The relentlessness of Maya Lin is something to be in awe of. The 52-year-old architect, artist and sculptor held an audience of about 1,200 Pittsburghers enthralled at Carnegie Music Hall the other night as she spent nearly an hour and a quarter describing the arc of her career, from young architect to mid-career artist and environmentalist. She moved briskly through 206 slides and three brief videos and spent the last 20 minutes of her talk giving a global perspective on environmental issues like the extinction of species. She exhorted her audience to greater environmental awareness. And, with it all, she related everything to her art. Lin's talk marked the opening this past week of a small but fascinating show of her sculpture at the Heinz Architectural Center at the Carnegie Museum of Art that will run through mid-May. Maya Lin is most famous, of course, for her 1981 design for the Vietnam Veterans Memorial at the Mall in Washington, DC. She won a major national design competition for the memorial (there were 1,441 other entries) while still an undergraduate at Yale. The memorial was dedicated 30 years ago this coming November. It says a lot about the memorial's enduring emotional resonance that -- as you will see if you go there -- it can still make visitors cry. But the exhibition at the Carnegie is not about the Vietnam memorial or other moving memorials she has done since. Instead, this is a straightforward presentation that wants to lead us to meditate about rivers, seas, lakes, land forms and other elements in the natural environment. What we see here are her persistent efforts to find sculptural forms that will get us to care more about the world around us. All of her recent work, in fact, seems to be an outpouring of her concerns for the environment. One work was done especially for the Pittsburgh exhibit. It's called "Pin River -- Ohio (Allegheny & Monongahela)." A 15-foot-long wall sculpture made entirely of long, steel straight pins (and their shadows), it plots the whole meandering irregularity of the Ohio River system, from the beginnings of the Allegheny and the Mon all the way to Cairo, Ill. The pins vary in height, and they cluster at times to mark the effects of dams. It's important to view our rivers as a total system, she noted in her talk, "because what we don't see, we tend to pollute." She has done similar "Pin River" representations of other rivers for exhibitions in other towns. Two of her most striking pieces here are wall-size sculptures of the paths of the Hudson and Colorado rivers made by casting molten recycled silver. Contemplating the complexity of these river systems is worthwhile. You readily see Lake Mead and Lake Powell in the Colorado piece, and "Silver River -- Hudson" shows New York harbor, leaving blank spaces as the silver flows around the places where Manhattan, Staten Island and Long Island are supposed to be. The river sculptures are essentially two dimensional. But three-dimensional representations of the Caspian Sea and the Red Sea might also please. She uses layers of stacked and shaped birch plywood to follow the exact underwater contours of the two seas -- though she exaggerates the vertical dimension. All this gives us an idea of the volume of these seas. Again, she wants us to contemplate the unseen in nature. Maya Lin has, over the years, always seemed to operate on the boundary between architecture and landscape, and she's had a great impact on the way design professionals think about architecture and landscape today. The Vietnam Veterans Memorial is a good example of that as you have the experience there of descending into a cut into the earth. "I didn't insert this memorial into the earth," she told her listeners at the Carnegie. Instead, she made a memorial "by cutting the earth and polishing it." She has gradually moved away from architecture toward art and sculpture, and has decided against maintaining the kind of large studio that architecture requires. Architecture, she observed, is like writing a novel; art is more like poetry. She didn't leave much doubt about which she prefers. And certainly the artwork gives her more of a chance to focus consistently -- and insistently -- on the environmental issues that she believes are so important today. A significant piece of current work, called the Confluence Project, involves landscape designs and art installations that will emphasize both the historical and environmental significance of seven sites along a 300-mile stretch of the Columbia River basin in the states of Washington and Oregon. These sites mark points of contact between the Lewis and Clark expedition and American Indians. Another major current concern is a sculpture and video project to be installed at multiple sites called "What is Missing?" It's about disappearing species and endangered habitats. Excerpts from the video are playing in the art museum's main lobby, near the gift shop. Lin perhaps summed up her work best herself when she quoted a prayer attributed to the Chinook Indians of the Northwest: "We call upon the Earth to teach us and show us the way."