Natural World

Nonnative Species

As human have traveled around the earth, they have purposefully and accidentally brought plants and animals with them and introduced them to new continents. Natural barriers to the movement of plants and animals include large bodies of water, large deserts, and mountain ranges. World wide, introduced species are directly behind habitat destruction in causing species extinction around the globe.

A healthy ecosystem requires a balance of plants, insects, herbivores, and carnivores which have evolved together in a system. Local habitats are the "ecological houses" of the world, and the loss of local plants and animals are a serious threat to global biodiversity. The San Francisco Bay Area is considered one of the top 25 biodiversity hotspots on the planet. Invasive species and their removal are an economic burden to land managers around the world, but here at Golden Gate they are also threatening native biological richness that rivals that of the Equatorial rainforests! Through educating themselves about invasive species, park visitors can truly learn to celebrate local nature.

Wetlands, Marshes and Swamps

Estuaries provide transition from ocean to land. Freshwater streams meet saltwater tides creating on of the most fertile habitats on earth. Fast growing marsh vegetation provide habitat for decomposers, and the system is based on detritus (decomposed plant materials) rather than live plants. Estuaries also act as a doorway for ocean fish who spawn in freshwater streams such as coho salmon and steelhead trout. For other fish and crabs, estuaries are the perfect place to spawn, leaving the juveniles in the protection of the vegetation and abundant food supply to grow up until they are ready to enter the open ocean. Estuaries also provide important stopovers for migrating ducks and shorebirds as they fly thousands of miles up and down the Pacific Flyway.

The life of marshes is determined by the tides that wash in and out twice daily., They are either submerged by salty water or left high and dry as thee tides recede. Marsh organisms are distributed at different tidal levels, depending on their ability to withstand the stress of tidal inundation. Eelgrass beds grow in channel bottoms and deep basins, supporting sponges, tunicates, and moss animals. Most of these animals are filter feeders or graze on microscopic algae that grow on the leaves of the eel grass. In the mudflats (shallow areas where little vegetation grows), are worms, clams, and snails. Most of these animals are detritivores, filtering organic particles from the water like crabs and shrimp. Raccoons and fox also regularly come down to water’s edge looking for a snack.

Over 90% of the Bay Area’s salt marshes have been developed. Restoration has become a buzzword as more regions of the bay become public lands administered by the Fish and Widlife Service and state and county parks, or nonprofit land trusts. In Golden Gate, Crissy Field is an example of a successful salt marsh restoration project, with many species of invertebrates and fish long gone from the area returnign to the site. Salt marsh vegetation occurs in areas with perennial soil saturation and regular tidal inundation. Adaptations of salt marsh species include mechanisms for salt extrusion, stems that have large air spaces to allow oxygen from the air down to the roots in anoxic soils, and floating seeds. Some species are also succulent to retain water in their tissues. Salt marshes have three distinct elevation zones: 1) the low marsh which is completely submerged daily and dominated by cord grass and pickle weed, 2) the middle marsh which is partially submerged daily and has the highest diversity of species, and 3) the high marsh which is only inundated only during very high tides and dominated by marsh gumplant and marsh rosemary.

The threatened Marsh bird's beak ranges along the central California coast, but due to widescale development of marshlands, numbers have quickly dropped. This small annual was introduced to the recreated salt marsh at Crissy Field in 2001.

The endangered California sea blite, although once abundant from Morro Bay up the central California coast, has been extirpated from San Francisco Bay. This small succulent shrub was also introduced to the recreated salt marsh at Crissy Field in 2001.

Natural Features & Ecosystems

From the first explorers landing on the Pacific, to modern day scientists studying endangered species, to a small child seeing a sea star for the first time, the coast is a place of discovery. Golden Gate covers seven different ecological zones starting with the open ocean and the Gulf of the Farallones National Marine Sanctuary. Rocky coasts shelter tide pools and show off the wide variety of geologic formations in the park. Sandy beaches are playgrounds to the public as well as home to many plants and animals. Estuaries begin the transition from sea to land, mixing fresh and salt waters and terrestrial and marine species. Marshes such as Crissy Field, Rodeo Lagoon, and Big Lagoon spread out in their mucky green glory, marshes being one of the most productive habitats on earth. Where low coastal hills meet the ocean as in the Marin Headlands and Pacifica, scrublands and grasslands cover the rocky earth like a skin. Looking out over the grassy hills and the dark green patches of scrub, you may think that you are in the Mediterranean, except for the fog blowing in to the coast. Always moving through the land are the green ribbons of creeks such as Redwood, Lagunitas, and Lobos Creek. Come discover the nature of Golden Gate. There is more than meets the eye!

Lightscape / Night Sky

Darkness is a valuable resource for visitors, and critical to the welfare of the park's wildlife. High points in open areas within the park provide excellent opportunities to view the night sky. In particular, such areas that have little or no artificial lighting are sought by visitors to practice amateur astronomy close to home. Wildlife habitat is more valuable when unimpaired by artificial light. While high open areas in the park may provide opportunities to view the night sky, most of these locations are subject to light pollution from the surrounding Bay Area. Lighting within and adjacent to the park also reduce the darkness of the night sky. Wildlife habitat is impaired by artificial lighting. The park does not have a plan to address preservation and restoration of dark habitat.

Lower Redwood Creek

The Redwood Creek watershed encompasses a breathtaking swath of habitat bordered by the peak of Mount Tamalpais to the north and extending south to Muir Beach where the creek empties into the Pacific Ocean. It also provides crucial habitat for a number of protected species under the Endangered Species Act including Coho salmon, steelhead trout, and the northern spotted owl. A good chunk of the watershed is within the boundary of Muir Woods National Monument, a very popular old growth coast redwoods forest that averages over one million guests per year. As with many National Parks, it is a balancing act between providing for the enjoyment and protection of these resources.

Redwood Creek is the southernmost range of Coho salmon, an amazing fish species that journeys from the oceans and back to the streams and rivers of their birth. This epic adventure begins as the sand bar at Muir Beach is breached by floods from the winter rains. Mature salmon fight their way upstream over log jams and through shallow spots to find a gravel bed near a nice deep pool. They do not eat at all during their voyage, and they die as soon as they have mated. All of this work is so they can successfully lay the eggs of the next generation. The salmon are very picky and can only make their nests, or reds, in areas where there is the right size of rocks and not too much silt, where the water is moving and not stagnant. After the eggs hatch, salmon need deep pools that remain aerated and cool throughout the summer for the juvenile salmon. Throughout their range in the Pacific Northwest, salmon habitat has been degraded through deforestation, unsustainable agriculture uses, and road building. There are very few places where an entire watershed is protected like at Redwood Creek.

Redwood Creek was within the boundaries of the Mission San Rafael, and extensive use began after the Mexican government took control and deeded the land to individuals. William Richardson bought parts of the Marin peninsula and began grazing Longhorn cattle on what he named Rancho Sausalito. Richardson also began logging the redwoods from the east side of Mount Tamalpais, and Portuguese dairy farmers moved into the region. The Banducci family moved to a site along the banks of Redwood Creek in 1850, and began growing flowers for the cut flower trade, mainly bulbs such as gladiolas. In order to avoid the flooding of their fields during the rainy season, the Banducci family installed dikes against the stream bank and channelized it, causing the water to flow fast and straight through that stretch, previously known as "the bowling alley."

