Colstrip Strip Mine

February 21, 2001


Provided by: Rod Benson
Summary authors: Rod Benson

This photo shows a coal strip mine located near Colstrip, in southeastern Montana. Known to geologists as the Powder River Basin, this region contains thick seams of coal that formed as coastal swamps were buried by sediments millions of years ago. Strip mining can be used in this area because the coal seams are closer to the surface than seams located in the eastern United States where underground coal mines are more common.

The majority of coal mined in the U.S. is from seams varying in thickness from 3 to 10 feet, although the seams in the Powder River Basin of southeastern Montana and northeastern Wyoming average 40 feet in thickness. Mining of this region’s low-sulfur coal increased drastically in the early 1970’s when the Clean Air Act mandated that industries decrease emissions of sulfur dioxide, a gas that contributes to acid rain.

In the photo, a large truck sits atop a seam that is ready to be mined. The long pile of rock (spoils) to the left is material recently removed from the strip of coal. The spoils sit atop the seam that was mined before the strip shown in the photo was exposed. After the exposed strip of coal is removed, the area to the right will become the next strip. Its topsoil has already been taken away and stored. Next, the sedimentary rock (overburden) will be broken up using explosives and piled onto the strip where the truck sits. Eventually, the broken up rock will be smoothed out, the topsoil put replaced, and the area will be seeded with native plants.

Source: Montana Power Company’s “Coal Fact Sheet” 1990

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