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Cambrian Rock Exposures

Posted Mar 19 2013 12:00am
Unlike strata from the other Paleozoic Periods, Cambrian rock exposures are relatively rare throughout the United States.  Deposited from 542 to 488 million years ago (MYA), when the variety of shelled and soft-bodied marine life was exploding on Planet Earth, Cambrian sediments lie deep beneath most regions of our country and are exposed primarily in association with ancient Precambrian basement rocks.

Where Precambrian domes or ridges have pushed up through overlying Paleozoic, Mesozoic and Tertiary strata, Cambrian sediments, usually metamorphosed by heat and pressure, may be found along the primary uplift.  Examples include Cambrian strata ringing the Black Hills, St. Francois Mountains (southeast Missouri) and Adirondack Mountains and seams of Cambrian deposits along the Blue Ridge of the Carolinas and northern Georgia; scattered Cambrian exposures are also found in the Rockies and throughout the Great Basin where fault blocks ranges brought deep basement rocks to the surface (e.g. in the Inyo and Nopah Ranges of southeastern California).  As one might expect, Cambrian rocks lie relatively close to the surface along the outer edge of some structural geologic basins; though often covered by younger sediments, they have been exposed by glacial erosion in Upper Michigan and eastern Wisconsin (along the northwest rim of the Michigan Basin).  Finally, Cambrian sediments are exposed deep in the Grand Canyon, lying just above the ancient Vishnu schist at the bottom of the chasm.

While Cambrian rock exposures are rather limited throughout most of North America, our Continent does harbor Earth's most famous Cambrian time capsule.  In the Canadian Rockies, west of Calgary, the Burgess Shale, deposited some 505 MYA, is renowned for its cargo of early marine fossils, including trilobites, brachiopods and soft-bodied organisms.
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