Riparian areas are ecosystem pathways, moving energy, materials, and organisms through the landscape. Riparian habitats give wildlife a shady place to forage and rest, and an available water source. The long green ribbon of a creek is an ecotone where habitats come together, providing a variety of plants and insects to eat, as well as different types of shelter for birds to nest in. The complexity of microhabitats created by the layering of trees, shrubs, vines, and aquatic vegetation promotes high bird diversity especially. Along the banks are trees such as red alders and willows. In the understory are flowering shrubs like elderberry and thimbleberry mixed with ferns and bleeding heart.

The Park Service purchased the flower farm in 1986, and the restoration of the site began in 2003. First the dikes were removed in order to restore the historic floodplain. Then invasive vegetation such as cape ivy and Eucalyptus trees were removed. On a few big construction days in 2004, log jams and weirs were placed in the streambed to create deep pools as refuge for the juvenile salmon, and to restore the natural meandering bends in the creek. During this process, all of the water actually had to be removed, and any juvenile salmon in the creek were caught and moved further up or down stream. In 2007, the second phase of construction continued with more woody debris installation, and the creation of a frog pond as potential red-legged frog habitat onsite.

Other project designs included, willow mattressing and bundles along creek banks as a living barrier to erosion, and lots of native plantings! Willows, alders, and other creek trees such as dogwoods and myrtles were planted to provide shade over the water. Thousands of fruiting shrubs, ferns, and wildflowers were planted to add food and cover for birds such as the Wilson's warbler and small mammals such as the dusky-footed wood rat. Flooding downstream has been reduced, and salmon numbers are up. The Banducci project is a testament to the fact that restoring ecological processes and allowing nature to take its course improves the habitat not only for the Coho salmon, but for all of the creek creatures that depend on a healthy watershed.

Weather

The coastal areas of central and southern California have a Mediterranean climate. Mediterranean climate regions occur on the west coasts of continents at mid-latitudes throughout the world. Other regions that share our climate include the Mediterranean Basin of Europe, Chile, South Africa, and Western Australia. In these regions, temperate wet winters contrast with warm or hot dry summers. The average annual rainfall in central California ranges from 15 to 55 inches, with almost all rain occurring between November and April and an extended drought during the summer months. Vegetation in these climates have undergone convergent evolution, developing many of the same structures and adaptations. Early European explorers were struck by the similarities in appearance of vegetation between the California coast and their European homelands.

Coastal mountains and valleys create microclimates in the region. Mountains parallel to the coast produce rain shadows and drier interior valleys. During the summer, gaps in the coastal ranges permit ocean fog to penetrate inland, providing some relief from summer heat and drought. Fall and Spring actually have the warmest weather of the year.

This contrast between summer and winter and the many microclimates noticeably affect the ecology of the Bay Area. Plants and animals have evolved many different adaptations to deal with the extended summer drought and the localized climatic zones have led to the evolution of numerous endemic species with very limited geographic ranges. These climatic conditions, coupled with the local geology, have contributed to the high species diversity in the San Francisco Bay Area number of rare or endgnered species.

Nature & Science

As most of California and our nation become developed, urban national parks and their resource management issues become increasingly important. The San Francisco Bay region has notably rich biological diversity due to the variety of habitat and unique geology.

Over half of North American avian species and nearly one third of California's plant species are found in the park. 24 federally threatened and endangered species exist within lands that the Park Service manages, and a total of 33 threatened and endangered species exist within Golden Gate's legislative boundaries.

The Recreation Area is spread through a 60 mile swath of coastal lands, with inholdings by other agencies and private owners incorporated into the park’s legislative boundaries. Because of the multiple landowners, many Bay Area residents and park visitors think of our park in terms of isolated sites such as Muir Woods, Stinson Beach, the Marin Headlands, and the Presidio. The bigger picture is one of wildlife corridors, secret nature spots, and refuges for rarities. Traveling through the park connects you with windswept beaches, dramatic cliffs, coastal chaparral, low mountains, swaths of prairie, live oak woodlands, salt marshes, freshwater wetlands and creeks, and redwood forests. All you have to do is get out and explore!

Cape ivy control

Cape Ivy is the California coast's biggest and baddest weed. If the Park Service had their way, we would be circulating a mug shot of this bright green vine. It is such a pain that it is the first wildland weed that the California Department of Agriculture has decided to develop a biocontrol for, it has cost so much money for land management agencies to combat it. Cape ivy currently covers 500,000 acres throughout the state! Golden Gate preserves some of the only remaining high quality coastal habitat in the country. Conservation International and The Nature Conservancy consider the California Floristic Province to be a global biodiversity hotspot: one of 25 regions worldwide where biodiversity is most concentrated and the threat of loss most severe. The San Francisco Bay area is also a United Nations Biosphere Reserve, one of 400 sites worldwide where governments, scientists, and NGOs are working together to protect critical ecosystems.

Cape ivy was introduced to the eastern United States in the 1850s as a common house plant. It's pretty yellow flowers are proof positive that looks can kill. It was introduced to California a century later in the 1950s. It is a now a serious threat to most of the western coast of North America as well as Italy and Australia. In its natural range in South Africa, Cape ivy has a relatively restricted distribution, growing in moist mountain forests. Central coastal California shares a Mediterranean climate type with only four other regions in the world: South Africa, the Mediterranean basin of Europe, Western Australia, and Chile. Crossing the world's oceans from continent to continent through the movements of mankind, these plants have landed in areas that they never would have been able to breach on their own.

In Golden Gate National Recreation Area cape ivy has invaded disturbed sites with year-round moisture, including riparian habitats along stream banks, coastal forests, and soils with a high water table. The ivy is also very good at colonizing new areas due to the fact that it can reproduce from stem fragments as small as half an inch. Carried by runoff, wildlife, or humans, it can root and grow rapidly. To illustrate the point, in 1987 there were 9 acres of Cape ivy in the Marin Headlands. In the next decade, the ivy gobbled up 67 acres! The ivy forms impenetrable mats as it climbs native shrubs and trees to form a solid layer that blocks out light and smothers other vegetation. It drapes from tree to tree, aptly nicknamed the "kudzu of the west."

Cape ivy is unsuitable forage for most wildlife due to the presence of strong chemical compounds (alkaloids and xanthones) in its leaves. It is known to be toxic to mammals and spiders, and there is some evidence that it is harmful to aquatic organisms. This monoculture of a bully plant reduces habitat for pollinators, and drastically alters bird diversity. Cape ivy most likely alters ecosystem level functions like nutrient cycling and food web dynamics as well.

Currently Golden Gate and Point Reyes National Seashore to our north are working together to remove 188 acres of ivy patches in both parks. The basic plan is to first create containment lines around the patches to stop the spread. Then all vegetation in the patch must be completely removed and raked to bare ground, and tarped to compost on site. Native shrubs and trees will resprout and refill the area over time, with staff consistently following up on ivy resprouts for the next 3-5 years. Different methods have been used in removal, mainly manually or with power tools, but also employing judicious herbicide application and innovative methods such as goat grazing and prescribed fire. Staff have also monitored different treatment plots and mapped ivy areas to be contained or removed in the future.

