Glen Canyon Park

O'Shaughnessy Hollow is a rugged, undeveloped 3.6 acres (1.5 ha) tract of parkland that lies immediately to the west and may be considered an extension of Glen Canyon Park.

The park and hollow offer an experience of San Francisco's diverse terrains as they appeared before the intense development of the region in the late 19th and the 20th centuries.

The park incorporates free-flowing Islais Creek and the associated riparian habitat, an extensive grassland with adjoining trees that supports breeding pairs of red-tailed hawks and great horned owls, striking rock outcrops, and arid patches covered by "coastal scrub" plant communities.

[4] An additional building about halfway up the canyon near Islais Creek serves the Silver Tree Day Camp and the Glenridge Cooperative Nursery School.

The bottom of the canyon, where Islais Creek flows, is irregular but moderate in slope, dropping 350 feet (107 m) over a distance of about 1 mile (1.6 kilometer).

[9] The creek has a meager but year-round natural flow, and the water and resulting vegetation provide a habitat for animals, including skunks, opossums, raccoons, red-tailed hawks, red-shouldered hawks, great horned owls, coyotes, and the rare native San Francisco forktail damselfly, Ischnura gemina.

The terrane is a large packet of rock that extends diagonally from the Marin Headlands (just north of the Golden Gate), through the Twin Peaks and Glen Canyon area, and on to the southeast.

[15] The upper slopes and cliffs are of layered chert, which hardened into rock from the ooze of remains of countless radiolarian creatures that accumulated on top of the lava.

During subduction - 160 to 80 million years ago - and the subsequent uplift, the terrane was twisted, broken and disrupted, and the chert was deformed into the tight folds now visible in road cuts along O'Shaughnessy Boulevard (see photo).

On November 26, 1869, an explosion completely destroyed the entire facility, turning every one of the buildings on the site and the surrounding fencing, into "hundreds of pieces", according to a newspaper account.

As Jeanne Alexander has written,[19] Crocker installed a mini-amusement park with an aviary, a mini-zoo--bears, elephants and monkeys, a bowling alley, and, for extra thrills, hot-air balloon rides, and an intrepid tight-rope walker who performed on a wire stretched across the canyon.

Neighborhood children had to find other places to play.The Crocker era ended in 1922 when the City of San Francisco purchased the 101-acre (0.41 km2) Glen Canyon Park and Recreation site.

[26] For example, the plan envisions the removal of some large, mature trees (often eucalyptus, which was imported from Australia) to favor smaller native plants.

The plan also includes new restrictions on recreational use of the park, such as the closure of some trails and of some areas for rock climbing, and prohibitions against unleashed dogs.

In Glen Canyon Park, about 20 mature eucalyptus trees were removed in 2004 as part of an effort to increase the diversity of species living along Islais Creek.

Glen Canyon Park used to support populations of two rare species: the "vulnerable" San Francisco forktailed damselfly[27] and the "endangered" Mission blue butterfly.

Here the main issue appears to be re-establishing the native plant "silver bush lupine", whose leaves are the larval food of these butterflies.

This section presents an important opportunity for urban design because the properties along the north side have remained undeveloped since their acquisition by the City in the 1950s.

Photograph of a steep slope.
Spring in Glen Canyon Park. Field mustard in flower clings to the stony canyon walls; Islais Creek and its willow thickets lie at their base.
Photograph of a steep slope and the canyon bottom.
Glen Canyon Park in winter. The photograph shows three characteristic features of the park's landscape. The vividly green wild oat ( Avena fatua ) and slender wild oat ( Avena barbata ) grasses on the left cover the steep eastern slope of the canyon. The line of blue gum eucalyptus trees is a windbreak planted in the 1850s following Adolph Sutro 's purchase of this land, which he named his "Gum Tree Ranch". The willow thickets in the bottom and bottom-right of the photograph surround Islais Creek and a small wetland; the wetland is traversed by a boardwalk leading to the trail seen emerging from the thicket.
Photograph of a rock formation with a road and a car at its bottom.
Roadcut showing Franciscan chert rock in O'Shaughnessy Hollow. [ 11 ] The remarkable folding of the stacked layers indicates the tectonic forces that lifted up the coastal mountain ranges, and which warped the originally planar layers of this rock into the fantastic shapes they now present. The chert itself in this area is rich with fossils of radiolarian creatures. [ 12 ] The land above and around the roadcut has a " coyote brush scrub" community of plants that is typically found on thinner, drier soils in this region. [ 3 ] The road is O'Shaughnessy Boulevard, which was constructed around 1935.
Map of Glen Canyon Park showing the route proposed in 1958 for the Crosstown Freeway. A citizen movement stopped freeway construction before it had reached Glen Canyon Park.