Can you picture San Francisco, that bastion of urbanity, as a naturalist’s paradise?

Well, try again.

Bird watchers, beachcombers, hikers, cyclists, surfers, surf fishers, picnickers and nature lovers flock to both sides of the Golden Gate and the coastal range to the City’s south.

This phenomenon is the result of a stunning environmental breakthrough.

Congress, in October, 1972 enacted legislation creating the Golden Gate National Recreation Area (GGNRA) and mandated the National Park Service to develop and administer it. Today, the GGNRA is the largest urban park in the world and the most popular in the national system. In 2007, more than 14 million visitors explored its 114 square miles.

It’s hard to conceive of a preserve of this scale in the fourth most populous metropolitan complex in the country. Much of it derives from the U.S. Army’s determination to make the San Francisco isthmus impregnable. The GGNRA encompasses ten obsolete military installations.

If you’re familiar with San Francisco and its surrounds, you can look at it this way.

Imagine yourself atop Telegraph Hill facing the Pacific. Down to your right are the islands of Angel and Alcatraz. Ahead the Golden Gate Bridge stretches toward the chaparral-covered headlands of Marin County. To the left is a metropolitan greenbelt realtors had been rubbing their hands over for decades. It runs along the San Francisco shoreline from Aquatic Park to Land’s End, taking in the Maritime Museum, Fort Mason, the Marina Green, the Presidio, Baker Beach, a piece of Lincoln Park and Fort Miley.

Around the corner to the south are the Cliff House, Ocean Beach’s 8.5 miles of pounding surf, and the sandstone cliffs and dunes of Fort Funston. The park’s San Francisco segment totals 4,800 acres.

Directly across the Golden Gate are the rolling hills, valleys and inlets of Fort Baker, Barry and Cronkhite.

All of this is part of the Golden Gate Recreation Area. The rest is out of sight.

The Marin County portion of the park reaches north for 20 miles. In a swath the size of a junior sheikdom, you’ll find sandy beaches, lagoons, marshes, redwoods, ranchlands, rugged promontories, streams, fortifications, wildlife sanctuaries, picnic facilities, 100 miles of trails and five camping sites. (Phone 415-331-1540 for information.)

Rodeo, Tennessee, Muir and Stinson Beaches, Muir Woods National Monument and Mount Tamalpais are part of this protectorate. Counting the adjoining Point Reyes National Seashore, also under the NPS pennant, 100,000 Marin acres have been turned over for all time to people, flora and fauna.

On October 1, 1994, administration of the historic Presidio military post was officially transferred from the Sixth U.S. Army to the GGNRA, creating the nation’s newest national park. A military post in continuous use since 1776 -- by the Spanish and Mexicans followed by the U.S. Army -- the 1,400-acre waterfront site was the only one of 86 U.S. military installations on the 1988 base closure list to become a national park.

Future plans call for further developing recreational space throughout the Presidio, including 11 miles of hiking trails and 14 miles of bike routes; rehabilitation of historic buildings for new park purposes; conversion of some of its 800 buildings into an international center for research and education on environmental and cultural issues; and replanting and restoration of the historic forest and 285 acres of native plants. The new park will retain its military tradition as the Sixth U.S. Army continue will retain its headquarters here.

Sweeney Ridge, site of the discovery of San Francisco Bay by the land expedition led by Gaspar de Portola in 1769, became part of the GGNRA in 1980. This 1,047-acre parcel is in San Mateo County.

As the chains have been dropped on hitherto restricted roads, dazzling vistas have opened on one of the most spectacular coastal configurations in the world.

The first link in the park chain was Alcatraz. Phased out as a federal penitentiary in 1963, the once-ominous Rock attracts more than 1,354,714 visitors annually on sea-land excursions from Pier 33. (Phone 415-981-ROCK (7625) or www.alcatrazcruises.com for information on how to get there.)

Alcatraz’s bigger neighbor, Angel Island, is a state park. This former army post and immigration station offers 740 acres of trails, coves, bike paths and picnic sites. Launches serve it from Fisherman’s Wharf and from Tiburon on the Marin shore.

City side, the NPS has created Golden Gate Promenade between the Presidio’s Fort Point beneath the Golden Gate Bridge, a pre-Civil War fortress with National Historic Site status (open from 10 to 5, Wednesday through Sunday), and Aquatic Park. The footpath follows the bay for 3.5 miles by way of Crissy beach, the Marina Green and yacht harbor and Fort Mason’s cypress-lined bluffs to Hyde Street Pier with its flotilla of historic coastal vessels.

Throughout their diversified domain rangers are conducting nature walks, historical tours, environmental field trips and school group programs. Maps and information on park facilities, including hostels in San Francisco and Marin, and public transportation may be obtained from GGNRA Headquarters, Fort Mason, San Francisco, CA 94123, 415-561-4700 or www.nps.gov/goga.

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