Nonnative Plants

Nonnative plants thrive in the park, particularly in areas subject to intensive historic land use related to grazing or military occupation, or adjacent to urbanized areas that are a constant source of weed invasion. The spread of non-native plants represents the most significant threat to the biodiversity of the park. One or several of the park’s 21 most invasive non-native pest plant species invade approximately 85 percent of the park’s estimated 48 plant communities. Research has shown that introduced species alter community composition and reduce the diversity of native plants, insects, and small mammals. Invasive non-native species are also found within all nine Special Ecological Areas designated as the most biologically intact and diverse areas within the Golden Gate. Non-native species also directly threaten habitat for the federally endangered mission blue and San Bruno elfin butterflies, Raven’s manzanita, Presidio clarkia, and San Francisco lessingia, as well as 12 other special status plants (Department of Fish and Game and Native Plant Society listed).

Golden Gate has currently targeted the 22 most invasive non-native species for control. These species include: Monterey pine, blue gum eucalyptus, Monterey cypress, black acacia, thoroughwort, cotoneaster, helichrysum, Himalayan blackberry, tall fescue, European harding grass, French and Scotch broom, Cape ivy, Ox-eye daisy, pampas grass, yellow star thistle, periwinkle, gorse, capeweed, English ivy, and calla lilies. These invasive plant populations are considered under control due to a decade of volunteer, staff and grant expenditures. And despite the extensive urban perimeter around the park, only two new invasive species have established small populations within the park within the last decade.

Endangered Scrubland Plants

The endangered Raven's manzanita is one of the San Francisco peninsula's unique subspecies of Hooker's manzanita. Discovered by Missouri Botanical Gardens curator Peter Raven as a young man, it is now reduced to a single plant near the World War II memorial in the Presidio. Cuttings from the mother plant have been grown and outplanted in the area. A Fish and Wildlife Service Recovery Plan came out in 2002, calling for more cuttings planted in the Presidio and other natural areas of the San Francisco peninsula.

Endangered Grassland Plants

The endangered Presidio Clarkia occurs in serpentine soils created from California’s state rock Serpentinite. Serpentenite occurs in fault zones and tends to have high levels of heavy metals such as zinc and magnesium and low levels of nutrients. Most of the Presidio’s grasslands have been developed or overrun by invasive European grasses. There is an additional population in the Oakland Hills.

The endangered White-rayed pentachaeta is a small annual plant with yellow disk flowers surrounded by white to purple ray flowers. This member of the sunflower family (Asteraceae) is currently known from a serpentine bunchgrass community and native prairie in two small areas of San Mateo County, both on San Francisco Water District lands. It was formerly known from Marin to Santa Cruz counties.

The endangered Fountain thistle is an herbaceous perennial with several stout, erect, reddish stems and large white to pinkish, nodding flowering heads. This member of the sunflower family occurs only in the extremely restricted serpentine seeps of the Crystal Springs region, San Mateo County. It sometimes grows with other rare plants like fragrant fritillary (Fritillaria liliaeca) and San Francisco wallflower (Erysimum franciscanum). The few existing fountain thistle occurrences are on public land owned and managed by CALTRANS and SFWD. An occurrence previously known from Edgewood County Park is thought to be extirpated; no plants have been seen there since one plant was observed in 1993. Construction of Interstate 280 contributed to the decline of fountain thistle by destroying habitat and altering the drainage patterns feeding the seeps in its serpentine grassland plant community; subsequent invasion of pampas grass into several of the colonies further threatens the species.

The threatened Marin dwarf-flax is a delicate annual plant in the flax family, with congested clusters of small rose to whitish flowers. It is found on serpentine ridges covered with bunchgrass from Marin County to San Mateo County and in a serpentine chaparral association in Marin County. There are now 20 known existing occurrences, ranging from land owned by the Marin Municipal Water District to the Presidio of San Francisco to Edgewood Park in San Mateo County to land owned by the San Francisco Water District. Residential development and road and freeway construction have eliminated five of the historically known populations.  of Marin western flax.

The endangered San Mateo thorn mint is an aromatic annual herb of the mint family. The small plants have white flowers, sometimes tinged with lavender, in tight clusters. It is restricted to serpentine soils of grasslands in San Mateo County. The species occupies slopes and flats with deep clay areas. The only remaining remnant population is in Edgewood County Park, and there is an introduced population at Pulgas Ridge. The extant populations are threatened by development and off-road vehicles.

Endangered Birds

The endangered California brown pelican have been observed roosting at Seal Rocks, Alcatraz Island, Hyde Street Pier, Bird Island, and Kent Island in Bolinas Lagoon. Brown pelicans feed on small fish such as the anchovy along the Pacific coast and in Bolinas and Rodeo lagoons. Threats to this species inlclude boating and active recreation in roosting areas, pollution and oil spills, and climatic factors affecting anchovy availability.

The endangered American peregrine falcon has historically nested at three sites in Golden Gate between the Golden Gate Bridge and Muir Beach since 1990. Peregrines are also known to over‑winter on Bolinas Lagoon. Between 15 and 30 peregrine falcons of all three subspecies — tundra, Peale’s, and the continental — have been observed in the GGNRA by the Golden Gate Raptor Observatory. Threats to this species include visitation by fishermen and adventurers, and toxic contaminants. Peregrine falcon decline is linked to the organochlorine pesticide DDT, which thinned egg shells, causing parents to cruch their own offspring. DDT was banned in 1972, but other organochlorines still enter the local environment. The peregrine falcon has been proposed for de-listing by the U.S. Fish and Wildlife Service (USFWS). 

The threatened northern spotted owl was listed by the USFWS in 1990. Northern spotted owls are widely distributed in forested regions from southern British Columbia through Washington, Oregon, and northwestern California. They reach the southern limit of their range in Marin County, where they occur in Golden Gate National Recreation Area, Muir Woods National Monument, Point Reyes National Seashore, and other parts of the county. These three national park units began a joint systematic survey for spotted owls in Marin County in 1993. Preliminary results of these surveys indicate that the county may support the highest density of spotted owls nationwide. Northern spotted owls are typically found in old- and mature second-growth forests, but in Marin County they reside in second- and old-growth Douglas fir, bishop pine, coast redwood, mixed conifer-hardwood, and evergreen hardwood forests. Threats to this species include urban development along protected-area boundaries, intense urban recreational pressures, potential for catastrophic wildfires due to unnatural fuel buildup, possible genetic isolation, and range expansion of the barred owl.

The threatened marbled murrelet are found in forest stands with old growth characteristics, and are extremely sensitive to disturbance and noise in the vicinity of nesting areas. A few unverified inland sightings have been reported since 1990. Systematic surveys have been conducted in Muir Woods National Monument and no murrelets have been detected. Marbled murrelets are infrequently seen in nearshore waters from mid-summer through winter. Detection of breeding murrelets in Marin would be extremely significant as there is a geographical gap between breeding populations in San Mateo and Santa Cruz counties to the south, and Mendocino County to the north. Threats to this species include range expansion of ravens, urban development, and urban recreational pressures.

