High above the Pacific Ocean in La Jolla, the cliffs reveal more than just a beautiful view — they mark the boundary of a tectonic fault that continues to shape Southern California. The Rose Canyon Fault Zone, which runs through coastal San Diego County, is an active strike-slip fault that juxtaposes distinct sedimentary rock formations along the shoreline. It is the southern portion of the much larger Newport-Inglewood Fault which extends all the way to the Santa Monica Mountains, and it itself if part of the larger San Andreas system. This EarthCache invites you to explore the geologic record exposed in the cliffs and learn how tectonic motion has altered and reconfigured the landscape over millions of years.
🧭 What is the Rose Canyon Fault?
The Rose Canyon Fault is a right-lateral strike-slip fault, meaning that as the two sides move past one another, the far side of the fault shifts to the right. It is part of the broader San Andreas Fault system and serves as the primary active fault beneath the city of San Diego. Although not as long as some of California’s larger faults, the Rose Canyon Fault (and to an extent the larger Newport-Inglewood-Rose Canyon Fault system as a whole) is capable of producing significant earthquakes, with estimated magnitudes up to 7.0. The fault stretches offshore from north of La Jolla, passes inland through the city, and connects with other faults to the south near San Diego Bay.
At this coastal location, the fault provides a clear and accessible look at the dramatic contrasts it has produced in the geologic record, placing sedimentary rocks of different ages and origins side by side. These formations are visible in the sea cliffs and offer valuable clues about how the region’s landscape formed and evolved.
🪨 Geologic Formations on Either Side of the Fault
This juxtaposition of Late Cretaceous Point Loma Formation and Quaternary Bay Point Formation sedimentary units across a fault zone illustrates the horizontal displacement that the Rose Canyon Fault has accommodated over time. The rocks we see here were not originally deposited next to each other — they’ve been brought into contact by tectonic motion.
West Side
On the west side of the fault — the side closer to the ocean — the bedrock consists of Cretaceous-age sedimentary rocks that belong to the Rosario Group, primarily the Point Loma Formation. This formation is approximately 70–75 million years old and was deposited in a deep-marine environment on the ancient continental margin of what is now California. It consists primarily of fine-grained sandstone and siltstone, usually gray to olive in color, with some interbedded shale. These rocks were laid down by turbidity currents and other deep-sea processes, and they often display features like graded bedding, soft-sediment deformation, and fine lamination. It may show bedding (dark and light contrasting lines that can be inches to feet thick) and occasional fossil traces, but is generally coarse-grained and durable, forming much of the steep cliff faces that is a near vertical drop.
East Side
In contrast, the east side of the fault contains the much younger Bay Point Formation, a Quaternary marine terrace deposit that is less than 250,000 years old. These sediments were laid down in shallow marine or beach environments during periods of higher sea level. The Bay Point Formation is typically composed of silty to sandy layers, sometimes containing marine shell fragments, and is far softer and more erodible than the older Point Loma sandstones. Its light brown to grayish tones and looser texture make it visibly and physically distinct from the bedrock to the west. it contains little to no bedding and is more crumbly, hence why the cliffs on the east side are more shallow and gradual (albeit they are still steep!).
⚒️ The Fault in the Landscape
The Rose Canyon Fault does not always produce dramatic breaks in the ground surface, but its effects can often be seen in subtle shifts and changes in rock type or landform. In the La Jolla cliffs, the fault is expressed as a relatively narrow zone — often only meters wide — but it has brought rocks of different ages and depositional histories into direct contact.
Few places in Southern California offer such a visible and accessible window into tectonic processes. The cliffs of La Jolla present a striking example of how Earth’s crust is broken and rearranged over time. The presence of sedimentary rocks of different ages and compositions placed side by side is a direct result of movement along the Rose Canyon Fault — a reminder that even here, along a tranquil coastline, the land is still in motion. This EarthCache gives you the chance to experience geology in real time and space — and to better understand the active processes shaping the region.
Works Cited
https://www.sandiegouniontribune.com/2022/11/23/beneath-la-jollas-shores-oceanography-talk-in-la-jolla-will-take-a-dive-into-underwater-canyons-and-more/
https://www.youtube.com/watch?v=vD_RPgHuP1s
https://ngmdb.usgs.gov/Prodesc/proddesc_344.htm
https://www.sandiegouniontribune.com/2022/11/23/beneath-la-jollas-shores-oceanography-talk-in-la-jolla-will-take-a-dive-into-underwater-canyons-and-more/
TO LOG A FIND ON THIS CACHE YOU MUST ANSWER ALL THE QUESTIONS BELOW. YOU CAN CONTACT ME THROUGH MY EMAIL OR THE GEOCACHING MESSAGE CENTER TO SEND YOUR ANSWERS. ANY INCORRECT ANSWERS MAY RESULT IN A CLARIFICATION RESPONSE FROM ME.
1. "Rose Canyon Fault - La Jolla" on the first line of your email AND list all geocaching names of your party so I can match your answers to them. If you all want to learn something, I would prefer each cacher send me individual emails in the spirt of earthcaching.
2. Take a photo of you (or your signature item if you don't want to show your face) with your back facing the cliffs towards the west at the posted coordinates. This photo MUST be uploaded to your "found it" log. Please do not send any photos through the message center as Groundspeak compresses images. Each log MUST have a unique and different photo. No two logs may contain the same photo.
3. Describe the Point Loma Formation to the west. Tell me the (a) colors, (b) average thicknesses of the beds (layers), and (c) if the beds are horizontal or tilted.
4. Describe the Bay Point Formation to the east. Tell me the (a) colors, (b) average thicknesses of the beds (layers), and (c) if the beds are horizontal or tilted.
5. How are these two formations (a) similar and (b) different? Both visually and in the way they originally formed.
6. Besides the stark difference in rock types, what other peices of evidence do you see that a fault is here? Explain your reasoning. Hint: think really obvious...and don't jump to any conclusions!