Belt Rocks of Montana

April 04, 2006

Slabs

Provided by: Rod Benson, Helena High School
Summary authors & editors: Rod Benson

Outcroppings of the Belt Supergroup can be seen throughout much of western Montana, but one of the most interesting exposures, shown above, is located between Philipsburg and Anaconda. Although sedimentary rocks are formed in horizontal layers, these layers of Belt rocks were disturbed as colliding plates formed the northern Rocky Mountains, roughly 80 million years ago. The mountains of Glacier National Park formed when a large portion of Belt rocks were thrust up and over younger sedimentary rock.

Recent radiometric measurements indicate that the sediment was deposited from 1.470 billion years ago to 1.400 billion years ago. Over 15,000 ft (4,572 m) of sediment was deposited in basins in what is now western Montana. Layers of beach sands, clear water limestones, shallow water sands, fine sand flats, and mudstones indicate that a variety of environments existed here during the Precambrian Era.

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