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environment leading to a layer of slightly different rock. In the case of a really deep section
of ancient rock, such as that seen in the Grand Canyon in Arizona, hundreds of millions
of years of deposits are represented. It is a natural human instinct to divide up and classi-
fy things, and sedimentary rock with its many layers is an obvious candidate. But, when
viewing a spatially narrow cliff face of flat layers, it's easy to forget that the layers are not
continuous around the world. The entire globe was never covered by a single shallow ocean
depositing similar sediments! Just as today, there are rivers, lakes, and seas, deserts, forests,
and grasslands, so in ancient times there was a panoply of sedimentary environments.
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