At right is the first stratigraphic section of the Grand Canyon, from John Wesley Powell’s 1875 report, showing what would later be termed the Great Unconformity. Letter A is the metamorphic basement—the oldest rocks. B is the Precambrian Grand Canyon Supergroup, which is composed of tilted sedimentary rocks that lack fossil assemblages. C indicates Paleozoic rocks, which contain fossils marking the explosion of life. Two unconformities can be seen at x and y, with the former marking the Great Unconformity. At left is a recent photograph of Grand Canyon from Walhalla Plateau, with the red line showing the Great Unconformity. Credit: Annie Scott/USGS
Originally uploaded in Integrating Research and Education:Montana Geoheritage Project:Montana Geoheritage Sites.
Image 350534 is a 1080 by 1920 pixel WebP
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Nov21 24