The threatened bank swallow colony at Fort Funston is the largest nesting colony of bank swallows in the San Francisco Bay Area. Bank swallows migrate from South America to nest in the hundreds of burrows that cover the sandstone cliffs. Watching these aerialists capture insects on the wing and then feed their young while hovering in front of the burrows is a sight to see. Threats to this species include burrow invasions from European starlings, predation by American kestrels, and conflict with recreational uses such as hang gliding and rock climbing. The sandstone bluff is extremely erodible, and intense storm events are also a threat. 

The threatened western snowy plover was listed in 1993. This small bird over-winters on Ocean Beach in San Francisco from July through early May. Hiding in small depressions in the sand just beyond the tide line, many visitors who recreate at this beach never knew they were sharing space with the plover. The park established a snowy plover management area from Sloat Boulevard in the south to Stairwell 21 in the north along the O’Shaughnessey seawall, based on several years of monitoring data. Threats to this species include recreation such as off-leash dog walking and intense storm events.

Birds

The most visible wildlife in Golden Gate are definitely the birds. You will see them rustling in the bushes near a path, soaring overhead, or floating across a marsh. An astonishing number of avian species live in or migrate through Golden Gate. A variety of habitats ranging from open water and protected bays, to rocky and sandy shorelines, to tidal marshes, coastal scrub, grasslands, and forests create many different habitats that support over 250 different birds. Golden Gate lies along the Pacific Flyway, and is also host to a variety of transient birds that stop over to rest and feed in their amazing journeys from equatorial regions as far south as the South American rainforests to polar regions as far north as the Canadian arctic. This combination of factors makes Golden Gate a birder's paradise. The Audubon Society, California Partners in Flight, the California Department of Forestry, and the Fish and Wildlife Service all monitor bird activity in the park for species of concern.

Park creeks attract neotropical migrants: small songbirds making their way up and down the coast each fall and spring. Along Redwood, Lagunitas, and Lobos Creeks riparian songbirds such as olive-sided flycatchers, Swainson’s thrush, Wilson’s warbler, tanagers, and grosbeaks can be heard trilling their territorial and reproductive calls. Two species, the willow flycatcher and the bank swallow are considered threatened by the state of California.

The old growth forest of Muir Woods represents a fragmented island of the redwood stands that existed 150 years ago. Muir woods is now the last remaining contiguous stand of old growth coast redwoods in Marin County. Underneath the dappled sunlight, high in the branches of these giants, federally threatened species such as the northern spotted owl nests. Marin County is home for a fairly large population of these small owls that have found themselves spotlighted in the media.  At least 69 bird species occur within Muir Woods, the majority of which are small neotropical migrants such as the Pacific-slope flycatcher, winter wren, golden-crowned kinglet, and chestnut-backed chickadee.

The precipitous cliffs and offshore rocks that flank the park are a haven for colonial seabirds. Bird Island off of Rodeo Lagoon is one of the largest roosting sites in northern California for the endangered brown pelican, with several hundred making a splash in Rodeo Lagoon each Fall. Brandt’s cormorants nest at Lobos Rocks and Seal Rocks along Land’s End in San Francisco, turning the rocks bright white with their strong-smelling guano. Pelagic cormorants nest in very small colonies on steep cliffs and sea stacks from the Golden Gate north to Stinson Beach. Peregrine falcons are seen in their kamikaze dive, foraging along the coastal cliffs and have nested from the Golden Gate Bridge north to Muir Beach.

Colonial nesting waterbirds have found not a prison on Alcatraz Island, but inviting habitat in which to nest during the Spring and Summer months. In one of the most internationally visible settings within the National Park Sysytem, Alcatraz supports black-crowned night-herons, Brandt’s cormorants, pelagic cormorants, and pigeon guillemots - the only colonies found in San Francisco Bay for these species. Most colonial nesting waterbirds breed on offshore islands.  The island’s large western gull colony represents a significant portion of its coastal breeding population in northern California.  

Sandy beaches and mucky estuaries provide important habitat for migrating and wintering waterbirds and shorebirds. Tomales Bay, Bolinas Lagoon, Stinson Beach, Muir Beach, Big Lagoon, Rodeo Lagoon, Crissy Field and Ocean Beach, provide habitat for loons, grebes, scoters, numerous species of dabbling and diving ducks, gulls, terns, willets, sanderlings, and sandpipers. Federally threatened western snowy plovers overwinter in Ocean Beach. The park's mudflats provide plenty of tasty invertebrates, and estuaries provide fish and crabs galore. The San Francisco Bay is an important stopover for migrating species both due to its size and diversity of suitable habitat. Nearshore marine waters just outside the Golden Gate also provide foraging sooty shearwaters and pigeon guillemot, with thousands of birds rafting together on open water.

An island of green in an urban matrix, the Presidio’s location on the northwest tip of the San Francisco peninsula offers a stopover location to many birds before crossing the Golden Gate Straights, or as an entrance gate to the rich wetlands of the San Francisco Bay. Native habitats and introduced forest are regionally important to locally declining species such as the California quail, western screech owl, wrentit, and Hutton’s vireo. The landscaped areas of the Presidio also attract some interesting birds. The hooded oriole reaches the northern limit of its breeding range in the Presidio due to the royal palms planted throughout this former Army Post.

Amphibians

Hidden under logs or in dense vegetation, amphibians are among the least seen wildlife of the park. You may have heard a Pacific chorus frog chirping near a creek, or seen the California newt on its yearly migration. Sodden areas near creeks and marshes are the best place to find amphibians. In locations such as Big Lagoon near Muir Beach, soft clear balls of amphibian eggs attach to emergent vegetation, and male frogs call out to potential mates. Or in forested areas, a check under fallen logs will most likely turn up slender salamanders. Amphibians are highly sensitive to pollution, and a close eye on these animals can alert park managers to potential contaminants in wetlands and in the atmosphere.

The threatened California red-legged frog has been extirpated from 70 percent of its former range. Threats to this species include urban encroachment, construction of reservoirs and water diversions, introduction of non-native predators and competitors such as the bullfrog, livestock grazing, and habitat fragmentation.

Animals

Golden Gate's urban features may seem to suggest a lack of wildlife, but there is much more than meets the eye. Golden Gate's coastal ecosystems supports a rich assemblage of wildlife including 387 vertebrate species. The park's grasslands, scrublands, wetlands, and forests also support a rich diversity of invertebrates, although these species have not been well inventoried. The Recreation Area is home to nearly 53 species of mammals, 250 species of birds, 20 species of reptiles, and 11 species of amphibians.  

Some animals are picky about the place they call home, while for other animals anywhere with food, shelter, and water will do. Habitat generalists occur across a broad range of plant communities found in the park. Habitat specialists are adapted to the conditions of one particular habitat. Chaparral or prairies may provide a glimpse of rabbit, quail, or bobcat. Forests and woodlands may lead you to a black-tailed deer, banana slug, or spotted owl.  Marshes and creeks host an array of amphibians, fish, and reptiles including the red-legged frog, San Francisco garter snake, and Coho salmon.  All habitats support a variety of insects, and bees and butterflies abound.

