These days, as the National Park Service celebrates its 100th birthday on Aug. 25, taking and sharing photographs of America’s national parks is nothing special. In 1861, when Carleton Watkins created this portfolio of 30 images of Yosemite Valley, the situation was very different.

Creating the images required lugging thousands of pounds of equipment via mule to the remotest, and often steepest, California wilderness.

But it was worth the schlep.

“As specimens of the photographic art they are unequaled, and reflect great credit upon the producer, Mr. Watkins,” the New York Times wrote in December of 1862, when the images were on view at a New York City gallery. “The views of lofty mountains, of gigantic trees, of falls of water which seem to descend from heights in the heavens and break into mists before they reach the ground, are indescribably unique and beautiful. Nothing in the way of landscape can be more impressive or picturesque.” (The day’s other news? The Civil War.)

And the pictures did more than get a good review. As argued by Weston Naef, curator emeritus for the Department of Photographs at the J. Paul Getty Museum in Los Angeles and an expert on Watkins, they were the spark that started the national parks movement in the United States, the inspiration for Abraham Lincoln’s signing of the Yosemite Grant Act in 1864, which led the way to the preservation of Yellowstone, which in turn led to the creation of enough federally conserved areas that eventually a National Park Service was necessary.

“Knowing what we know about Abraham Lincoln’s method of thinking, there is no way that he could have declared Yosemite Valley a place to be preserved unless he saw something,” Naef says. “So how would he have seen the something and what would the something have been?”

Read more about the answer here.