Over Labor Day weekend, while most people were cracking beers and barbecuing with family, Teresa Gergen, 55, was clinging to a jagged granite face 13,553 feet above sea level, crossing off the peak that would make her the first person in history to summit all 846 thirteeners in the lower 48 and Hawaii.

Gergen, who’s from Denver, completed most of the thirteeners on the list by 2008—except for 134 in California and Wyoming. So when she retired from her job as a library technician in 2013, she turned her focus to summiting the remaining ones, spending summers living out of her car and climbing as much as possible.

“I’d go in on a backpacking trip to climb peaks for anywhere from two to twelve days, come out and take a rest day or two in Bishop [in California], then go back in and do it again from another trailhead, all summer long, for as long as I could manage,” she says.

Though her achievements are many (like being the first person to summit all 2,311 Colorado peaks above 10,000 feet and climbing each state’s high point), Gergen’s mountaineering career didn’t begin until her mid-thirties, when a coworker at the University of Colorado casually suggested she climb a fourteener. “He had to explain what that meant,” she says. She bought a guidebook and set out but accidentally hiked up a nearby thirteener instead.

“I decided I had no business dreaming about climbing peaks, nor did I have the training to do so, and wrote them off,” says Gergen. Two weeks later, though, she was back at it and hasn’t stopped since. She completed the 53 Colorado fourteeners in three years, at the age of 37, and immediately set out to check off the next list: Colorado’s 584 thirteeners.

At the start of Labor Day weekend this year, Gergen only had two peaks left on her enormous thirteener list: Norman Clyde and Palisade Crest, both in California. ​​She began on August 26 by soloing a cross-country route out off South Lake Trailhead, near Bishop. She left the trail not long after Bishop Pass and cut across the barren, high-altitude plateau of Dusy Basin, scrambling up and over three seldom visited high passes: Knapsack, Potluck, and Cirque. The two-day traverse required careful route finding to avoid icy slopes (a result of the heavy snow year) and cliffing out.

After camping one night under the toothy ridge leading to the peak of Norman Clyde, Gergen was relieved to learn via an InReach message that the inconsistent weather forecast had evened out.

Gergen laced up her shoes and picked her way through Class 3 and minor Class 4 terrain until she was, at last, standing on top of 13,854-foot Norman Clyde Peak. “To my relief, the climbing ended up being easier than any I’d done all summer, and I summited and descended without issue,” says Gergen.

Last on her list was Palisade Crest, a series of 12 stone pinnacles named after characters in J. R. R. Tolkien’s Lord of the Rings series, the highest of which is 13,553-foot Gandalf Peak. Many routes on the ridge get technical, so she partnered up with fellow mountaineers John and Alyson Kirk.

“I knew that, with good weather and John’s leadership, my final thirteener, Palisade Crest, was just a matter of putting in the work—and being careful, since it’s a very intricate route across a lot of exposed technical rock,” she says.

On the morning of August 30, the Kirks and Gergen began their ascent of Gandalf Peak. After seven hours of climbing on high-elevation granite, the three topped out around 1 P.M. The Kirks presented Gergen with a custom banner commemorating her achievement, snapped a few photographs, and started their descent.

“I felt happy and relieved and grateful on the summit,” says Gergen. “But the full force of the accomplishment didn’t really hit until the long backpack out over the next couple of days.”

After two decades of obsessive climbing and checking off peaks from lists, Gergen says that this August’s triumph felt bittersweet: “Unlike my past completions, it’s not clear to me yet that I’ll just march right on to the next list. I will have to think hard about whether I still have what it takes to both climb hard and climb safe.”