Many wildlife have evolved a preference for specific vegetation to provide their food or shelter, such as the endangered mission blue butterfly and its host plant the silver-leaf lupine. Other wildlife use the park as a natural corridor in their yearly migration routes. Smack in the middle of the Pacific Flyway, hundreds of bird species use Golden Gate as open space to rest and refuel. Marine mammals such as whales, seals, and sea lions make use of the park's varied coastal habitats, with numerous haul out and pupping sites spread throughout the length of the park. Migratory insects such as the monarch butterfly return to the same tree stands year after year to overwinter.

Whether using park resources broadly or specifically, year round or for less than twenty four hours, all of these animals depend on the Recreation Area as a refuge from expanding development outside of park boundaries. There are over 80 rare or special status wildlife species currently identified as permanent or seasonal residents of the park, or dependent upon park lands and waters for migration. Of these, 12 are listed as federally endangered, and 12 are federally threatened.

Other Invertebrates (corals, sponges, worms, etc.)

More than 24 miles of ocean and bay border Golden Gate. Coastal and bay resources comprise biologically diverse and complex ecosystems, which contain a rich array of marine invertebrates and algae. Intertidal communities within or adjacent to park boundaries include natural islands, reefs, offshore rocks, straits, lagoons, mudflats, and beaches. Man-made habitat include piers, wharves, and shoreline rubble. The Gulf of the Farallones Marine Sanctuary is just offshore, and the San Francisco Bay-Estuary empties through the straits of the Golden Gate. Slide Ranch, Muir Beach, Pirates Cove, Kirby Cove, Fort Point, and Alcatraz display a rich diversity of sponges, bryozoans, tunicates, limpets, mussels, anemones, and sea stars on their rocky headlands. Barnacles and isopods cover the splash zone. Sea caves abound in the park and have yet to be inventoried.

At Rodeo Beach Bird Rock (a guano-covered sea stack), marine invertebrates include large chilipepper shrimp, California mussels, and green anemone and purple seastars. Less common invertebrates include abalone and sea urchins. Rocky intertidal areas are interspersed throughout Marin County at Stinson Beach, Slide Ranch, Muir Beach, Tennessee Cove, Rodeo Beach, Bonita Cove, Kirby Cove, and Lime Point. The intertidal zone along the coast of Marin County is generally steep and rocky, with small beaches occurring adjacent to watershed drainage areas, and not easily accessible by the public. In San Francisco County intertidal areas are primarily beach or pier habitat including Fort Funston, Ocean Beach, Land’s End, China Beach, Baker Beach, Fort Point, Crissy Field, Fort Mason, Black Point, and Aquatic Park. Many of the intertidal areas serve as living outdoor classrooms for Bay Area residents and visitors. The state‑protected Dungeness crab breeds in the Pacific Ocean just offshore, and many Dungeness juveniles, as well as other species of shore crabs, can be spied in the park's rocky areas and mud flats

The California freshwater shrimp is endemic to Marin, Sonoma and Napa counties, but only remains in portions of 16 coastal streams. Lagunitas Creek in Marin County contains the most viable population of the shrimp and it is the only site occurring on protected lands. The shrimp is threatened by water diversions on Lagunitas Creek, watershed erosion, stream sedimentation, riparian vegetation removal, agricultural development, grazing, and urbanization.

Mammals

These furry members of Golden Gate's wildlife seem to attract the most interest from park visitors. Despite the urban development and habitat fragmentation that continues to occur in central coastal California, the park supports a diversity of mammals. The fauna of today are very different than the variety of species found here before the arrival of the Europeans in the 1700s. After the Gold Rush, local grasslands and scrublands were quickly replaced with dairies and farms to support this emerging metropolitan mecca. Long gone are the large mammals such as the grizzly bear and tule elk of early California. But high densities of mesocarnivores, including the gray fox, bobcat, and the recently reestablished coyote inhabit coastal scrub and grasslands across Marin and San Mateo Counties. Mountain lions sightings regularly occur throughout undeveloped areas of these two counties. These carnivores feed on a variety of small and large mammals such as the black-tailed deer, broad-footed mole, pocket gopher, deer mouse, western harvest mouse, California vole, badger, and brush rabbit.

The shady forest world of Muir Woods supports 30 species of mammals, ranging from the vagrant shrew and Trowbridge's shrew to the Sonoma chipmunk, western gray squirrel, opossum, and black-tailed deer. Spotted owls feed primarily on dusky-footed wood rats, known for their large wood pile nests. Carnivores include the raccoon, striped and spotted skunks, long-tailed weasel, gray fox, and the recently returned river otter. Bats are always an intriguing group, instilling awe into those who admire and fear them. Nine different species have been identified roosting in the fire scars of old growth redwood trees. Three of the species, the Pacific western big-eared bat, fringed myotis, and Yuma myotis, are federal or state species of concern.

Isolated coastal rocks and beaches serve as haul-outs for harbor seals and California sea lions. Although Bay shores were once filled with pinipeds, concetrations such as the hundreds of harbor seals that haul out in Point Bonita Cove at Marin Headlands can still be found. Significant harbor seal pupping areas are found in Bolinas Lagoon and Tomales Bay. The northern elephant seal population has been rapidly increasing, leading to more encounters on sandy beaches throughout the region, including a fairly large colony at Point Reyes National Seashore. California gray whales, humpback whales, and harbor porpoises use nearshore waters. Young whales occasionally wander into San Francisco Bay. Southern sea otters are infrequently seen offshore with numbers increasing as the population spreads north from Monterey Bay.

Insects, Spiders, Centipedes, Millipedes

Golden Gate is simply teeming with insects, spiders, and other many-legged crawlers. Every habitat has its crew of characters, from bugs thriving in the tops of tall trees, to soil dwellers milling beneath the earth's surface. The sight of early morning dew on a spider web, or a bumble bee lumbering from flower to flower, or a centipede crawling through leaf litter can be an introduction to an entirely new world. Social insects such as ants, bees, and wasps are often introduced species from other parts of the world, but native representatives still thrive in the park. Grasshoppers jump in front of visitors in the grasslands, and beetles trekk across the sand dunes. Even the fly, a creature we often think of as ugly, comes in a rainbow of colors and can be seen pollinating flower clusters in the spring. Bugs even show up in wetland areas, water striders gliding across a creek surface, or dragonflies and damselflies alighting on a sedge. There are as many insects to learn as the visitor has time to explore.

The most beautiful insects of the park are no doubt the butterflies. In late spring and early summer, the iridescent wing scales catch the visitor's eye as they move from flower to flower spreading pollen. Grassland wildflowers attract the majority of species, but butterflies abound in any of the park's ecosystems. At least 44 species of butterflies occur in the Marin Headlands and 34 species occur at Milagra Ridge, inlcuding species of skippers, swallowtails, hairstreaks, blues, ladies, admirals and crescents. Such high species diversity illustrates the importance of habitat fragments within largely developed landscapes.

Endangered Fish

The threatened steelhead trout is found in many perennial coastal streams within the park. In addition, the offshore waters along the Pacific coast as well as estuarine areas in San Francisco Bay and Tomales Bay provide rearing habitat for steelhead. Threats to this species include degradation of spawning gravels, habitat simplification, and water diversions.

