With a shed roof that rises at a shallow angle from a brick base, and a curtain-wall facade festooned with eye-catching stained glass, the Lewis and Clark Branch Library in Moline Acres, Missouri, is one of the more striking midcentury-modern structures in the St. Louis area—and also one of the most endangered.

Commissioned as part of an ambitious midcentury construction program, the library building was designed by the architect Frederick Dunn and completed in 1963. Now, however, the St. Louis County Library system is planning to raze the 16,000-square-foot library, which it says is antiquated, and replace it with a 20,000-square-foot new building, to be paid for with $6.5 million in proceeds from a tax increase that voters approved in a 2012 referendum.

A demolition date has not been set, but the plan has drawn the ire of architects, preservationists, and other guardians of the St. Louis area’s rich midcentury-modern architectural heritage. The preservationist group Modern STL, which is leading a grass-roots and public-relations offensive to save the 51-year-old building, plans to present an alternative proposal to the St. Louis County Library board on Monday night for a 3,500-square-foot addition that would leave Dunn’s building largely untouched.

“Frederick Dunn is one of the unsung modernists of St. Louis who did a lot of really great buildings here, a number of which of course have been modified or demolished,” said Andrew Raimist, an architect and an authority on Harris Armstrong, another notable midcentury St. Louis architect. “But this is a public building that’s in great condition and there’s really no reason to take it down. There’s no logic, in my mind. It’s an open floor plan; there’s no structural problems. If they need more space, it’s on a big piece of property. I think it can be easily added onto.”

Lindsey Derrington, an architectural preservationist and an STL Modern board member, credits Stewart W. Smith, a former jazz musician who was named the director of the St. Louis County Library system in 1951, with recognizing an opportunity “to build new libraries for a new America” that emerged in the boom years after World War II. Under Smith’s leadership, the library system embarked on an expansive building program, completing two branches in 1958 and a new headquarters branch that opened in 1960. For the Lewis and Clark branch, the final building to be constructed as part of this effort, the library system turned to Frederick Dunn & Associates.

A native of St. Paul, Minnesota, Dunn studied architecture at Yale University before moving with his mentor, Charles Nagel Jr., to Nagel’s native St. Louis. The pair founded their own firm and fell in with a group of young creative types, including a pre-Cranbrook Charles Eames. Dunn returned to St. Louis after wartime service in the Navy and established his own firm, which designed modernist gems across the St. Louis area, most notably the Steinberg Skating Rink in Forest Park.

“The vision and the foresight and the will to engage an architect like Dunn for a project like this—he was very well established in St. Louis; he’s one of the major architectural forces to have worked here in this period—does speak to an understanding on the part of the library director that an architect can put in place a vision that can do a great deal to help the vision of the library,” said Mary Reid Brunstrom, a St. Louis-based architectural historian.

The Lewis and Clark branch, named for its proximity to the starting point of the Corps of Discovery Expedition, consists of a main reading room with stacks, a 250-seat auditorium and a pair of meeting rooms. The simple design calls attention to the building’s most arresting feature, the stained-glass panes designed by Robert Harmon of Emil Frei & Associates. Concentrated at the children’s reading corner at the northern end of the library, the windows imbue Dunn’s rigidly symmetrical building with what Derrington calls “a delicate asymmetry.” The panes feature Meriwether Lewis, William Clark, and their interpreter and guide Sacagawea, as well as abstract patterns and silhouetted animals.

“The most eye-catching thing about the building is the stained glass,” Raimist says. “If it didn’t have the stained-glass windows, it wouldn’t be such a remarkable building. The way the stained-glass design is worked into the fenestration as it wraps around the building, it emphasizes the children’s reading corner, and it also tells the story of movement and exploration, which is metaphorical of the Lewis and Clark Corps of Discovery. So to me, it seems that when the library commissioned Dunn to do this, they obviously put a lot of thought into it. They spent extra money to have these beautiful, unique glass pieces put in, and I think the design of the whole thing is a real work of art.”

A library spokeswoman, Jennifer McBride, said that library officials recognize the building’s architectural significance, but she added that the library system is making good on a promise to the residents of north St. Louis County to replace the aging Lewis and Clark branch with a new building with more computers and larger meeting rooms.

“We’re sensitive to that discussion,” she said of the preservation campaign, “but again we are following through on our promise to the community. We went to the voters and told them that if they approved the tax increase, we were going to give them a new building. So we’re moving forward with that plan.”

McBride noted that the library intends to preserve Harmon’s stained-glass windows, which she said “will feature prominently in the new building.” But Derrington and other preservationists noted that the library has not said how or where the glass will be displayed, and that removing them from the window frames relegates them to relic status.

Asked to respond to the criticism that a part of St. Louis’ architectural heritage will be lost if the Lewis & Clark branch is razed, McBride said: “A lot of the preservationists who have been making that argument don’t live in that community. They don’t use the branch on a regular basis. We have gone to the Moline Acres community a number of times to seek input, and we’re doing what they’ve asked us to do.”

Derrington, Raimist, and Brunstrom acknowledged that they do not live near the library, but noted that Carole Wuesthoff, who has spearheaded a petition drive to save the building, lives in neighboring Jennings, Missouri, and uses the Lewis and Clark branch.

“There are a lot of old buildings in St. Louis, and every time an old building is demolished, people say it should be saved,” Raimist said. “There’s no question there is not enough money to save the hundreds of buildings that are demolished every year, so I think you have to kind of pick and choose what buildings are really worth investing in, and to me this one is absolutely worth it.”

Like Raimist, Brunstrom acknowledged that the fate of the Lewis and Clark library appears to be all but sealed. But she said it was worth it to keep up the fight, if only to change a lingering perception that midcentury structures are not old or significant enough to warrant preservation. “Over time we’ll get there,” she said, “but we’re going to lose buildings in the meantime. We are losing them.”

Click through he slideshow to view images of the library, preliminary renderings for a new library building, and Modern STL’s proposal for an addition to the existing building.