Manifest dirt

Nineteenth-century settlers left a dusty mark on the West. Rocky Mountain lake deposits reveal that America’s westward expansion kicked huge amounts of dirt into the air—probably from livestock grazing.

A team led by Jason Neff, a biogeochemist at the University of Colorado in Boulder, examined soil cores from the beds of tiny mountain lakes in Colorado’s San Juan Mountains. The cores captured soil and dust deposited in the lake over 5,000 years.