The greening of Pennsylvania Avenue continues, even as early snows arrive.

Landscape architects are putting final touches on the design for John Marshall Park across the boulevard from the National Gallery of Art. Construction will begin next month.

Staff people in the offices of the Pennsylvania Avenue Development Corporation (PADC) also are refining concepts that will govern the design of Market Square, the large plaza across from the National Archives, and Indiana Plaza, an informal park nearby. Construction on those projects will begin in early 1983.

At 14th Street and the avenue the PADC continues to fiddle with Robert Venturi's Western Plaza, the platform-map of the District's monumental core. Some lively improvements may be made there next year.

And then, even as he packs up his office in preparation for his move to San Francisco (where he will cross the bridge into private enterprise), PADC executive director Andrew Barnes still finds himself making little changes in the strings of lights -- he calls them "Tivoli lights" -- adorning the honey locust trees clustered in Pershing Park at 15th Street and the avenue.

The point is to make Pershing a welcoming space even in wintertime, and at night. To that end the rectangular pond in the center of the park has been converted into an ice-skating rink. For the next few months, skaters will be welcome from lunchtime through early evening during the work week and from morning to evening on weekends, with special "moonlight" sessions on Fridays, Saturdays and holidays.

Admittedly it still is a matter of years before Pennsylvania Avenue becomes the magnet for people "day and night and emphasizing night" foreseen by Max Berry, chairman of the board, and others associated with the PADC, but things are moving rapidly. Next year, including new sidewalks, trees, street furnishings and the parks, the corporation will commit more than $11 million to the construction of street improvements.

The parks are a crucial part of this vision of humanizing the great boulevard. The idea of scattering five of them along the avenue, each quite different, was developed when the first of the modern plans for the ceremonial street was scuttled in the early 1970s. Symmetry and monumentality were the watchwords of the original scheme, and accordingly its parks were grandiose. The vista from the Capitol to the Treasury Building was to terminate in a great National Square that would have destroyed the Willard and Washington hotels, among many other buildings.

Pershing Park and the Western Plaza have been built. Conceived as a tandem of opposites -- the flat open platform of Western Plaza playing to Pershing's multi-leveled textures -- they speak directly to ordinary human experience in the more or less ordinary language of the street. More intimate in scale and not so centrally located, Marshall Park promises to be an inviting oasis for visitors to the National Gallery and for workers in the complex of courts and District office buildings to the east and north.

Each of these spaces was formed by different designers for different sites with different needs. Despite this, there is a clear family resemblance between the spaces: Each is rather formal, linear and rectangular in plan. When asked about this, Ron Eichner, the PADC project manager for Marshall Park, responded, "Oh, yes, the architects tried all sorts of curves and circles, but in the end, somehow, they didn't seem to fit."

Thus does the long shadow of Charles Pierre L'Enfant, the city's first designer, continue to fall upon all who try to fit spaces into his grand plan. This is not necessarily a good thing; misunderstood, it often leads to outrageous pomposity -- to wit, the absurd scale of the National Square. But, so far, in the case of the Pennsylvania Avenue parks, it has tempered egocentric creativity with a certain crisp discipline (which doubtless also reflects the influence of Sasaki Associates, general landscape consultants to the PADC).

Marshall Park, designed by Carol R. Johnson & Associates of Cambridge, Mass., and situated between the U.S. Court Building and the new Canadian Embassy site, will consist of a series of three rectilinear terraces stepping gradually up the gentle slope from Pennsylvania Avenue to C Street NW. It will take the place of the existing north-south street, John Marshall Place.

The sequence is beautifully modulated. Each space will be quite distinct -- two formally planted granite-paved platforms surrounding a grassy mini-mall -- and yet the flow from one to the other is effortlessly handled. Each space in its way will tempt pedestrians to dally for minutes or hours. Careful attention has been paid to seating, with a variety of benches, chairs and sittable stairs, although the designers do play it cute with their idea of putting a couple of life-size bronze chess players on the northern terrace.

Basically, though, the design is an ingenious intermingling of Washington formality and English-garden informality. The city's famous sightlines are rigidly respected -- its architectural centerpiece is the engagingly circumspect old City Hall building two blocks to the north -- and yet the density of plantings on the edges of the park will, in time, act as an alluring foil.

At the other end of the avenue the designers of Pershing Park -- Jerome Lindsey of Washington and M. Paul Friedberg and Partners of New York -- defuse the potential stiffness of their park with a multi-leveled pinwheel plan that spins around the rectangular pool/ice-skating rink. With its sequence of sloping green areas, cascading stairs and open terraces, Pershing Park is in fact a state-of-the-art demonstration of the lessons landscape architects have gradually been relearning about urban parks.

To be sure there is a certain gracelessness in the design, especially apparent in the two buildings on the site: an oddly off-putting steel-and-plastic-glass cupola structure where food is served and a lumpy stone "building" that does double duty as a summertime fountain and open-sesame wintertime house for the ice-cleaning machine. To compare these buildings to Frederick Law Olmsted's wonderful spring grotto on the Capitol grounds is to recognize just how far the skill has gone. Until it returns, landscape architects would be well advised to collaborate with architect architects to do the buildings in their parks.

Pershing Park is not, visually, an elating space, but in the end this will hardly matter. It will become one of the more active and interesting outdoor spaces in the city because it was superbly designed to attract and to accommodate people in large numbers. Although quite different in most respects from Rockefeller Plaza in New York, it promises to become our Rockefeller Plaza. As William S. White has pointed out, the success of that famous urban space is the result of a combination of events: The show goes on at ground level where people are eating or skating, and the show attracts and entertains thousands of others. With its clever succession of tiers, each with a good view of the show, Pershing Park will succeed in much the same way.

In any case, the revised Pennsylvania Avenue plan assigned the grand visual role to Western Plaza, next door to Pershing Park. The sad thing here is that although architect Robert Venturi brilliantly responded to the challenge the PADC board stripped his design of two essential elements. Of his three-level plan -- a map of the Avenue and its surround engraved into the plaza platform, models of the Capitol and the White House at the appropriate places, and two high pylons to frame the vista and to fix our sense of the scale of the great space -- only the base got built.

Working with the architect, the PADC staff continues to plug away at the resulting problem. Recently, it prepared for the board a scheme to build the model structures on the site and to substitute flagpoles, at least temporarily, for the pylons. The flagpole idea needs rethinking, but the buildings will do wonders for the park. If the PADC board can muster the courage to go forward with Venturi's original scheme, or a close echo of it, it will be doing itself and us a great favor.

Even as it is, we are being given five tremendous gifts in these parks along the avenue. The whole scheme, including not only design but creative ideas about uses and activities as well, is a model of sophisticated, enlightened, realistic urban planning. The parks will do a lot to temper the massive mediocrity of much of the new architecture along the avenue, and they can only get better as the years go by.