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Sea kayaker paddles past a cave opening on Santa Cruz Island, Channel Islands National Park, California.
NPS Photo by Hidekatsu Kajitani.
Introduction
Sea caves or littoral caves are formed primarily from erosion caused by waves. They can be formed along the ocean coast and lakeshores where water impacts bedrock. Most sea caves are formed along weaknesses in the rock, such as faults, fractures, or bedding/foliation planes and can occur in nearly every type of rock.
Sea Cave Development
1) Bedrock with fractures and large fault.
2) Caves develop as waves erode the faults and fractures near the edge of the water.
3) With continuing erosion, the cave becomes open at both ends forming a natural bridge or arch.
4 Illustrations above by Paul Burger, NPS.
4) Arch roof collapse leaves behind a sea stack and further erosion turns another cave into an arch.
Painted Cave, Santa Cruz Island, Channel Islands National Park, California.
NPS image.
Coastal Erosional Features
As erosion progresses, the cave can become open at both ends to form a natural bridge or arch. When the arch roof collapses, it leaves behind a sea stack, no longer connected to the mainland above mean high tide. Caves and arches will continue to form as waves erode more of the bedrock.
Illustration of coastal erosional features. Sea caves are erosional features that form in high energy tidal environments. Waves and the sediments they carry exploit and enlarge weak zones such as joints, faults, dikes, veins, and layers of soft rock in otherwise erosion resistant rock.
NPS illustration by Trista L. Thornberry-Ehrlich (Colorado State University).
Sea stack in Olympic National Park, Washington.
NPS photo (March 05, 2021).
Habitats
Sea caves can be important habitat to many types of animals. Sea Lions use caves as haulouts that provide shelter and protection from predators. Many marine fish and invertebrates also use the dim, dark, environment for shelter.
Many types of sea birds use caves for nesting, shelter, and to avoid predation from other birds and land carnivores.
Bird Rock in Golden Gate National Recreation Area, California.
NPS Photo by Paul Burger.
Marine isopod (~2.5 cm in length) from a sea cave in Golden Gate National Recreation Area, California.
NPS Photo by Paul Burger.
Invertebrates
The unique environment of sea caves provides home to several invertebrate species including crickets, spiders, and marine invertebrates, including an undescribed troglobitic isopod known to occur in only two caves in the Golden Gate National Recreational Area (Elliot et al. 2017).
Cultural Resources
Humans, modern and ancient have made use of sea caves for shelters and other purposes. Objects such as rafts, wooden kayaks, and other items have been found cached in caves. Most of these caches have been found in caves now well above sea level due to the retreat of glaciers. Petroglyphs, and pictographs suggest some sea caves may have been used as temporary shelters or hiding places, and possibly ceremonial sites.
More modern humans have made use of sea caves as well. One of the small sea caves on Alcatraz was reportedly used by an attempted escapee to hide until he became too cold and hungry and gave himself up. At Sutro Baths in Golden Gate Recreational Area, tunnels, canals, and other works were used to modify sea caves to bring water into an indoor pool.
A blasted tunnel (right) and concrete canals (left) were used to bring seawater from natural sea caves to the Sutro Baths, Golden Gate National Recreation Area, California.
NPS photo.
Interior image of Sutro Baths, circa 1900. Golden Gate National Recreation Area, California.
Photo courtesy of Gary Stark, The Cliff House Project.
