New York,s Museum of Modern Art is currently exhibiting the master plan for Liberty State Park, a vast and varied waterfront recreation area facing the Statue of Liberty, Ellis Island and the Manhattan skyline.

It is a splendid plan.Moreover, it is another indication that Americans may be learing to appreciate and perhaps love their cities.

Only a few years ago, almost everybody solemnly pronounced our old industrial cities dead of heart failure. There seemed to be no cure for the ghettos and skid rows of cities like Baltimore, Pittsburgh and Philadelphia. g

The most respectable establishments on Scollay Square, in the heart of Boston , were tatoo parlors and nudie shows. The noble Quincy Market halls were deemed ripe for the wrecking ball.

Only the affluent could get out of the asphalt jungle. Parks and playgrounds, never abundant in cities that need them most, are still woefully overcrowded and worn out. A chance to frolic on a meadow, stroll in the woods, dally by a stream or play on a beach costs ever-longer hours of ever-more expensive travel.

Today, Scollay Square has been rebuuilt into an imposing event called Government Center, bustling with art displays, concerts, and other attractions. Quincy Market, a potent concoction of old architecture and new shops, earns $65 million in gross sales per year.

More importantly, Quincy Market's thousands of visitors each day exult in the kind of public happiness that seemed to have vanished from American life along with old-fashioned clam bakes and county fairs. The new amenities of downtown Baltimore, Pittsburgh and Philadelphia are not far behind. Fabulous skyscrapers are sprouting in bankrupt New York City. The Fulton Street Fish Market is coming back and a new Quincy-type marketplace if being built in the South Street Seaport area under the Brooklyn Bridge.

There is no conventional explanation for this renaissance. The jobless still don't have jobs or even new hope for finding work. Industry has not moved back to the cities. Economics have not changed.

What seems to be changing is people's attitudes. We seem to be learning that perhaps we can do more with our affluence and our lives than watch the tube.

There may even be a growing feeling, reinforced by the new minority militancy, that there is more to democracy than voting every so often (which only half of us do, anyway).

Liberty State Park seems to reflect the conviction that if we cannot bring all of the people to our great parks, we should start bringing great parks to the people. It seems to reflect a dawning recognition that cities should be fun.

The site of the park is a 600-acre disaster area on the western shore of New York Bay's inner harbor. It was one of those outhouses of the railroad age that most people chose to ignore: coal dumps, railroad tracks, oil tanks, garbage heaps and a slaughterhouse. During World War 1, some bright railroad moguls illegally stored two million pounds of munitions there. A German sabateur ignited them and the blast emanating from Black Tom Island shattered windows as far away as Philadelphia. A baby in Jersey City was thrown from his crib.

Since the demise of the railroads (the last of them, the Lehigh Valley freight operation, was shut down in 1977), the area became even worse than an industrial outhouse. It became an abandoned industrial outhouse, overgrown with weeds and litter.

The idea of turning this delinquent real estate into a glorious park was first proposed over 20 years ago. But it was not until we waxed patriotic for the 1976 Bicentennial that a beginning was made. New Jersey wanted access to the Statue of Liberty. The ferry landing was turned into a little 30-acre park. It became the most popular park in New Jersey and encouraged state authorities to approve the entire 600-acre, $160 million master plan.

The plan was designed by Robert Geddes, head of the architecture and urban planning school at Princeton, and principal in Geddes Brechedr Qualls Cunnningham, an architectural firm that shuns commas, hyphens and fashionable architectural stunts.

The Geddes plan combines our newly gained insights into ecology with environmental design, the creative re-use of historic buildings, landscape architecture and engineering. Its boldness approaches that of Frederick Law Olmsted's design for New York's Central Park more than a century ago.

The basic element of the design, structurally and esthetically, is what Geddes calls, Liberty Walk, a gracefully curved, 1-1/2-mile-long embankment promenade. It walls the land behind it and embraces the Bay with the Statue of Liberty and Ellis Island in front of it.

The 30-minute walk along this tree-lined promenade, with its views of the skyline, bridges, monuments and shipping, will be among the most exciting in the world. Liberty Walk will also be a place to fish and watch the fireworks and sound-and-light shows on Ellis Island.

The area contained by the crescent-shaped Liberty Walk consists of a large greenpark with gently rolling grasslands, woodlands, lagoons and two large wetland areas, which will remain as a wildlife preserve for waterfowl and various species of mammals living in the salt marshes.

There will be an amphitheater for festivals. The old Railroad Maritime Terminal, built with whimsical imagination in 1889, is being restored and will open next summer to music and dance performances, exhibitions and restaurants. Geddes hopes that the park will also include a farmer's market.

There will be bicycle paths and boardwalk vehicles, hiking trails and picinc areas, overlooks, safe overnight parking for recreational vehicles, an educational playground, such as a Sesame Street park, a lodge for overnight guests and conferences, and, close to Jersey City, tennis courts and sport fields.

The hope is that the entire park will be completed in 1986, in time for the 100th anniversary of the Statue of Liberty.

The estimated $160 million total cost is being shared by the State of New Jersey and various federal agencies. Congress has authorized much of the federal share and the Heritage Conservation and Recreation Service, the Economic Developement Administration, and even the Army Corps of Engineers are enthusiastically supporting the project.

The Fish and Wildlife people in the Interior Department, however, have so far withheld their approval because they worry that the proposed embankment might disturb the marine ecology that evolved in the littered swamps and backwaters left from the railroad and garbage dump days.

Liberty State Park will bring nature, art and beauty to the lives of the 16 million people in the New York-New Jersey urban region who will have easy access to it. It would be ironic, indeed, if this vast and magical environmental improvement were held up or sabotaged by a bureaucracy of zealous environmentalists.