Precambrian Greenstone

Precambrian Ely Greenstone, Lake Michigan's Left Coast, lakemichigansleftcoast.com, Elizabeth G Fagan

Precambrian Greenstone

The Canadian Shield constitutes the largest mass of exposed Precambrian rock on the face of the Earth. Extending into Minnesota and Wisconsin, the Canadian Shield is a collage of rock categories. One type teal-hued Ely Greenstone, named for its source in northeastern Minnesota. It is one of the oldest rock types on Earth.

Ely Greenstone dates from the Precambrian Age, Archean Eon, Neoarchean Era, some 2.722 billion years ago. It is a type of of Banded Iron Formation (BIF), formed when metamorphosed basalt oozed onto the ocean floor into bulbous forms or “pillows.”

Precambrian Ely Greenstone, Lake Michigan's Left Coast, lakemichigansleftcoast.com, Elizabeth G Fagan
Precambrian Ely Greenstone, Lake Michigan’s Left Coast, lakemichigansleftcoast.com, Elizabeth G Fagan

Iron-rich Ely Greenstone has BIF’s characteristic “banding,” in most cases rusty red or pearly white. It is fine-grained and heavy. The slab shown here weighs 19.8 pounds (8.981 kg). Its origin as “pillow lava” gives this piece an odd, serpentine shape.

Great Lakes rock hounds are particularly grateful for the glaciers that dumped myriad rocks and fossils in their wake. On the beach at Lake Michigan’s west coast, the gorgeous deep teal of wet Greenstone stands out among the millions of plain limestone rocks.

We can’t wrap our minds around the deep time 2.722 billion years represents. Holding in your hands something so old is a transcendent, mystical experience. It reminds us that humans are a only a smudge on Earth’s timeline.


About the Author

Elizabeth G Fagan is an artist who resides on Lake Michigan’s shoreline in southeastern Wisconsin—a place she calls Lake Michigan’s Left Coast. She was a professional writer based in Chicago for much of a professional career that spanned more than 25 years.

Rosie Fagan, Montrose Dog Beach, Chicago
Rosie Fagan, Montrose Dog Beach, Chicago

The 2012 superstorm Hurricane Sandy is remembered for devastating New York City. Sandy was so massive she caused 25-foot waves on Lake Michigan in Chicago, some 900 miles to the west. Millions of stones washed up onto Montrose Dog Beach, the place where Fagan regularly walked her German Shepherd Rosie. One of Fagan’s early interests was revitalized as she collected rocks and fossils on that beach.

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