The threatened coho salmon is in its southernmost range in the Lagunitas, Olema, and Redwood Creek watersheds. Juveniles are often found in deep pools with abundant cover in the form of undercut banks, overhanging vegetation, and woody materials. In addition, the offshore waters along the Pacific coast as well as estuarine areas in Tomales Bay could provide rearing habitat for coho salmon. Threats to this species include degradation of spawning gravels, habitat simplification, and water diversions.

The endangered tidewater goby currently lives in Rodeo Lagoon and at Joacomini Ranch in the north lands near Point Reyes. Historic records indicate that the small estuarine fish occurred in at least 9 other locations within the San Francisco Bay Region, such as Lake Merced and Corte Madera Creek. Threats to this species include loss of habitat through excessive sedimentation, poor water quality, and non-native competitors.

Endangered Butterflies

The endangered mission blue butterfly inhabits Milagra Ridge in Pacifica, Sweeney Ridge in San Bruno, Twin Peaks, and portions of the Marin Headlands. One of the first invertebrates to be protected under the Endangered Species Act, this small butterfly is an important component of area grasslands. Its host plant is silver-leaf lupine, and it is only in its mature flight stage for three weeks. Several butterfly habitat restoration projects are currently underway in the park involving non-native plant removal and native plant restoration. Threats to this species include development, trampling by excessive foot traffic, illegal off-road vehicles, and non-native plant invasion.  

The endangered San Bruno elfin butterfly occurs at Milagra Ridge and Montara Mountain near Pacifica, San Bruno Mountain in San Francisco, and near Alpine Lake and Dillons Beach in Marin County. Its host plant is stonecrop. Young larvae are tended by several species of ants that protect them from predators. These ants also groom the larvae and feed on a honeydew substance produced by the larvae.  Threats to this species include non-native plant invasion, trampling by people, lack of proper fire management, and development.

The Bay checkerspot butterfly inhabits Edgewood Park in the San Francisco Watershed. It has a wing span of little more than 2 inches. The forewings have black bands along all the veins on the upper surface, contrasting sharply with bright red, yellow and white spots. Their habitat is on serpentine soild where their host plant, dwarf plantain, grows. Males typically emerge four to eight days before females. Males can mate many times, while most females mate only once. Threats to this species include development and non-native plant invasion.

Fish

Golden Gate National Recreation Area cannot escape the influences of the ocean and bay waters that surround it. Notice the fisherman on park beaches or piers, and you will begin to realize the bounty of fish in the park. Starting from the offshore waters of the Pacific ocean, a multitude of species travel along currents past and through the Golden Gate. The SanFrancisco Bay is a world of its own, supporting estuarine species that can handle the fluctuating salinity levels. Intertidal areas of the park provide important spawning and rearing habitat for fish.

Commercially important species such as the Pacific herring spawn in Tomales Bay, the intertidal rocks of Alcatraz, and other central bay rocky shorelines. Anchovy are the most abundant fish in the Bay, entering seasonally to forage and spawn, and are important to the economy of West Coast fisheries. 

The intertidal zone supplies fishermen with surf perch, cabezons, blennies, rock fish, pricklebacks, mussels and sea urchins. Typical estuarine fish include brown smoothhound, pile surfperch and white croaker. Coho salmon, lamprey, steelhead trout and maintain their annual migrations up Redwood Creek, Olema Creek and Lagunitas Creek. Green and white sturgeon can still be found in lower Lagunitas Creek, Tomales Bay, and the San Francisco Bay-Estuary. Limited information about fish species and abundance is available from beach seines and trawls conducted by the California Department of Fish and Game.

Nonnative Animals

Non-native animals are often introduced to an area as hunting stock for pioneers settling an area. Others like rats were brought over by accident in the hulls of ocean-crossing ships. Invasive animals tend to be generalists that compete with local wildlife for food and territory. They can also be a threat to local vegetation if they overforage in delicate habitats. Sometimes invasive wildlife need to be removed from parklands.

A number of birds have become park pests. Brown-headed cowbirds parasitize open-cup nests of birds. Neotropical migrants and riparian nesting birds are particularly susceptible. Cowbird parasitism is widespread throughout the park, but the level of parasitism and the lack of concentrated foraging areas make cowbird control unrealistic. Neotropical migrants are threatened by elevated predation levels (probably resulting from habitat modification), loss of habitat and parasitism. Wild turkeys were recently introduced into Marin County by the CDFG. Wild turkeys feed on a wide variety of foods including native frogs and native plants and seeds. Peacocks have similar habitat impacts. European starlings are cavity nesters that compete with and displace native species from limited nesting habitat. American kestrels, bank swallows and other cavity nesters are impacted by the widespread occurrence of starlings.

Feral hogs were widespread in the park during the 1980s but appear to have been successfully eradicated through hunting and trapping efforts by the NPS. Only a few unconfirmed sightings have been reported over the past 5 years. Feral hogs have potential to seriously degrade habitat and native animals populations through soil disturbance, uprooting of native plants, competition for foraging resources, particularly acorns, predation on small animals, and disease transmission. Feral hog populations could rapidly increase again at any time in Marin or San Mateo counties.

Norway and black rats are known to occur in various locations throughout the park, including Muir Woods, Alcatraz, Olema Valley and Marin Headlands. Rats prey on native wildlife and their young. They were found preying on the Townsend's big-eared bat in Olema Valley, where steps were taken to discourage and exclude them from the maternity roost. They are also a threat to burrow-nesting birds (such as pigeon guillemots on Alcatraz) that leave their young unattended while the adults forage at sea. Black rats are excellent climbers and will take eggs and young out of nests in tall shrubs and trees. Rats also carry diseases and constitute a human health threat wherever they occur.

A major threat to aquatic systems is the bullfrog. It is found in inland lakes, ponds, and wetlands such as Big Lagoon. It is a treat to local amphibians, eating the eggs and juveniles of other frog species, including the endangered red-legged frog.

Pillow Basalt FAQ

Where does pillow basalt form?

Pillow basalt is a volcanic igneous rock that forms when lava of basaltic compositionis erupted underwater. The rapid cooling of the lava by cold water on all sides forms the pillow-shaped bodies, which can then break open and extrude more of the hot lava from inside. The rapid cooling also creates pillows that are composed of volcanic glass on the outside and that have very tiny, almost invisible crystals on the interior. Pillow basalt typically forms at volcanoes at mid-ocean ridges or at oceanic hot-spot volcanoes, such as those that formed the Hawaiian Islands. Basalt forms the crust of all the ocean basins and is therefore the most common rock in the Earth's crust.

What gives basalt its color?

Basalt has a lower percentage of silica and a higher percentage of iron and magnesium than other volcanic rocks. These characteristics give it a very dark, almost black color. Hot, mineral-rich seawater flowed through much of the basalt that is part of the Franciscan Complex, changing some of its minerals into chlorite and other green minerals. This altered basalt is called greenstone.

What makes basalt sometimes appear speckled?

