Rose Canyon Fault - Old Town EarthCache

Welcome to Old Town

In Old Town San Diego, the landscape you see today has been shaped in part by the Rose Canyon Fault. This active fault passes through the area, slowly shifting the ground over thousands of years and leaving behind subtle clues in the terrain. Gentle slope changes, offset stream channels, and breaks in the landscape mark its trace. Layers of ancient sandstone and gravel tell the story of shifting seas and repeated earthquakes. Here, human history and geologic history overlap as centuries of settlement have taken place atop land still being reshaped by the movement of Earth’s plates. This EarthCache invites you to discover how the fault has influenced the land beneath Old Town and to see firsthand the signs of an active tectonic boundary.

The Formation of the Old Town Fault Zone

The Rose Canyon Fault is a right-lateral strike-slip fault, meaning that if you stood on one side and looked across, the land on the opposite side would appear to have shifted to the right. This motion results from the Pacific Plate and North American Plate sliding past one another, with the Rose Canyon Fault acting as one of several connected faults that carry this tectonic stress through the region. In the Old Town area, the fault passes through land that has been inhabited for thousands of years, from ancestral Native peoples to Spanish settlers and early American communities. The relatively level ground here is part of a marine terrace formed during a past high sea level. Over time, movement along the fault has offset streams, created subtle slope changes, and shaped the boundary between flat terrace surfaces and the steeper ground above them. Geologic studies reveal evidence of multiple large earthquakes here in the past few thousand years, including one likely associated with the 1862 San Diego earthquake. Though the fault moves only a few millimeters per year, these small shifts accumulate over thousands of years, creating scarps, fractured rock, and altered drainage patterns that mark its path. The Old Town landscape today reflects both its deep human history and the ongoing forces of plate tectonics that continue to shape it.

Fault Exposure in Old Town

The landscape of Old Town San Diego preserves clear evidence of long-term tectonic activity, particularly through its marine terraces and fault scarps. Marine terraces are broad, flat or gently sloping surfaces formed during periods of higher sea level when the ocean deposited sand, gravel, and other sediments along the coastline. Over thousands of years, these terraces have been slowly modified by the movement of the Rose Canyon Fault. As the fault shifts the ground laterally, it tilts and offsets portions of the terraces, creating subtle changes in slope and breaks in what was once a continuous, level surface. These changes are not dramatic, but they reveal the steady influence of tectonic forces over geologic time, recording both the rise and fall of ancient shorelines and the persistent motion of the fault beneath the region.

Fault scarps are another feature that provides direct evidence of tectonic movement. A scarp forms when one side of a fault is lifted relative to the other, creating a step-like feature in the landscape. In Old Town, these scarps are relatively small but still visible, marking locations where the ground has shifted due to repeated fault movement over thousands of years. Natural processes such as rain, wind, and surface erosion have softened their edges, but the underlying displacement remains clear. Where fault scarps intersect the terraces, they can create abrupt changes in slope or subtle offsets in drainage patterns, providing tangible markers of the fault’s path and its long-term activity.

Together, marine terraces and fault scarps illustrate the interaction between tectonic processes and surface geology. The terraces show the ancient shoreline surfaces that were gradually reshaped by uplift and tilting, while the scarps indicate the slow but steady slip along the Rose Canyon Fault. Observing these features in the Old Town area offers a direct, real-world glimpse into how the movement of Earth’s plates has sculpted the landscape over thousands of years, preserving both the history of past sea levels and the ongoing activity of an active fault beneath the region.

Fault scarp (northeast).

Marine terrace (southeast).

Geological Significance

The Old Town segment of the Rose Canyon Fault is one of the most active in the San Diego region. Studying the uplift, tilting, and offsets here helps geologists understand earthquake hazards and the long-term development of the local landscape. Observing the terraces and fault scarps in this area allows you to see firsthand how tectonic motion, erosion, and sedimentation work together to shape the ground, creating subtle slopes, stepped surfaces, and linear breaks that record thousands of years of geological activity.

Tasks for This EarthCache

To log this EarthCache, visit the site and answer the following questions. Send your answers via Geocaching or email.

1. Include "Rose Canyon Fault - Old Town - GCB7AET" on the first line of your message.

2. Besides the marine terrace and fault scarp, what other features indicate that a fault lies beneath this location? How do you think these features formed?

3. Look northeast at the fault scarp. Describe its approximate length, height, and slope. What observations suggest that these features were formed mainly by tectonic activity rather than by other processes?

4. Look southeast at the marine terrace. Describe its approximate length, height, and slope. How could this terrace reflect both past sea levels and movement along the fault over time?

5. Compare the fault scarp and the marine terrace. What characteristics help you distinguish between them? Which feature do you think has been shaped more directly by movement along the fault?

6. In your log, attach a photo of yourself or a personal item with the Old Town sign at the posted coordinates in the background. (Note: photos predating the publication of this EarthCache are not accepted.)