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. The chemical makeup of the cores was nothing like the surrounding bedrock, suggesting that the dirt came from hundreds of kilometers away, Neff says. Winter winds are known to blow dust from California and Nevada deserts to Colorado.

Carbon and lead dating revealed that, after the 1800s, the average annual amount of dust deposited was 500 percent more than before that time, the team reports in the March Nature Geoscience.

The spike appears around the time ranching boomed across the West, as cattle and sheep munched on erosion-protecting plants. The Taylor Grazing Act of 1934 limited ranching, and Neff's team noted a subsequent drop in dust levels.

The dust hasn't settled. The past 100 years saw a jump in deposits of chemical fertilizer, which might disturb the delicate alpine ecosystem, Neff says.

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