Sometimes basalt in the Franciscan Complex will have small white or pink speckles or even small round holes in it. These are the result of gas bubbles that were captured in the lava when it cooled. Sometimes the gas bubbles are empty, but other time they get filled with quartz or calcite that precipitated out of mineral-rich water circulating through the basalt, producing the speckles. Sometimes the gas bubbles in Franciscan basalt get filled with a bright orange semi-precious form of quartz called carnelian. If basalt is erupted too far under the ocean, the pressure from all the overlying water will not allow the gas bubbles to form. This is the case for most Franciscan basalt, but the basalt at Point Bonita is an exception.

Graywacke Sandstone FAQ

Where is graywacke sandstone deposited?

Graywacke sandstone is a sedimentary rock that is made up mostly of sand-size grains that were rapidly deposited very near the source rock from which they were weathered. Graywacke is deposited in deep ocean water near volcanic mountain ranges, where underwater landslides and density currents called turbidites quickly transport sediment short distances into a subduction zone or ocean trench. This type of sandstone contains fewer grains made of quartz and more made of feldspars, volcanic rock fragments, as well as silt and clay than most sandstone. It is therefore also known as "dirty sandstone." The volcanic rock fragments give graywacke a greenish-gray color.

What makes the beds in graywacke?

Graywacke sandstone deposits display flat-lying beds, each composed of sedimentary particles of different sizes. The sandstone beds can be from inches to many feet thick and are often separated by thin, dark shale beds. Each sandstone bed was formed during a single turbidite or submarine landslide event and was deposited over a short period of time from hours to days. The thin shale beds formed between turbidite events, when mud particles slowly settled to the sea floor, and may represent thousands of years. Turbidites display graded bedding, that is, the grain size decreases upwards in the bed. During a turbidite event, the larger and heavier grains settle out first. As the energy in the landslide event decreases, finer and finer particles settle out to the sea floor.

Are there fossils in graywacke?

Graywacke sandstone occasionally contains fossil mollusks which sometimes can be used to tell when the rock was deposited. Sandstone deposits of the Franciscan Complex contain clams and ammonites from the Jurassic and Cretaceous periods. These provide ages for when the Franciscan oceanic rocks got close enough to North America for continental graywacke sediments to be deposited onto them. The shale layers between the graywacke beds may contain microfossils that also can be used to date the rocks and to determine the depth of water in which they were deposited. Sometimes trace fossils are also visible in graywacke. Trace fossils are the marks and tracks of animals that burrowed, fed, crawled and lived in the sediments. Trace fossils can provide information on how deep the water was and how much oxygen was present when the sediments were deposited.

Chert FAQ

Where is chert deposited?

Chert is a sedimentary rock rich in silica. Franciscan chert is formed from the tiny silica shells (0.5-1 mm) of marine plankton called Radiolaria. Radiolarian chert forms where two conditions are met. First, a deep, open ocean setting is required where there is little continental mud or carbonate sediment to dilute the "rain" of dead radiolarian shells settling to the seafloor. Second, the upper ocean waters need to be relatively rich in nutrients in order for abundant Radiolaria to thrive.

How do geologists get the Radiolaria out of the chert?

The beautiful and intricate Radiolaria tests can be extracted from the chert by crushing it into small pieces and then putting it in a solution of hydrofluoric acid. This is the same acid used to etch glass. The acid dissolves away the less durable rock matrix, leaving the exquisite three-dimensional shells exposed. The tests are then photographed using a scanning electron microscope.

What can the Radiolaria tell us?

Radiolaria are still living in the oceans today. Different species live in tropical oceans versus temperate or cold ocean water. By comparing the types of Radiolaria in local chert with modern forms, we know that Franciscan chert contains some tropical and subtropical forms. Based on this observation, it appears that the sediments forming the local chert were deposited far to the south of their current location around San Francisco. Bay Area chert possibly came from the north equatorial upwelling zone, at the latitude of present-day southern Mexico.

Scientists also study the various species of Radiolaria that are present in chert deposited at different times in the past. Through these studies, they have developed an evolutionary sequence for different species in the rocks. This evolutionary sequence, or biostratigraphy, is then linked to radiometric dates obtained from associated volcanic rocks. Then geologists can determine the age of the chert. The radiolarian species in the Franciscan chert in the Marin Headlands lived and died to form the rocks during the period from about 200 million to 100 million years ago.

What makes the chert bedded?

The prominent bedding seen in Franciscan chert leads to the name ribbon chert. The hard, silica-rich chert beds are separated by thin beds of soft, clay-rich shale. These dramatically alternating beds are the result of a process called diagenetic enhancement. When the chert-forming sediments were laid down, some levels had slightly more silica than others. When the sediments were transformed into rock in a process called diagenesis, the silica in the less silica-rich zones migrated into the more silica-rich zones, increasing the silica contrast between the levels and enhancing the bedding to form ribbon chert.

Why did some levels of chert-forming sediment have more silica than others?

There are two theories for the original differences in silica levels that led to the formation of chert beds. The first theory is that silica levels were controlled by changes in the oceanographic process called upwelling, which brings nutrient-rich water to the ocean surface and allows the Radiolaria to thrive. Some evidence indicates that upwelling in the world's oceans is cyclic and strongly controlled by the Earth's orbital cycles. The second theory is that the silica-rich beds were deposited by small underwater landslides that sorted the fine clays out from the heavier radiolarian shells. There is some microscopic evidence of graded bedding that supports this possibility. Both processes, cyclical upwelling and underwater landslides, could have played a role in the formation of the Franciscan ribbon chert.

Why is chert different colors?

Most local chert is red and less commonly green, but it may be a range of colors. The color reflects the amount of oxygen present in the sediment when it became rock. If oxygen is plentiful in the sediment, it oxidizes small amounts of iron present and the chert is red. If oxygen is scarce, the iron is reduced and the chert is green or black.

Why are the chert beds so bent and folded?

Many Franciscan chert beds are highly folded and contorted, but within a short distance they often appear to be unfolded. Some scientists believe that this type of folding is the product of the slumping of the soft gelatinous silica-rich sediments, contorting them before they are fully hardened into rock. This slumping may have occurred on the flank of an oceanic mountain range as earthquakes shook the tectonically active mid-ocean ridge. Another theory is that the chert beds were folded by compressive forces developed as the Franciscan Complex was created by the scraping and addition of oceanic rocks to the western margin of North America.

Diabase FAQ

Where does diabase form?

Diabase is an intrusive igneous rock with the same mineral composition as basalt. It cools under basaltic volcanoes, like those at mid-ocean ridges. Diabase cools moderately quickly when magma moves up into fractures and weak zones below a volcano. There, it forms dikes (tabular igneous rock bodies that cut across pre-existing rock layers or bodies) or sills (tabular igneous rock bodies that form parallel to pre-existing rock layers). The moderate cooling rate allows small visible crystals to form in the rock.

Why does the diabase have large and small crystals?

Igneous rock with some large crystals among the smaller crystals is called a porphyry. The different crystal sizes are the result of different rates of cooling as the magma body moved upward. The large crystals, called phenocrysts, in diabase are feldspar crystals that grew as the magma cooled slowly deep in a magma chamber. Later the magma with the large phenocrysts moved upward quickly, causing more rapid cooling of the rest of the magma and the formation of the small crystals that make up the rest of the rock.

