THE BLUE RIDGE Parkway's the best boon our government ever doggled. Conceived as a makework project during the Great Depression, the highway could have become a 469-mile scar on our eastern mountain majesty; instead it became one of the most beautiful and least damaging auto routes in the world.

Connecting the Shenandoah and Great Smoky Mountains national parks, the parkway is one grand and unified earth sculpture, combining the artful sciences of architecture, construction, landscaping and social engineering. It was also far ahead of its time in conserving a fragile ecosystem that otherwise surely would have been ravaged by development.

Few of the millions of motorists who annually enjoy the parkway are aware of the study and care that went into every inch of the roadway, every foot of the adjacent land and every mile of the magnificent vistas that continually reveal themselves. And it all was accomplished not in spite of but because of government bureacrats.

How this miracle came to pass is examined in an exhibit at the National Building Museum. Curator David P. Hill has resurrected original conceptual sketches, design drawings and construction blueprints, along with photographs of the mountaineers whose displacement was the single sad aspect of the project. While it gave many of them a chance to make more money than they'd ever dreamed of, the parkway put an end to an isolated, self-sufficient way of life that had endured for many generations.

Appalachia, not the West, was America's last frontier. The remote Blue Ridge was so poorly mapped when the project was launched in 1933 that Stanley Abbott, the parkway's pioneer designer, had to mark his route proposals on photographs taken on the spot.

Each tree, each rock outcropping, each line of sight was considered and reconsidered. Bridges, rest stops, lodges and other structures were designed to minimize intrusion and echo regional architecture. Farmlands were leased back to the original owners so that traditional uses could continue while assuring the integrity of the landscape. Creative new concepts such as scenic easements were widely used.

Local labor was used and landmarks such as Mabry Mill were preserved. The history of the land and people was recorded, so that visitors would know that this hollow had been held by the hardy Caudill family, and that cove by the Cades. That Negro Mountain is so named because as early as the American Revolution its summit cave was a refuge and hideout for runaway slaves, one of the original way stations on what became the Underground Railroad.

"Paint your parkway with broad strokes," Abbott told the young designers. They responded by creating overlook scenes that are compositions consciously modeled on the Hudson River School.

Construction took 52 years, ending in 1987 with the opening of a self-effacing 1,243-foot viaduct, an engineering marvel that skirts instead of scarifies the face of North Carolina's Grandfather Mountain.

We don't build 'em like this any more, and we should ask ourselves why.

THE BLUE RIDGE PARKWAY -- Through May 3 at the National Building Museum, F Street between Fourth and Fifth streets NW. Open 10 to 4 Monday through Saturday and noon to 4 Sundays and holidays. Metro: Judiciary Square. Good wheelchair access.