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Winter sunset from Yavapai Point. NPS photo by M. Quinn.
Grand Canyon is a place about time. Not the scenery, or not only the scenery —
the reason it stops people is that it shows you more of Earth's history, more
completely exposed, than almost anywhere else, and it does it all at once.
Walk the rim, hike an hour down, or run the whole 277 miles of river: the
thing that gets under people's skin is the arithmetic. Our timescales run to
hundreds and thousands of years. The wall in front of you runs to billions.
Three sets of rocks, bottom to top
The canyon's strata sort into three packages, and each one is a different
story rather than a different chapter of the same one.
| Set | Age | What it records |
|---|---|---|
| Vishnu Basement Rocks | 1,840–1,375 Ma | Crust forming, tens of miles down, as volcanic island chains collided with the young North American continent |
| Grand Canyon Supergroup | 1,255–729 Ma | Basins recording the assembly and break-up of an early supercontinent — and evidence of early single-celled life |
| Layered Paleozoic Rocks | 530–270 Ma | Layer by layer, dramatic geologic change and the evolution of animal life |
The Vishnu Basement Rocks: once tens of miles deep.
The Grand Canyon Supergroup.
The Layered Paleozoic Rocks.
What "best age" means
The numbers above are not the only numbers anyone has published, and this is
worth being plain about. A best numeric age is the most accurate and most
precise one currently available — which depends on the dating technique used,
on the geologic constraints the unit sits inside, on whether there is anything
datable in it at all, and on the fossil record of that particular rock unit.
They change as the work improves. That is not a weakness of the figures; it is
what having figures means.
Sources
Written from the National Park Service's *Telling Time at Grand Canyon
National Park*, a work of the United States government in the public domain.
Nestas coleçõesGrand Canyon National ParkTelling Time at Grand Canyon National Park
Licença: CC0 1.0 (domínio público) · Adaptado de www.nps.gov
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