The Geology of Cabrillo National Monument, California

Cabrillo National Monument occupies the southern end of the Point Loma Peninsula, about 8 km (5 mi) southwest of downtown San Diego, California. The rocks beneath it formed on the deep floor of the Pacific in the Late Cretaceous and now stand above the sea. This summary draws on the National Park Service's Geodiversity Atlas, which gathers what the NPS Geologic Resources Inventory knows about each park's rocks, sediments, landforms, fossils, soils and the processes that shape them.

The monument

Established on October 14, 1913, the monument commemorates the Iberian explorer Juan Rodriguez Cabrillo and his 1542 voyage, during which he claimed the coast for Spain. It covers about 65 hectares (160 acres) of land and 49 hectares (120 acres) offshore, preserving rare natural and cultural resources.

• The Old Point Loma Lighthouse is its cultural centerpiece and has become a symbol of San Diego.
• Visitors can see marine life from shore, and from December through February watch Pacific gray whales migrate past.
• It is an important stopover for birds on the Pacific Flyway, and popular with birders.
• Its views take in one of the world's most spectacular harbors and stretch from Mexico to the mountains east of Los Angeles.

Point Loma has long been a lookout over the Pacific and San Diego Bay, used for military, navigational and recreational purposes.

The Old Point Loma Lighthouse and coastal bluff. Image from the National Park Service's page.

From the deep sea to a headland

Only a few geologic units are found at the monument, all dating to the Late Cretaceous (100 to 66 million years ago), though they do not represent that whole span. They began as sediment shed from the Peninsular Ranges and carried through a submarine fan system to great depths in the Pacific. Tectonic deformation and faulting related to the Rose Canyon fault zone later lifted those deposits up.

These uplifted fan deposits, the Rosario Group, form the monument's bedrock. Two of its formations, the Point Loma and Cabrillo Formations, are exposed among marine terraces and wave-cut platforms. The Old Point Loma Lighthouse is partly built of local sandstone from these two formations.

Offshore, the area west of the peninsula is made of undivided Rosario Group rocks, while to the east the floor of the Pacific and San Diego Bay is made of Holocene marine deposits less than 11,700 years old.

The monument lies in the Lower Californian Physiographic Province, also called the Peninsular Ranges, and shares its geologic history and characteristic formations with a much larger region.

Geologic Resources Inventory map. National Park Service.

Maps and inventories

The Geologic Resources Inventory produces digital geologic maps, reports and posters for more than 270 natural resource parks, and a Soil Resources Inventory has been completed for Cabrillo. Both are available through the NPS DataStore.

Aerial view of Cabrillo National Monument. National Park Service.

Sources

• National Park Service, "NPS Geodiversity Atlas—Cabrillo National Monument, California": https://www.nps.gov/articles/nps-geodiversity-atlas-cabrillo-national-monument-california.htm
• The report cover shown on the source page is not reproduced.