Glen Canyon Natural Areas

Glen - Multiple Hiking Paths

Beyond Glen Canyon Park’s recreation center, ball field, and tennis courts lies a large urban canyon that has incredible spring wildflower displays, dramatic rock (chert) formations, and Islais Creek, one of the few remaining free-flowing creeks in San Francisco. This 60 acres of wilderness, formerly referred to as the San Miguel Hills, not only provides critical habitat for a wide array of wildlife, but serves as a relaxing sanctuary from the city’s urban bustle. An extensive network of hiking trails leads through a variety of habitats, from the lush creekside vegetation to the rocky grass- and scrublands of the canyon’s steep eastern slope, where a profusion of wildflowers blooms each spring.

Natural History

Islais Creek

Even if you don't at first hear the trickle of Islais Creek, the dense stand of willows west of the service road is a telltale sign that water is near by. Islais Creek supports a diverse streamside ecosystem characterized by water-loving plants such as willow trees, horsetail, seep monkey flower, and red columbine. These plants provide habitat for adult and larval insects, which in turn feed amphibians, reptiles, and birds, some of which travel from as far away as South America. The creek is important to migratory and resident songbirds, and is one of the few remaining spots in San Francisco where they can take advantage of a fresh water source, abundant insect life, and generous shelter.

Grassland

As you walk along the north-south road through the canyon, you may notice the steep eastern slope, home to a grassland community. During summer and early fall the land is parched and golden brown, and native grasses drop their seeds and await the winter rains. By February the hills transform to a verdant green. In spring a variety of native wildflowers, including the California poppy, blue-eyed grass, checkerbloom, and mule’s ears add oranges, blues, pinks, and yellows to the canyon.

Coastal Scrub

Farther north along the road you will encounter the dense shrubs that comprise the coastal scrub community. Here, coyote brush, coastal sage, bee plant, and California blackberry intermingle to provide habitat for a wide array of ground-nesting and shrub-loving birds.

Coastal scrub provides important structure and cover for bird nests, and the plants are an incredible food source for birds. Coyote brush alone provides habitat for more than 250 species of insects, which in turn are a critical food source for birds such as swallows and black phoebes, as well as California alligator and western fence lizards. The thorny strands of California blackberry often wind their way around the coyote brush, bearing fruit for birds, raccoons, and humans throughout the summer. Bee plant is aptly named, as it attracts not only bumblebees and honeybees, but more than 20 species of non-stinging solitary bees.

In recent years coyotes have been spotted in Glen Canyon; this native mammal, once prevalent in San Francisco, had gone extinct from the city but now is making its way back. If you are lucky enough to see one of these elusive creatures, please don’t try to approach or feed it, for its safety and yours. Remember that a fed coyote is a dead coyote: While not normally a threat to humans, coyotes that become too accustomed to being approached can become aggressive. Keep your dog on leash in areas that coyotes are known to frequent, for its own safety. For more information about learning to coexist with coyotes, see Project Coyote.

Urban Forest

Most of the forest at Glen Canyon Park is composed of blue gum eucalyptus, cypress, and pine trees, and accounts for over 17 acres of the park. These trees were planted throughout San Francisco beginning in the 19th century and have significant value for both wildlife and people, even though they are invasive species. Owls and hawks rely on them for nesting and hunting, and human residents appreciate the respite that the urban forest offers.

Geology

Throughout the park you will find outcrops of reddish rock formed from the shells of microscopic ocean-dwelling organisms called radiolaria. Millions of years ago these tiny animals lived, died, and drifted down through the deep ocean waters to the ocean floor. Eventually the combination of pressure and time turned them into the rock we call radiolarian chert. The movements of the tectonic plates eventually brought the chert close to the shore. Later, sand eroding from the Sierra Nevada washed into the ocean and turned to sandstone layered atop the chert. As the Farallon plate subducted under the Continental plate, the rocks of Glen Canyon were scraped off, scrambled up, and pitched upon the land.

