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Generalized Geologic Map of the Conterminous United States
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  The North American Tapestry of Time and Terrain

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Legend and Rock Ages
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  Legend and Rock Ages
 

Precambrian Rocks - 4 billion to 540 million years ago

map of Precambrian rocks

 

The Precambrian is actually a segment of time that includes two eons, the Proterozoic and the Archaean that span billions of years. The oldest rocks on earth formed during this time, as well as the first continents, and the earliest, simplest forms of life.

Many of the oldest rocks found on earth today are found in the Superior Upland, a part of the Canadian Shield, the broad swath Precambrian rocks that curves around Hudson Bay. These rocks make up the "core" of the North American continent. Other Precambrian rocks are found throughout the continent, in places such as the Adirondack Mountains, the Rocky Mountains, the Llano Uplift, the Black Hills, the Baraboo Range, and the Grand Canyon.