2017 San Diego County Cachies: WINNER Best Location & Nominated Most Educational
Sunset Cliffs Geology
The beautiful rocks of Sunset Cliffs take their name from the coloring of the various rock layers that were sedimentarily formed & later exposed by the erosion of the Pacific Ocean. The bottom layer of sandstone is known as the Point Loma Formation. This layer was formed around 75 million years ago during the late Cretaceous. It is topped by another late Cretaceous sandstone layer called the Cabrillo Formation. Following this is an unconformity, as the top layer of sandstone and conglomerate was deposited less than 1 million years ago during the Pleistocene. Geologically erosion is happening relatively fast at this location, around 18,000 years ago the shoreline existed around half a mile out to sea.
The Point Loma formation is unique because it is one of the few places in California where dinosaur fossils have been uncovered. At this particular spot are very good examples of another interesting geologic feature, concretions.
Concretions
Concretions are hard compact masses of rock that form around a “seed” which is often organic in nature. This nucleus can be an item such as a small bone, shell, or leaf, although modern concretions that have formed around human debris have also been described. Concretions form early in the deposition of a rock layer so they are usually harder than the surrounding strata. When a layer containing concretions is exposed to erosion the concretions are revealed as the surrounding rock erodes away. Because of the unusual, often spheroid, shape of concretions they are often initially misidentified as pseudofossils, such as dinosaur eggs or human artifacts.
Types Of Concretions
Concretions come in a wide variety of forms. Below are some of the more common types:
Septarian: Concretions containing angular cracks. Although the reason the cracks form is still unknown, the fact that the crack radiate toward the outside of the concretion indicate that the edges of the concretion were harder than the center while forming. A famous example of septarian concretions are the Moeraki Boulders in New Zealand.
Elongate: As the name indicates these concretions are elongate in shape. Elongate concretions formed in the presence of flowing groundwater and can be used to determine the direction and even velocity of the flow. Elongate concretions are common in the Santa Fe Group of New Mexico.
Cannonball: These are the most common form of concretion. Cannonball concretions can widely vary in size from several inches in diameter to over 18 feet. The Cannonball River of North Dakota takes its name from the widespread presence of this type of concretion in the area.
Hiatus: These concretions were formed, exposed, reburied, and then exposed again. This is most usually determined by the presence of marine organism fossils encrusting the outside of the concretion. The Menuha Formation in Israel contains examples of hiatus concretions.
Logging Requirements:
Send a message with:
- The text “Sunset Cliffs Concretions” on the first line.
- The number of people in your party.
- What type of concretion is at the posted coordinates?
- The coordinates of another concretion in the area.
Thanks to David Paczona, City Of San Diego Park Ranger & Marilou Fedalizo, Permit Center Supervisor for approving this Earthcache.