WHAT: “Lewis and Clark Revisited: A Trail in Modern Day,” a traveling exhibition now on display at Penn Museum. The exhibit includes 60 black and white photographs taken by Greg Mac Gregor as he retraced Meriwether Lewis and Williams Clark’s historic 1804 voyage across the continent.

ORIGINS: In the early 1990s, as the National Park Service prepared to open a new museum about the exploration of the American West, Mac Gregor—who had previously photographed the Oregon Trail—was contracted to photograph some of the sites Lewis & Clark passed during their approximately 4,000 mile journey. But after completing the job, Mac Gregor knew there was much more to see—and shoot. So he set out to document the entire Lewis & Clark trail, following in the explorers’ footsteps.

CHANGES: That was the plan, at least. “I didn’t realize how difficult it would be [to follow their course],” Mac Gregor explains. “I didn’t realize how much the Missouri River wandered back and forth across the original course. But today in Missouri they’ve diked it all the way across. So the river you’re looking at today isn’t nearly the same river they saw. This awareness eventually led to me change the way I was going to photograph it. I was kind of forced into it.”

METHOD: Lewis & Clark’s original path stretched from Saint Charles, Missouri to the Pacific Ocean, and Mac Gregor knew he couldn’t possibly hike or drive the entire trail. Instead, he flew to different locations along the path, rented a car, then drove up and down the trail, 200 miles in each direction—essentially covering it multiple times—all the while keeping an eye out for worthy sites. In total, he would travel 16,000 miles. The project took six years to complete.

A WELL-WORN PATH: Mac Gregor’s photographs capture some natural beauty, but plenty of industry and commerce, as well. Such is the state of the modern West. “Wherever Western rivers travel, that’s where the action is,” Mac Gregor says. “Besides, they’re running through arid country. So along the river and trail, you have big cities, little cities, cattle drinking from the river, fishing competitions, boats. That was a surprise. On Lewis & Clark’s path today, there’s a lot of commerce, a lot of dams. But I wanted the project to talk about contemporary America anyway.”

PHOTOGRAPHER’S FAVORITE?: Mac Gregor struggled to pick just one image to name his favorite, but eventually settled on his shot of Montana’s Eye of the Needle monument—or, more accurately, what is left of it. The naturally created “arch” overlooking the Missouri River, once a beloved landmark, was destroyed by vandals while Mac Gregor was working on his project. His photo shows what’s left.

DETAILS: The exhibit runs through Feb. 10 at Penn Museum, 3260 South Street. For more information, visit www.museum.upenn.edu or call 215-898-4000.