Scarce water supply and shallow soils make the rock outcrops a challenging habitat for plant life; only well-adapted species survive. Dudleya has adapted to this dry environment by storing water in its thick, fleshy leaves. Lichens offer another example of plant adaptation, releasing special chemicals to obtain nutrients from the rock, a process that also contributes to soil formation. Reptiles like California alligator lizards and San Francisco garter snakes take advantage of the solar exposure the rock outcrops provide and spend many afternoons basking in the sun.

Cultural History

Humans have used and enjoyed Glen Canyon Park's landscape for hundreds of years. Originally hunting territory of the local Ohlone Indians, it became grazing land for the longhorn cattle and sheep of Mission Dolores in the late 18th century. Grazing continued under Mexican rule from the 1820s to the 1840s. The park site was part of José Noe's Rancho San Miguel in the 1840s and ‘50s. In early American times the canyon was a haven for smugglers and cattle rustlers. Devil's Cave and Dead Man's Cave, in the rocky outcrops, were alleged hideouts.

In 1868, Glen Canyon became the site of the first commercial dynamite manufacturing operation in the United States, run by the Giant Powder Company. One year later, an explosion completely destroyed the facility. The Giant Powder Company's failure to transform the canyon into an industrial area gave developers the idea to turn it into residential properties. In 1889 the heirs of Adolf Sutro sold the land to the Crocker Real Estate Company. The developers attracted people to the area by constructing an amusement park that included an aviary, a bowling alley, and a small zoo. Balloon ascents and a tightrope walk across the canyon by Jimmy "Scarface" Williams were special attractions. The area was called the Glen Park Picnic Grounds, and was purchased from the Crocker Real Estate Company by the City and County of San Francisco in 1922 for $30,000.

In 1941 O'Shaughnessy Boulevard was completed, cutting off the watershed on the west side of the park and further diminishing Islais Creek. The Silver Tree Day Camp was established that year, and the Silver Tree building was built in 1956.

In the 1970s a plan to widen O'Shaughnessy Boulevard and make it part of the freeway system was defeated by community opposition led by the "Gum Tree ladies," a group of local women who united around efforts to save Glen Canyon from development. The Friends of Glen Canyon Park and the Glen Park Association continue to work to improve the park's recreational facilities and restore its natural areas.

Trail Map

An extensive 3.7 mile trail network leads through a variety of habitats, from the lush creekside vegetation to the rocky grass- and scrublands of the canyon’s steep eastern slope, where a profusion of wildflowers blooms each spring. The 1.2 mile Creek to Peaks trail starts along Islais Creek and rises up towards Twin Peaks, where sweeping views of San Francisco and the Bay can be enjoyed.

Islais Creek supports a diverse streamside ecosystem of willow trees, horsetails, seep monkey flower, and red columbine. These plants provide habitat for adult and larval insects, which in turn feed amphibians, reptiles, and birds, some of which travel from as far away as South America.

The north-south road through the canyon is home to a grassland community. During summer and early fall the land is parched and golden brown, and native grasses drop their seeds awaiting the winter rains. By February the hills transform to a verdant green and by spring a variety of native wildflowers, including the California poppy, blue-eyed grass, checkerbloom, and mule’s ears add oranges, blues, pinks, and yellows to the canyon.

Farther north along the road, a dense coastal scrub community of coyote brush, coastal sage, bee plant, and California blackberry intermingle to provide habitat for a wide array of ground-nesting and shrub-loving birds. Coyote brush alone provides habitat for more than 250 species of insects, which in turn are a critical food source for birds and reptiles. California blackberry wind their way around the coyote brush, bearing fruit for birds, raccoons, and humans throughout the summer. Bee plant is aptly named, attracting not only bumblebees and honeybees, but more than 20 species of non-stinging solitary bees.

Most of the forest at Glen Canyon Park is composed of blue gum eucalyptus, cypress, and pine trees planted in the beginning the 19th century and has significant value for both wildlife and people. Owls and hawks rely on them for nesting and hunting, and human residents appreciate the respite of the urban forest.

Glen Canyon Trail Map

Location Map