Granite and Granodiorite FAQ

Where do granite and granodiorite form?

Granite and granodiorite are intrusive igneous rocks that slowly cool deep underground in magma chambers called plutons. This slow cooling process allows easily visible crystals to form. Both rocks are the product of the melting of continental rocks near subduction zones.

What is the difference between granite and granodiorite?

These rocks are both classified as granitic, because they both are rich in quartz. Granite contains mostly potassium feldspars and has a low percentage of dark iron and magnesium minerals. In contrast, granodiorite contains more plagioclase (calcium and sodium) feldspar than potassium feldspar and has more dark minerals. Thus it is a darker color than granite. Chemical and x-ray analysis of granite and granodiorite can be used to "fingerprint" these rocks, telling their exact composition and where they may have formed.

Where do you find granite and granodiorite?

Granitic rocks are found on continents around the world near active or past plate boundaries. They formed as magma cooled many kilometers below the Earth's surface. The granitic rocks were then uplifted to the surface as the volcanic mountains above them eroded away. In California, granitic rocks form the core of the Sierra Nevada, cooled from rock melted during the subduction process that also formed the rocks of the Franciscan Complex. Granite and granodiorite are also found west of the San Andreas fault near Monterey, Pacifica, and Point Reyes, where granite from the south end of the Sierra range has been transported northward by San Andreas fault movement. Some granite was also carried to the California coast from China by humans. Chinese granite sometimes filled the holds of sailing ships on their way to San Francisco during the Gold Rush.

Why do some grantitic rocks have both large and small crystals in them?

This type for rock is called a porphyry. The different crystal sizes are the result of different rates of cooling as the magma body moved upwards. The large crystals, called phenocrysts, are usually feldspar crystals. Feldspar is one of the first minerals to form large crystals as magma solidifies. They grew as the magma cooled very slowly deep in the magma chamber. Later, the magma with the phenocrysts moved quickly upward into cooler rock, causing more rapid cooling of the remaining molten rock to form the smaller crystals that make up the rest of the rock.

Developing an Essential Question

The essential question of our geology curricula, How do I recognize evidence of geologic change in my environment?, is intended to be a theme or thread, linking all curriculum components together. The question has the following characteristics:

Transportable: The essential question can be asked in any educational setting: National Park, school, neighborhood, Earth, and during any part of the Rocks on the Move curriculum: pre-site visit, on-site program, and post-site lessons.

Multi-sensory: The essential question can be answered using many senses. We can observe geologic changes, visually, or by feel (shaking ground), smell (volcanic activity), taste (salt air), hearing (hear the rumble of an earthquake or landslide).

Universal: All students have experienced geologic change of some sort or another by being residents of planet Earth, so all students have personal experiences to draw upon and share.

Process-oriented: The essential question addresses process (geologic changes and how they occur) rather than solely observation/description (rock identification and mapping).

Limestone FAQ

Where is limestone deposited?

Limestone is a sedimentary rock rich in the mineral calcite, which is made of calcium carbonate. Franciscan limestone is formed mostly from the tiny carbonate shells of single celled marine animals called foraminifera. This type of limestone forms in ocean settings where there is not enough continental mud to dilute the slow "rain" of carbonate shells, and where the ocean is not so deep that all calcium carbonate dissolves into the water before it can be buried in the sediment to form rock. In today's oceans, all calcium carbonate dissolves below the depth (called Carbonate Compensation Depth or CCD) of about 4 km. Franciscan limestone is thought to have formed on the tops and sides of underwater volcanoes in water less than 4 km deep.

What can the fossil shells tell us?

Scientists have studied foraminifera found in sedimentary rocks. After identifying these fossils, geologists develop an evolutionary sequence of the species. This sequence, called a biostratigraphy, helps geologists determine the age of the rocks. The limestone in the Marin Headlands is too metamorphosed and recrystallized to contain original fossil shells, but other local Franciscan limestone contains fossils that tell us it was deposited from about 125 million ago to about 90 million years ago.

Serpentinite FAQ

Were does serpentinite form?

Serpentinite is a metamorphic rock that forms at tectonic plate boundaries deep within the Earth. In the Franciscan Complex, it formed when ocean water carried down with subducting ocean crust was heated and moved through upper mantle and basal ocean crust rocks, hydrating their magnesium- and iron-rich minerals, like olive and pyroxene, to form magnesium-rich serpentine minerals.

What makes serpentinite look and feel the way it does?

Serpentinite rocks are almost exclusively made of serpentine minerals. The most common serpentine mineral in Franciscan rocks is antigorite. This mineral gives the serpentinite its characteristic light to dark green color. Serpentine minerals are made of tiny sheets of silica tetrahedrons that are loosely held together. The weak bonds between these sheets gives serpentine its greasy or scaly look, and slippery feel (like a snake skin). Serpentinite often contains many veins, some of which may be filled with the fibrous mineral chrysotile (a form of asbestos). Chrysotile is a serpentine mineral in which the silica sheets are rolled into tiny tubes to form hollow fibers. Loose asbestos fibers cause lung disease if you inhale them. Pay attention to serpentinite - if you see fibrous asbestos, don't handle the rock.

How does serpentinite get to the Earth's surface?

Scientists do not fully understand how serpentinite makes its way from deep in the Earth, where it often forms, to the surface of the Earth. In the process of formation, serpentinite rock actually becomes less dense (more buoyant) as water is added. Thus it essentially floats upwards, buoyed up by the denser rock around it. Serpentinite is also a very plastic and greasy rock that can be easily squeezed into and even lubricate the many faults at plate boundaries. It then may get dragged or "squirted" to the surface along the faults.

How can water molecules become part of a rock?

Hot water forced through tiny fractures and pores in the mantle rock dissolves some silica from the rock into it. This silica-rich water then chemically reacts with mantle rock minerals like olivine to produce serpentine minerals by the formula below:

3Mg2SiO4 (olivine) + SiO2 + 2H2O converts to 2Mg3Si2O5(OH)4 (serpentine)

In this chemical reaction, the water is converted to hydroxyl groups (OH) that are part of the serpentine minerals.

Environmental Factors

The Golden Gate National Recreation Area is truly a park on the edge, located on the far end of the North American tectonic plate and flanking the Pacific Ocean. Situated above a large subduction zone where planetary crust is driven underground, destroyed, and recycled into new geologic resources, Golden Gate contains a wide variety of geologic features. From the Franciscan complex, representing tectonic events that took place hundreds of millions of years ago. To the Colma formation which tells of a time hundreds of thousands of years ago when San Francisco was an island and the Central Valley of California was an inland sea. To dune sands only a few thousand years old that migrated from the Sierra Nevada Mountains through the Sacramento River all the way to the coast. Whether scraped from the ocean floor under extreme heat and pressure like serpentenite, or built by millions of microscopic sea creatures under time and pressure like radiolarian chert, the Recreation Area has anything a rock lover could want.

Climatic shifts and geologic processes continue to shape this environment as they have for millennia. More recently, human-caused factors such as air, noise, light, and water pollution, have had a much greater impact on natural resources world-wide. The Recreation Area strives to keep a healthy environment for wildlife and world-weary humans alike.