Upper Fremont Glacier, high on the north slope of Fremont Peak in Wyoming’s Wind River Range, is one of the highest glaciers in the American Rockies and part of the largest glacier grouping in the U.S. Rocky Mountains. It has become especially important for climate research. Ice cores recovered in 1990, 1991, and 1998 preserved evidence of long-term shifts in climate and atmospheric chemistry, including a change linked to the end of the Little Ice Age and findings that paralleled results from Peru. The glacier’s layers also recorded nuclear testing fallout and mercury pollution, including declines after cleaner air regulations in the late 20th century.