologists know if the rock was deformed (such as being twisted or folded) since it formed. The image below shows fossils in the concave down (bump up) position and show that the rock had originally formed out of the page. This means, from the camera, we are looking down on the rock as it originally had formed. The rock here is right-side up because the fossils are concave down ("bum" up).
What kinds of fossils are found here:
Shells
Admission Fee: no
Link for more information about this site: [Web Link]
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