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Comment: 17:32 - 18:26 (00:54)
Source: Annenberg/CPB Resources - Earth Revealed - 8. Earth's Structures
Keywords: "James Sadd", "Grand Canyon", "geologic structure", unconformity, uplift, deformation, "tectonic activity", sandstone, "sea level change", erosion, "sedimentary rock", contact
Our transcription: Unconformities are formed when rock is first removed by erosion followed by burial of the erosion surface by younger sedimentary rocks.
The horizontal contact separating the lower dark rocks from the overlying chocolate brown rocks is the great unconformity of the Grand Canyon.
The lower rocks are over 1.5 billion years old.
After they were deposited as sediments, they were deformed and tectonically uplifted during a mountain building episode.
Erosion then carved away at these rocks until sea level finally rose and flooded the area about 500 million years ago.
The younger rock layers covering the unconformity's surface are sandstones that were deposited at the bottom of that sea.
The great unconformity represents about a billion years of geologic time.
It reveals an important chapter in the geologic history of the Grand Canyon.
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