Fault Outcrop - Old Town EarthCache

Welcome to Old Town

On Congress Street in Old Town San Diego, you can see layers of rock that tell a story millions of years old. These sedimentary formations, sandstones, conglomerates, and shell beds, were deposited during the Pliocene and Pleistocene epochs, long before humans arrived in the region. The rocks here were shaped by changing seas, rivers, and the shifting movement of the Rose Canyon Fault. Fossils of oysters, scallops, snails, and even whales and sharks are preserved in these layers, offering a direct connection to the ancient environments that existed here. By observing these rocks, you can witness how the land has been continuously reshaped by both tectonics and the natural forces of sedimentation. This EarthCache invites you to explore these formations up close and consider the deep geologic history beneath your feet.

Note: While it is a public road according to the San Diego Zoning Grid Map, parking is not permitted on Congress Street. Please be considerate of nearby residents.

Lindavista Formation

The Lindavista Formation, which caps the Congress Street outcrop, is a reddish-brown mixture of sandstone and conglomerate approximately 20 to 30 feet thick. This formation was deposited during the early Pleistocene, around 2.5 to 1.8 million years ago, a time when sea levels were higher than today, creating shallow marine conditions along the coast. Periodic river flows and wind-driven sands contributed to the deposition, producing a mix of fluvial, aeolian, and nearshore marine sediments. The beds in this outcrop dip steeply, a result of tectonic tilting caused by movement along the nearby Rose Canyon Fault. Fossils are uncommon in this unit, but when present they include marine invertebrates such as clams, scallops, sand dollars, and snails, along with occasional remains of sharks or baleen whales. These rare fossils provide clues about the changing coastal environments and the types of organisms that inhabited this region during the early Pleistocene, offering a window into the interplay between tectonics, sea level, and sedimentation.

San Diego Formation

Beneath the Lindavista Formation lies the older San Diego Formation, a marine sedimentary unit deposited during the late Pliocene, roughly 3.5 to 1.5 million years ago, in a shallow embayment similar in size to modern Monterey Bay. This formation consists of yellowish-gray, fine-grained sandstone, interlayered with gravel, cobbles, and distinct shell beds, reflecting deposition in nearshore marine environments with varying energy conditions. The San Diego Formation has produced a remarkably diverse fossil record, including marine organisms such as oysters, scallops, snails, brachiopods, sharks, rays, and dolphins, as well as the occasional terrestrial mammal like camels and horses, showing that this area was influenced by both coastal and land-based ecosystems. At the Congress Street outcrop, the shell beds preserve original shells and molds in situ, providing direct evidence of the early Pleistocene nearshore environment. These fossil-rich layers allow geologists and paleontologists to reconstruct ancient coastal habitats, track changes in sea level over time, and understand how tectonic activity, particularly along the Rose Canyon Fault, has influenced the preservation and orientation of these sediments.

Formational contact (solid line) between Plio-Pleistocene San Diego Formation (Tsd) and the Pleistocene Lindavista Formation (Qvop11).

Geological Significance

The steeply dipping beds at Congress Street, which include the layered Lindavista and San Diego Formations, provide a clear record of the ongoing influence of the Rose Canyon Fault, an active right-lateral strike-slip fault in the region. Over millions of years, tectonic forces along this fault have caused the Earth's crust to deform, tilting originally horizontal sedimentary layers and producing fractures and small offsets within the rock. This tilting occurs because the movement along the fault creates stress in the surrounding rocks, forcing them to bend, rotate, or shift as the Pacific Plate slides past the North American Plate. As a result, the once-flat marine and nearshore deposits are now inclined at extreme angles, making the fossil-rich layers more visible and accessible. By examining these tilted layers, visitors can see direct evidence of how tectonic motion interacts with sedimentation, while fossils within the shell beds connect us to past sea levels and ancient coastal ecosystems. Together, the shell beds and steeply dipping rocks at this site preserve a detailed record of environmental change, sea-level fluctuations, and the cumulative effects of tectonic activity over the Pliocene and Pleistocene epochs.

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 "Fault Outcrop - Old Town - GCB7AER" on the first line of your message.

2. Inspect the layers of rock in the outcrop. Estimate how steeply the beds are dipping and in which direction. How might movement along the nearby Rose Canyon Fault have caused this tilt over time?

3. Observe the Plio-Pleistocene San Diego Formation. What types of fossils or sedimentary features can you see in this layer? What do they tell you about the ancient marine environment that deposited this rock?

4. Examine the basal conglomerate of the Lindavista Formation. How would you describe the size, sorting, and composition of the clasts? What does this suggest about the energy and environment of deposition when this layer formed?

5. Look at the sandstone above the conglomerate in the Lindavista Formation. How does its grain size, color, and bedding differ from the conglomerate below? What can these characteristics tell you about how and where this sediment was deposited?

6. How do you think tectonic activity, natural erosion, and human construction combined to expose these rock layers at the surface?

7. In your log, attach a photo of yourself or a personal item with the outcrop in the background. (Note: photos predating the publication of this EarthCache are not accepted.)