The rocks highlighted here are the oldest in the United States. They formed in Precambrian time, between 4.5 billion and 560 million years ago. Precambrian time is divided into Eras:
- Proterozoic Era (2500 to 560 million years ago)
- Archaean Era (3800 to 2500 million years ago)
- Hadean Time (4500 to 3800 million years ago)
The oldest Precambrian rocks shown here are about 2.6 billion years old, meaning
that they formed during the Archean Era. Even older rocks are exposed
in Canada; the Superior Upland
in Minnesota and Wisconsin is a younger part of this ancient "core"
of North America, called the Canadian Shield. Other ancient rocks
are exposed in various "windows" into the past across the country,
like the Adirondacks in
New York and the Llano Uplift
in Texas.
Precambrian features of the United States:
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