Geology Reference
In-Depth Information
Geo-inSight
North America's Precambrian Rocks
Precambrian rocks underlay much of North America
(Figure 19.5), but they are exposed only in the vast
Canadian shield and in areas where uplift and erosion has
taken place, as in the Appalachian and Rocky Mountains
and the Grand Canyon. Many of the older precambrian
(Archean) rocks are metamorphic, but large areas of younger
precambrian (Proterozoic) rocks show little or no effects of
metamorphism.
1. View of one of the Twin Lakes in the Beartooth Plateau on the
border between Montana and Wyoming. The Beartooth Scenic By-
way is advertised as the most beautiful in the United States. Most
of the rocks of the plateau are Archean-age gneisses.
2. Shell Falls plunges about 36 m into Shell Creek
Canyon in the Bighorn Mountains of Wyoming. The
gorge is incised into 2.9-billion-year-old (Archean-age)
granite.
3. This view from the inner gorge of the Grand
Canyon in Arizona shows the Vishnu Schist (black)
that was intruded by the Zoraster Granite. The schist,
which is about 1.7 billion years old, was originally
sedimentary rocks and lava fl ows.
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