On a bright, late-summer day in southwest Montana, Julio Betancourt gazes through binoculars across a dry slope rimmed by limestone cliffs. A cave-like divot in the rock catches his attention.

“I see middens,” he says, meaning packrat middens — the nests that the long-tailed nocturnal rodents construct with material from trees and other vegetation. Excited, he uses a technical term: “I can actually see the amberat.”

For Betancourt, a U.S. Geological Survey senior scientist who studies climate variability and ecological change, middens provide a window into the past. Packrats drink no water, but produce a viscous plant-derived urine, which they excrete on their nests. When this dries, it forms amberat — an asphalt-like crust that can preserve plant material for tens of thousands of years. A well-preserved midden is a snapshot of plant communities from as far back as the last ice age.

Over the past decades, Betancourt has chiseled samples from hundreds of middens, some more than 50,000 years old. Back then, glaciers were scouring Western mountain ranges, global temperatures were several degrees cooler, and familiar trees like ponderosa pine hadn’t yet spread across the West. By noting middens’ age and location, scientists can map how plant species migrated as the ice age transitioned, starting around 12,000 years ago, to the warmer, arid climate of the modern West.

Now, middens may help us understand how plant communities will respond to human-caused climate change. “Most places in the world lack the detailed historical knowledge that middens provide here,” says Betancourt. “We’re damn lucky” to have them.